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Biotech. Adv. Vol. 6, pp. 725-770, 1988 0734-9750/88 0.00 + .50
Printed in Great Britain. All Rights Reserved Copyright (~) 1988 Pergamon Press plc
THE POTENTIAL OF MICROALGAL
BIOTECHNOLOGY: A REVIEW OF PRODUCTION AND
USES OF MICROALGAE
JOEL DE LA NOUE and NIELS DE PAUW
Groupe de Re c h e rc he e n Re c y c lage B io log ique Un iv e r s i t~ L av a l Quebec,
C a n a d a G 1 K 7 P 4
L abora tory
fo r
M a r i c u l tu r e S t a t e U n i v e r s i t y o f G h e n t G h e n t B e l g i u m
ABSTRACT
An overview of the various aspects, promises and limitations of microalgal
biotechnology is presented. The factors of importance n microalgal cultivation
as well as the culture systems are briefly described. Microalgal biomasses can
ful f i l the nutritional requirements of aquatic larvae and organisms. The
biochemical composition of algae can be improved by the manipulation of culture
conditions. The nutritive value of the microalgal biomasses for human and animal
consumption is also commented upon as well as some socio-economical aspects.
Among the sources of required nutrients N, P), wastewaters and manures can
upgraded as culture media for microalgae he safety of which has to be evaluated.
Harvesting of the biomass is one of the bottlenecks. The various techniques,
physical, physico-chemical and biological are outlined and their feasibility and
economic interest examined. Microalgal biomasses can be submitted to various
technological transformations. Various processes are reviewed in the light of
their effects on safety and nutrit ional value. The possible extraction of fine
chemicals and the preparation of protein concentrates is also reported on. The
various uses of microalgae lead to a possible competition, to be evaluated,
between systems for the production of food, energy and chemicals. The review
finally covers the application of genetic manipulation to microalgae.
725
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7 6
J D E L A N O U E a n d N . D E P A U W
KEYWORDS
Microalgae - Solar biotechnology - Culture conditions Microbiological
safety Harvesting Nutr it ional value Technical transformations - Fine
chemicals - Feeds/Foods - Genetic manipulations -
IMPORTANCE AND POTENTIAL OF MICROALGAE
Microalgae, as well as bacteria and yeasts, belong to those promising
microorganisms which deserve attention within the vast array of tradi tional as
well as new biotechnologies. Along with basic research, applied algology has
been developing rapidly over the last 40 years, star ting in Germany and extended
in the United States, Japan, Israel , I tal y, with the aim of producing single cel l
protein (SCP} and fat (see Burlew, 1953). As a result, already in the 1960s, the
commercial production of Chlorella in Japan and Taiwan as a novel health food
item was a success (Kawaguchi, 1980; Soong, 1980). In the 1950s the idea of
using microalgae fo r wastewater treatment was launched and in the 1960s interest
grew in developing ex tra ter rest ri al l i fe support systems. In the 1970s attention
went to the production of microalgal biomasses for fuel and fer t i l i zers. A new
trend in the 1980s is to use microalgae as a source of common and fine chemicals.
Furthermore, microalgae have since the 1940s been playing a role of increasing
importance in aquaculture. For histor ica l reviews on applied algology, the
reader is referred to Goldman (1979a) and Soeder (1980, 1986).
An important feature of microalgal systems is thei r v er sat i l i ty, making i t
possible to l ink di ff erent applications within the same process, for example
wastewater treatment and production of food, feed and chemicals. Another
at tracti ve characteristic of microalgae, in comparison with other microorganisms,
is the ir photosynthetic capabi li ty to convert solar energy into valuable biomass
with an interesting biochemical composition. As such, microalgae could play an
important role in solar biotechnology. Hereby, annual yields of 25 T and more,
even up to 200 T dry weight algae per hectare have been forwarded (Dubinsky e_tt
a ..., 1978; Goldman, 1979a; Shelef and Soeder, 1980; Soeder, 1980; Sant il lan,
1982; Richmond, 1986c). Moreover,more than 60 of the dry weight can be made up
by protein. Under favourable conditions, microalgal cultures can produce up to
20 to 35 times more protein than soybean and more than 50 times more than rice,
wheat or maize for the same area (Switzer, 1982; Ci ferr i , 1983). Of part icular
importance is also that microalgae can be grown yearound and harvested on a
continuous basis and be cultured on marginal lands in arid regions of the world,
ut i l iz ing waters unsuitable for conventional agriculture (Thomas, 1983; Cife rr i
and Tiboni, 1985; Gauthier et al_~.., 1985; Hal l, 1986; Richmond, 1986c). Algal
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MICROALGAL BIOTECHNOLOGY 7 7
cultures also have a lower water consumption than that required by tradit ional
cultivars (Heussler e_t_t al___:., 1978c). I f one considers that the water used for
algal cultures can be used afterwards for i r r igat ion, algal cultures are even
more advantageous.
Although there is yet no real breakthrough in this f ie ld , due to a number
of fundamental drawbacks (Benemann and Weissman, 1984; Benemann e .t al_._~., 1986;
Soeder, 1986), interes t in applied algology has never decreased i f we look at the
recent appearance of several important books on this matter: Algae Biomass
(Shelef and Soeder, 1980), Biotechnology and Exploi ta tion of Microalgae (Becker
and Venkataraman, 1982), CRC Handbook of Microalgal Mass Culture (Richmond,
1986a), Algal Biomass Technologies (Barclay and Mclntosh, 1986), Algal
Biotechnology (Stadler e_t_t al___~., 1988) and Microalgal Biotechnology' (Borowitzka
and Borowitzka, 1988). In connection one can mention the establishment in 1980
of an International Association fo r Applied Algology (IAAA) on ,the occasion of
the International Conference on Microalgae Production in Tru j i l l o , Peru (G.
Shelef, person, commun.) and the French Association fo r Applied Algology ,
active since 1982 (R. Fox, person, commun.). Also a newsletter cal led Applied
Phycology Forum , edited by W.R. Barclay in the US, is dist ri bu ted worldwide
since 1984 (Barclay, 1984-1985).
PRODUCTION OF MICROALGAE
Figure l schematically shows the major pathways followed in microalgal
production and ut i l iza t ion of the biomasses for di ff erent purposes. Production
of microalgae f i r s t involves the cu lt iv at io n, followed in most cases by harvest-
ing and processing o f the algae (Soeder, 1980; Becker and Venkataraman, 1982).
From a systematic point of view, the microalgal group includes several
thousand species belonging to two major groups: the prokaryotes inc luding
blue-green algae (cyanobacteria) and the eukaryotes, including a.o. green algae
(Chlorophyta), red algae (Rhodophyta), and diatoms (Bacil lariophyta). Of these,
only 30-40 species have been considered for mass cu lt iv at ion and only few are
presently of real commercial importance (Borowitzka, 1988a; Richmond, 1986c). To
these belong representatives of the genera Chlorella, Scenedesmus, (green algae),
Spirul ina (a blue green alga) and a number of phytof lagellate and diatom species
which are used as live food in larval mariculture (De Pauw and Persoone, 1988).
Other algae of commercial in terest in the future could be Dunaliella (a green
flagel late), Porph~ridium (a red alga) and Botr¥ococcus (a green alga) (Richmond,
1986c). Expressed in quanti ties, the world production of microalgae (mainly
Spirul ina and Chlore lla) only amounts to about one thousand tons per year
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7 2 8
[ DE LA NOUE and N DE PAU W
~I ~ CULTIVATION POND
m c cc ~ ~ 0m;~nolm.ra~ ~
~ ccMP~ I
l i
F ~F ~
Figure I. Flow diagram of algal mass cultivation systems (from Becker and
Venkataraman, 1982).
(Kawaguchi, 1980; Soong, 1980; Ci ferr i and Tiboni, 1985) but is expected to
increase markedly in the near future (Venkataramanand Becker, 1988).
Determinants of Algal Growth
Major factors of importance in the cult ivat ion process are l ight,
temperature, nutrients, pH and agitation (Becker and Venkataraman, 1982).
Although maximum growth rates are achieved in conditions of l ight saturation,
maximum yields wi l l be obtained only when l ight is the l imi ting nutrient since
production is proportional to solar energy conversion eff iciency. Light
limi ta tion is established as a function of irradiance by adapting the areal
density of the culture and thus the algal concentration. In continuous cultures
this is achieved by changing the detention time or d ilu tion rate of the culture
(Goldman, lgTgb). The maximal theoretical conversion efficiency for total l ight
energy has been established at 6.6 (Shelef and Soeder, 1980). However,
sustained efficiencies of conversion of total incident radiation are more l ikely
to be within the range of l to 2 , or expressed in PHAR (Photosynthetic Active
Radiation) about 2 to 4.5 (Benemann eta)..., 1977; Goldman, 1979b; Shelef and
Soeder, 1980; Soeder, 1980; Pouliot and de la NoUe, 1985). Translated into dai ly
yields, these conversion ef fic iencies correspond to less than lO g up to 30 g and
in exceptional cases even 50 g dry weight per m (Goldman, 1980). Althoughunder
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MICROALGA L BIOTEC HNOLOG Y 7 9
l ight limiting conditions, the effect of temperature is lessened with regard to
the growth rate, too low temperatures may also become limiting especially during
the daytime or in winter Toerien and Grobbelaar, 1980; Vonshak e.t_t al., 1982;
Bedell, 1985). For this reason heating up of the cultures may be beneficial
mainly in areas with a l ot of incident radiation. Optimal temperatures for most
species range between 15 and 30°C.
Several kinetic as well as empirical models predicting algal productivities
as a function of irradiance and/or temperature have been developed Goldman,
1979b; M~rkl, 1980; Toerien and Grobbelaar, 1980; Hil l and Lincoln, 1981; Grobbe-
laar e al_~., 1984). Other models also take nutrients Shelef, 1981, in Grobbe-
laar et a l. , 1984) and mixing into account Erickson and Lee, 1986). In open air
cultures, yields are pract ically linearly correlated with the incident radiation
Paelinck, 1978, in De Pauw and Van Vaerenbergh, 1983; Castillo e_t_tal_~., 1980).
For optimal growth, the culture must be provided with nutrients in adequate
amounts Borowitzka, 1988b). These include several macronutrients such as
carbon, nitrogen and phosphorus, sulfur , potassium, for diatoms also si licon)
and a number of trace elements like minerals e.g. Co, Mo, Mn) and several
vitamins e.g. Bl2, thiamin). Besides quantit ies, the right proportions among
nutrients e.g. N:P; N:Si) are also important. Basic information on algal
nutr ition is given by Kaplan et al. 1986). With regard to the source of carbon,
apart from some algae like Spirulina which are capable of using bicarbonate at
alkaline pH values Ci fe rri, 1983), most photo-autotrophically growing algae
prefer to ut i l ize free CO2 Heussler e al., 1978a; Richmond e al., 1982; Kaplan
e_t_t al.__~., 1986). Yields may, however, be increased two- or threefold by using
organic sources of carbon like glucose or acetate mixotrophic growth) which of
course adds substantially to the production costs Soong, 1980).
Present attention is also directed towards growing microalgae
heterotrophically in the complete absence of l ight of which certain species e.g.
Chlorogonium elongatum and Chlorella p~renoidosa) are capable Kawaguchi, 1980;
Becker and Venkataraman, 1982; Kreuzberg et al. , 1985; Kaplan et al ., 1986).
This process could also be exploited under conditions of low illumination where
glucose can replace l ight as the energy source Folmann et al., 1978). The type
of C-source is related to the pH Richmond, 1986b). This factor substantially
controls the bio-avai labil ity of nutrients. A too high pH for example wi l l ,
through the photosynthetic activity, make the free carbon dioxide unavailable to
most algae. Phosphorus, too, be may precipitated and ammonia nitrogen is also
stripped to the air De Pauw and Van Vaerenbergh, 1983). For this reason,
addition of free carbon dioxide is beneficial to algal growth Becker and
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730 J DE LA NOUE an d N DE PA UW
Venkataraman, 1982). Moreover, norganic carbon levels play an important role in
affecting individual species growth rates (Novak and Brune, 1985).
Agitation is also very important to microalgal cult ivat ion, not at least to
avoid sedimentation, photoinhibit ion, nutrient limita tion and thermal
st ra tifi cation but also to increase the l ight conversion efficiency (M~rkl, 1980;
Persoone e_t_t al. , 1980; Richmond, 1986d). Since mixing requires energy, however,
the most economical regime wi l l have to be sought for each cult ivat ion system,
for which the algal output is maximal (e.g. De Pauw et al . , 1983; de la NoUe e.t_t
al~, 1984). Even i f the above conditions are optimally fu l f i l led, a number of
biological problems can and wi l l arise in the mass cult ivat ion of microalgae.
These include contamination, grazing, diseases, premature collapse and lack of
species control (Shelef and Soeder, 1980; Becker and Venkataraman, 1982; De Pauw
e_t_t al~, 1984; Richmond, 1986d). Control measures for avoiding contamination by
bacteria and other algal species are ster il izat ion and ul tr af i lt ra t io n of the
culture medium (Ukeles, 1976). Grazing by protozoans and diseases like fungi can
eventually be treated chemically (Heussler et al . , 1978b; Becker and
Venkataraman, 1982; De Pauw, pers. commun.; Becker, 1986). Larger zooplankters
and insects can be removed mechanically by screening (De Pauw et al . , 1983;
Becker, 1986), eradicated chemically (Loosanoff et al . , 1957) or by changing the
culture conditions (Lincoln et al . , 1983; SchlUtter and Groenweg, 1981).
Premature collapse of pure algae cultures during upscaling can be avoided by
establishing balanced growth conditions. A protocol for defining such conditions
has been presented by Pruder (1981). Control over the species composition of
large scale cultures on the other hand can be obtained to a certain extent by
proper operational management (Azov et al ., 1980; Richmond et al . , 1982; Vonshak
et al. , 1982; De Pauw et al . , 1983).
Cultivation Systems
Depending on the purpose of the mass production, the technology employed
may vary from a state close to agriculture to elaborate biotechnology (Soeder,
1980).
With regard to the origin of nutrients involved in the cultivation, a
dist inct ion can be made between clean water and wastewater-based production
processes (Soeder, 1980). In the former case str ict ly defined media are usually
employed in which bacteria are of no significant metabolic importance; in the
latter case (Fig. 2) less defined media such as sewage and manure are used in
which mixed cultures of bacteria and microalgae are act ive. While for small
scale culturing one usually relies on media which are complex and expensive, in
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MICROALGAL BIOTECHNOLO GY 7
good effect. I t has also been shown that greenhouse technology could make
yearound algal production feasible in northern climates with low temperatures but
suffi cient solar irradiance Pouliot and de la NoUe, 1985). Recently proposals
have also been made to ut i l ize geothermal water Bedell, 1985; Goldstein, 1986).
With regard to the sophistication of the cul tivation technology, the process may
be semi-natural, without inoculation of algae, and a natural bottom, e.g. the
cult ivat ion of Spirulina at Sosa Texcoco) or a r t i f ic ia l , with inoculation of
precultured algae and lining of the bottom e.g. Spirulina cultivation in Taiwan)
Ciferri and Tiboni, 1985).
Finally, i t must be mentioned that the cultivation process can be
batchwise, semi-continuous or continuous Vonshak, 1986).
Harvestin9 of microalgae
One of the major bottle-necks, limi ting the further expansion of most
microalgal biomass applications, is the cost-effective harvesting Table l ) .
Except for a few larger algae like Spirulina, which can be easily recovered by
simple gravity f i l t rat ion and inexpensive microstraining Becker and
Venkataraman, 1982), this is not at all the case for most species which are
indeed small in size less than 20 ~m). For this reason, many efforts have been
devoted to the development of suitable technologies for harvesting these small
particles Mohn, 1980; Richmond and Becker, 1986). Though technically solved,
the handicap s t i l l remains the incompatibil ity between the eff iciency of the
proposed methods and their cost-effectiveness Benemann e_t_tal~, 1980).
Table I.
Advantages and disadvantages of different harvesting methods
modified from Benemann e _tal.~., 1977, 1979 in De Pauw and Van
Vaerenbergh, 1983).
Method Re liabil ity Cost energy Quality for
requirement bioconversion
Centrifugation good high good
Chemoflocculation good high poor
Sandfiltration fai r low poor
Ultrafi l tration good high good
Microstraining poor low poor
Bioflocculation poor low good
The most successful techniques are centrifugation, f i l t rat ion and
flocculation Mehn, 1980). In practice, a combination of techniques is often
used to preconcentrate and/or concentrate.the algae. For preconcentrating the
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734 J DE LA NOUE an d N DE PAU W
algae, chemical flocculation eventually followed by flotation is common practice
in many operations Becker and Venkataraman, 1982; De Pauw and Van Vaerenberg,
1983; Lavoie et al., 1984; Richmond and Becker, 1986). However, a number of
problems with the use of flocculants, including toxicity and carcinogenicity,
have been recognized Dodd, 1979). This aspect is certainly important when the
objective is to use the algal biomasses as food or feed. A promising non-toxic
flocculant in this regard could be chitosan which is a natural product derived
from chitin, avail able worldwide Nigam et al., 1980; Becker and Venkataraman,
1982). Presently n its disfavour, is its high price. Process optimization and
recycling of the flocculant, however, could increase the cost-effectiveness of
chitosan Lavoie and de la NoUe, 1983; Lavoie et al., 1984; Morales et al.,
1985). To reduce the high inorganic flocculants demand, the use of polymers or
ozone treatment prior to the flocculation process has also been recently proposed
Shelef et al., 1986).
Another important step forward would be to get control of the process of
bioflocculation autoflocculation) of microalgae without addition of chemicals
Benemann e___al~., 1980). Bioflocculation is the formation of cellular aggregates
by means of exocellular polymers. Research is in progress to unravel the
mechanism of this phenomenon Sukenik and Shelef, 1984; Lavoie, 1985; Sukenik e_.tt
al., 1985). Bioflocculation can also be enhanced by continuous mixing of the
cultures, promoting the succession of readily settl ing self-flocculating species
or inhibiting photosynthesis in standing populations Lincoln and Koopman, 1986).
Recently the great potential of tangential flow fi l tration for concentrating
marine microalgae has also been demonstrated Welsh e_t_t al., 1985). The equipment
is, however, st i l l expensive. Finally, for harvesting Dunaliella, several
procedures have been proposed which exploit the high salinity-dependent
physiological and behavioural characteristics of this species Borowitzka and
Borowitzka, 1988). Another method is exploiting the salinity-dependent
hydrophobicity of the Dunaliella cell membrane Curtain and Snook, 1983).
UTILIZATION OF MCROALGAE
MicroalBae for Aquaculture
Microalgae are one of the live foods which are essential in aquaculture for
hatchery rearing of bivalve molluscs and peneid shrimp as well as the culturing
of several zooplankters rotifers, cladocerans, brine shrimp, copepods) which are
themselves live food organisms for larvae of marine fish and curstaceans De Pauw
and Pruder, 1986). Numerous, more or less sophisticated systems, have been
developed for culturing some 40 algal species to feed these larvae and
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MICROALGAL BIOTECHNOLOGY 7 5
zooplankton organisms (Ukeles, 1976; Watson, 1979; De Pauw, 1981; De Pauw and
Pruder, 1986; De Pauw and Persoone, 1988). In particular cases where pure
nutrient-rich well water or deep sea water is available, i t has been shown that
large scale production of monospecific algal cultures to feed oysters and clams
up to market size would be feasible (e.g. Roels et al., 1976; Scura e_tt al_~.,
1979). Also, i t has been demonstrated hat closed-cycle rearing of the American
oyster, from the larva up to the market size, is possible (Pruder and Greenhaugh,
1978)o However, in the latter case, the economic feasibility can be put in
question. For this reason, when large quantities of microalgae are needed, the
alternative for pure algae cultures may consist of bloom induction of natural
phytoplankton (De Pauw, 1981; De Pauw and De Leenheer, 1985).
De Pauw et al. (1983) demonstrated hat with proper operational management,
i t is possible to steer the composition of the natural assemblages towards
species suited to the consumers. Based on that principle, an industrial model
for nursery culturing of bivalve molluscs has been worked out (Claus et al.,
1984). The cultivation of microalgae none the less requiring specific skills,
the harvest and eventually the storage of algae grown at latitudes with ample
sunshine would represent a major breakthrough in aquaculture hatchery and nursery
operations (De Pauw et al., 1984). Apart from the constraints of economical
harvesting, however, are the processing and storage of the algal harvest and the
(re)treatment of the stored algae to make these acceptable again to the
consumers. More specifically, microalgae entangled n the matrix of a flocculant
are of too large particle size to be ingested by most filter-feeders, and
techniques for declustering such algal masses need to be developed (COST,
1983).
Apart from technical and economic problems involved with the mass
production of microalgae, the major problems in aquaculture are nutrition-
related (De Pauw e_t_tal_~., 1984). On the one hand, there is the lack of knowledge
of the nutritional requirements of the microalga consumers (molluscs,
crustaceans, fish) which are diff icult to assess, and on the other hand, there is
the biochemical composition of the microalgae which is determined by the culture
conditions (Webb and Chu, 1982). Of particular importance here is the presence
of essential fatty acids and the degree of fatty acid unsaturation. These
quantities can be modified by changing the culture conditions (Samson, 1980;
Enright e_t al.__~., 1986b). Of importance, for example, are l ight, temperature,
N-source, N:P ratio, etc (Sansregret, 1986). The same is true of the content of
amino acids and carbohydrates which are also of importance (Enright e_tt a__~.,
1986a; Terry e_t_tal_~., 1983; Terry, 1986).
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736 J DE LA NOUE an d N DE PAU W
Microal~ae for Wastewater Treatment
The increasing deterioration of our environment and the need for energy and
food is forcing us to explore the feasibi l i ty of wastewater recycling and
resource recovery. Within this context, bio-treatment with microalgae is
part icular ly at tract ive because of thei r photosynthetic capabi li ties, converting
solar energy into useful biomasses and incorporating nutrients such as nitrogen
and phosphorus causing eutrophication (Fig. 2). This fascinating idea launched
some thirty years ago in the U.S. by Oswald and Gotaas (1957) has since been
intensively tested in many countries (see examples n Goldman, 1979a; Shelef and
Soeder, 1980; De Pauw and Van Vaerenbergh, 1983). Thereby, emphasis may be put
on wastewater treatment and/or algal biomass production. Depending on the
options taken, deep oxidation ponds or shallow high rate oxidation or algal ponds
(HIROP, HRAP) are used for this purpose. For a review on the variables playing a
role in the design and operation of microalgal wastewater treatment systems we
refer to Azov and Shelef (1982), De Pauw and Van Vaerenbergh (1983) and
Abeliovich (1986). Processes involved in the removal of nutrients are
precipitat ion, stripping and (luxury) uptake by algal biomass. The efficiency of
removal of a part icular nutrient (for example N or P) wil l also depend on whether
or not these nutrients are limi ting in the wastewater to be treated. In this
regard i t was shown by de la No~e et al. (1980) that a two-phase culture system
with preconditioning of the algae could increase the nitrogen uptake of the
starved algae. In practice, with adequate stirring more than 90 nitrogen and/or
phosphorus can be removed (e.g. De Pauw e__t_tal__~.., 1978; Lincoln and Hi l l , 1980;
Shelef e_t_tal__~., 1980; Martin et al . , 1985a,b).
Provided suff ic ient solar irradiance and space are available, (non-toxic)
wastewaters of various or igin and nature (municipal, industr ia l, agricultural ,
aquacultural) may be treated with microalgal systems. Part icular ly promising
seem to be the combined treatment and upgrading of animal manures from
bio-industries with algal biomass production possibly in combination with biogas
production (Chung e_t_ta___~., 1978; Taganaidese__t_ta___~., 197g; Lincoln and Hi l l , 1980;
Groeneweg e_t.tal___~., 1980; De Pauw et al . , 1980a; Pieterse and Le Roux, 1980; Soong,
1980; Duerr, 1985; Martin e_t_tal~, 1985b). Light and temperature being the major
factors determining yi eld, wastewater treatment with algae wi l l be part icular ly
suited for application in tropical and subtropical countries (Soeder, 1984).
Light often being the limiting factor in Northern climates during winter time,
the use of ar t i f i c ia l il lumination to increase the performance of wastewater
treatment in Quebec, Canada, has also been considered (Pouliot and de la NoUe,
1985). However, the cost of such a process is presently prohibi tive, i f not
forever. I t is also important to stress that in the treatment process, a large
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MICROALGAL BIOTECHNOLOG Y 7 7
array of microalgal species developing in different aquatic environments e.g.
fresh-, brackish-, sea-, alkaline-, brine-water) adapted to specific high
nutrient loads can be utilized De Pauw and Van Vaerenbergh, 1983).
To be effective in wastewater treatment, the algal biomasses must be efficiently
removed. Different pathways may be followed to this end. Usually, harvesting
includes a solid-liquid separation followed by dewatering and drying Moraine e_t_t
a_._~., 1980; Richmond and Becker, 1986). Although harvesting is not presently
cost-effective, i t has been shown that under tropical and subtropical conditions
the cost-benefit of microalgal wastewater treatment processes may compare
favourably with classical wastewater treatment systems such as activated sludge
systems. Particularly in favour of microalgal wastewater treatment systems is
the fact that the harvested algal biomasses can be upgraded n numerous ways, for
example to animal feed, and that no sludges have to be handled Shelef e_t_t al.,
1978).
Wastewater grown microalgae may be used not only as a supplement n animal
feed Chung e_t_t a.__~., 1978; Lincoln and Hil l , 1980; Sandbank and Hepher, 1980;
Soong, 1980; Saxena e_t_t al_~., 1983) but also in aquaculture for feeding fish,
molluscs and crustaceans see several examples in Grobbelaar et al., 1981; De
Pauw and Van Vaerenbergh, 1983 and de la NoUe et al., 1986), and as a source of
energy, fuel, fertilizers and chemicals Benemann e_t_t a___~., 1977). Indirect
harvesting of the microalgae by fil terfeeders through art i f icial aquatic food
chains including zooplankton, bivalve molluscs or fish, has also been a promising
and realistic alternative e.g. Dinges, 1974; Goldman and Ryther, 1976; De Pauw
et al., 1980a,b; Pieterse and Le Roux, 1980; Edwards, 1980; Groeneweg and
Schl~tter, 1981; Tarifeno-Silva e_t_tal...~_., Ig82a; Proulx and de la NoOe, 1985a,b).
Depending on the destination of the algal biomass, different criteria will
have to be taken into account with regard to possible contaminants such as heavy
metals, pesticide residues, pathogenic bacteria and viruses. However, the few
results already available indicate that fear concerning potential risk might be
excessive Yannai e_t_tal., 1980; Tarifeno-Silva et al., 1982b; de la NoOe e___al.,
1986; Gauthier et al., 1985; Becker, 1986). Pathogens and bacteria eventually
remaining will be eliminated from the algal biomass at the processing stage. In
this context, microalgal production could also be used as a trap for toxic ions
and molecules Aaronson e_t_ta_._~., 1980).
Since the land-space requirements of microalgal wastewater treatment
systems are substantial De Pauw and Van Vaerenbergh, 1983) efforts are being
made to develop wastewater treatment systems based on the use of
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7 38 J D E L A N O U E a n d N D E P A U W
hyperconcentrated algal cultures. This process, called activated algae
(McKinney et al . , 1971), is highly efficient in removing N and P within very
short periods of time, e.g. less than one hour (Lavoie and de la No~e, 1985).
Concentrated algal cell suspensions experiencing severe light l imitat ion appear
to be more promising for tertiary treatment than for actual biomass production
purposes. The design of workable systems, not only with free microalgae, but
also with flocculated or algae immobilized for example n carrageenan beads, has
also been shown possible (de la NoUe et al . , 1983; Chevalier and de la NoUe,
1985a,b; 1986). Also, recycling part of the algae produced, to operate at the
shortest possible retention time, could increase system performance (de la No~e
and Ni Eidhin, 1988). The challenge will be to make these systems not only
reliable but also economically competitive with conventional wastewater treatment
systems. I t could well be that the fi rs t applications to effluent treatment wi l l
occur in the field of toxic metal removal from industr ial eff luents. Some
promising systems are currently under investigation (see, for example, Kosaric
and Ngcakani, 1988).
Microalgae as Human Food
Many ef forts have been made to promote microalgae as food directly for man.
This idea has been even more supported by the discovery that several native human
populations, at Lake Chad in Africa and Lake Tezcoco in Mexico among others, have
subsisted part ia ll y on the nutr it ional qualit ies of the naturally occuring
blue-green alga Spirulina (Durand-Chastell, 1980; Ci ferri, 1983; Bourges, 1986).
Representatives of this prokaryotic alga are indeed very rich in protein (Table
2) and except for some deficiency of sulphur-containing amino acids (methionine,
cysteine), have a f ai r ly balanced composition (Table 3) comparing favourably with
egg and milk protein. The same is true of essential fatty acid content
(Sant illan, 1982; Ci fe rr i, 1983; Bourges, 1986). The nut ri tional qual ity of
Spirul ina protein is even superior to that of soybean (Jaya et al . , 1980).
Table 2. Basic chemical composition of the microalgae, Scenedesmus obliquus
and Spirulina maxima, as compared to soya bean (whole seed) and
wheat (whole gra~-~. Modified af ter Soeder (1980).
Components Scenedesmus Spirulina Soya seed Wheat
Crude protein 50-60 56-62 34-40 13.4-13 .5
Water 4-8 I0 7-10 12 .8-13.5
Li pi ds 12-I 4 2-3 16-20 2. 1-2.4
Carbohydrates l O-I 7 16-I 8 19-35 78.6-80.5
Crude fibre 3-I0 0.I-0.9 3-5 2.1-2.4
Ash components 6-I0 6.4-9.0 4-5 1.6-2.8
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Table 3.
M CROALGAL BIOTECHNOLOGY 739
Essential amino acid composition of Scenedesmus species compared o the
FAO pattern (g/16 gN).
Amino acid
S. acutus S__~.obliquus2 FAO3
VAL 4.7 5.7 5.0
LEU 7.0 8.3 7.0
ILEU 3.1 4.1 4.0
PHE+TYR 6.0 lO.l 6.0
LYS 4.6 5.9 5.5
MET+CYS 3.2 2.9 3.5
TRY 1.7 n.d. l.O
THR 4.9 8.6 4.0
l Art i f ic ial medium (Becker et al . , 1976).
2 Urban wastewater (Proulx a~dTla NoUe, 1985b).
3 FAO/WHO (1973).
Furthermore, representatives of eukaryotic microalgae such as Scenedesmus,
Chlorella and Coelastrum have more or less the same nutri tional characteristics
(EI-Fouly e _t al__~., 1985). Microalgae are also rich in vitamins and other growth
factors. For these reasons, several research groups have been endeavoring to
establish production units for these algae in tropical or subtropical
(developing) countries l ike Peru, Thailand, India, and Egypt but also in the USA
and Israel (Soeder, 1980; Castillo e_t_tal__~., 1980; Becker and Venkataraman, 1982;
El-Fouly e_t_t al . , 1985; Richmond, 1986c). In most of these projects, serious
attention has been paid to the nut ri ti onal qual ity and the possible toxicological
effects of the algae (Payer e al . , Ig80; de la NoUe e al.__~., 1986). The tests
involved humans as well as animals (Pabst, 1978; Becker, 1980; Ci ferri, 1983).
Extensive testing has indicated that microalgae are a valuable and safe source of
protein. Testing with malnourished children also showed promise (Gross et al . ,
1978). The high nucleic acid content, however, limi ts the admissible daily
consumption to about 5 of the human requirements (Becker, 1986). Based on a
recommended maximum daily supply of 2 g nucleic acids from SCP per adult
(FAO/WHO, 1973, in Becker, 1986), 46 g of dry Spirulina or 15 g Scenedemus would
pose no problems (Becker, 1978a, 1986; Soeder, 1980; Bourges, 1986). Some
sensi tivi ty reactions have been reported with Chlorella in humans but no major
adverse effects have been reported (Shubert and Larsen, 1985; see also Scrimshaw,
1986).
I t is also of importance that the di gestibi l i ty and the nut ri ti ve value of
microalgae are influenced by the processing technology used (Becker, Ig80~
Venkataraman et al . , 1980; Becker and Venkataraman, 1982). I t has been shown
that d igestibi l i ty especially of chlorococcalean algae could be markedly
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7 40 J D E L A N O U E a n d N D E P A U W
increased by cracking the cell wall with appropriate treatments such as drum
drying, spray drying, etc (Mitsuda etal . , 1977a,b; Becker, 1980). However, he
high cost of production including the harvesting and processing, and the
potential consumer nonacceptance (determined psyche- culturally, poor sensorial
properties) are the major obstacles to a defini tive breakthrough. Direct
consumption is therefore presently limited to the use of expensive health food
sold for US $ 60 per kg and more on the international market (Bourges, 1986;
Richmond, 1986). Total production amounts to about one thousand tons per year
mainly involving Spirulina from Mexico and the U.S.A. (Ciferri and Tiboni, 1985)
and Chlorella from Japan and Taiwan (Kawaguchi, 1980; Soong, 1980). In contrast
with the health food cultus (Hills, 1980; Switzer, 1982), a different philosophy
is followed by several other scientists who endeavor to exploit inexpensive
algoculture systems to combat malnutrition in developing countries (Fox, R.,
1980, 1983, 1985; Olguin and Vigueras, 1981; Becker and Venkataraman, 1982). The
integrated vi llage health and energy systems involve cultivation of Spirulina
along with the production of biogas and compost.
Micro~lgae as Animal Feed
As in the case of human food, microalgae have also been successfully used
as an animal feed ingredient. Feeding experiments with rats, mice, poultry,
pigs, sheep and carp, demonstrated unequivocally that microalgal meals produced
from various strains or species of Chlorella, Scenedesmus and Spirulina are
valuable protein sources lacking any acute toxicity (Becker, 1980, 1986). As
already mentioned s the utilization of wastewater grown microalgal biomasses for
which hygienical cr iteria are not so stringent as for human food part icularly
promising (see examples given in section on Microalgae for wastewater
treatment ). Primary and secondary toxicological testing of sewage grown algal
biomasses with regard to heavy metals demonstrated the likelihood that routine
use wi ll turn out to be toxicologically safe (Becker, 1980, 1986). Moreover
these biomasses are a by-product of wastewater treatment and thus cost-
competitive in with conventional feeds (Shelef et al., 1978). In contrast,
though the potential is there, the cost of pure microalgal biomasses like
Chlorella and Spirulina presently prohibits their extensive application in
aquaculture (De Pauw etal____~., 1984).
Microalgae as a Source of Energy
At pilot scale i t has been demonstrated that microalgal biomasses can be
converted by fermentation into energy-rich products such as methane gas, alcohol,
or liquid fuel as vegetable oils or hydrocarbon (Benemann et al., 1977, 1986;
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MICROALG AL BIOTEC HNOLOGY 74
Mitsui, 1980; Cohen, 1986). A promising alga in th is regard seems to be
Botrxococcus braunii for the production of long-chain hydrocarbons Gudin e al . ,
1983; Benemann and Weismann, 1984; Richmond, 1986c). Though technically
feasible, a major drawback to practical appl ication is that enormous algal plants
are needed o ensure even a small percentage of our energy requirements. Concepts
have been developed for the exploi tat ion of algal energy farms up to lO0 square
miles in size in which unreclaimable water and wastewater nutrients could be used
Oswald et al . , 1977). The microalgal fuel economics and engineering of such
systems are described by Goldman and Ryther 1977) and Benemann et al. 1986).
Aside from the fact that no one is presently running algal farms larger than a
few thousand square meters, major constraints to further development are the
economical harvesting of the algal biomasses and the lack of control over
unwanted algal consumers like zooplankters. Moreover, n the present situation,
methane from microalgal biomasses is not at all competitive with conventional
energy sources Dubinsk.v e__t_tal__=, 1978). Other problems are related to the actual
methane fermentation of the algal biomass Benemann et al . , 1977).
Many algal species can also be induced to produce hydrogen through
biophotolysis. Cultures of ni trogen-f ix ing heterocystous e.g. Anabaena) as well
as non-heterocystous blue-green algae e.g. Spirul ina, Osci llator ia) can be used
for this purpose Benemann and Weissman, 1976; Hallenbeck e_t_t al.__~., 1978; Mitsui,
1980; Kumazawa and Mitsui, 1982, Karube et al~, 1986). A team from the
university of Miami Mitsui and co-workers) demonstrated hat a part icular strain
of marine blue-green non-heterocystous alga Osci llator ia) in a chamber 20 feet
square and three feet deep, could produce enough hydrogen to yield lO00
kilowatt-hours of elect r icity per month. Technical as well as economic
constraints, however, presently rest rict the practical application of these
systems. Immobilization of microalgae or cyanobacteria by entrapment in various
matrices polyurethane and polyvinyl foams, for example) might prove to be an
interesting solut ion to the problem of production of fuels, energy and chemicals
Ha l l , 1 9 8 8 ) .
Microal~ae as Fertilizers
A promising idea gaining more and more interest is the production of easi ly
harvestable nitrogen-fixing blue-green algae in conjunction with wastewater
treatment which could be converted into organic nitrogen f er t i l i zer Benemann,
1979; L i, 1981; Padhy, 1985; Venkataraman, 1986). I t has also been shown that
unconsumed microalgae in a wastewater-fish production system could be directly
upgraded agriculturally for the production of maize Edwards e al~, 1981). This
could be part icular ly rewarding in tropical countries where microalgal wastewater
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7 42 J D E L A N O U E a n d N D E P A U W
treatment systems could be easily applied. Presently, several companies nvolved
in algal biotechnology, are exploring the potential of blue-green algae as
agricultural fertilizers and soil conditioners (Applied Phycology Forum, 1985;
Metting, 1985). According to Curtin (1985) one company in the US has already
started producing and selling fertilizers composed of blue-green algae, which are
competitive with conventional agricultural fertilizers. One kg of algae could
replace 60 kg of conventional nitrogen ferti l izer.
Microal~ae as a source of common and fine chemicals
Our modern industrial world depends heavily upon petroleum and its
derivatives to obtain a vast array of useful chemicals. Production of microalgae
in arid and sunny parts of the world may be a solution to the economic and
material stress raised by the needs of the industry. It has been suggested that
those who wi ll benefit f irst from commercial marine biotechnology wi ll be the
producers of sugars and polysaccharides, pharmaceuticals, dyes, bioflocculants,
pigments, vitamins, lipids, oi l , etc. One of the most convincing examples is
that of phycobiliproteins which are fluorescent dyes used in certain immunoassays
and cell separation and worth 75/mg (Curtin, 1985). Two categories of products
can be obtained f~om microalgae (Gudin e_t_t al_~., 1983): endocellular substances
that act as osmoregulators in the cell (glycerol, sorbitol, mannitol, etc) or do
not (starch, amylase, amylopectin, glycogen) and exocellular products, mainly
polysaccharides (glucan, mannan, chitan), hydrocarbon or polyacrylates.
Obviously, the latter can be recovered more easily, and usually without
destruction of the cells.
The many products of interest find applications in the chemical, food,
and pharmaceutical industries and medecine (Tables 4 and 5). For reviews on
chemicals and products from microalgae, the reader is referred to Aaronson et
al. (1980), Benemann and Weissman (1984), Borowitzka (lg88a), Cohen (1986). The
diversity of exo-polysaccharides produced by microalgae is impressive and
undoubtedly represents considerable potential for the food industry as gums,
thickeners, gelling agents and stabilizers and many other diverse uses (Weiner e__
al_~., 1985). A carrageenan-like polysaccharide has been extracted from the marine
microalga Porph~ridium cultivated in outdoor saltwater ponds (Curtin, 1985) by
Israeli workers (Arad et al., 1986). It has been shown by Weissman and Benemann
(1980) that polysaccharide productivity can be markedly enhanced by nitrogen
starvation. This alga is currently one of the most promising, and efficient
culture systems have been designed (Gudin e_t_t al., 1983).
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M I C R O L G L B I O T E C H N O L O G Y
Table 4. Application of microalgae as a source of chemicals.
7 4
Chemicals Example Application
Proteins Protein concentrates Food and feed industries
Lipids Glycerides (glycerol) Fuels, food additives
Pigments B-carotene, Phytol Precursor vitamins
Phycobiliproteins Dyes, cosmetics
Vitamins Biotin Vitamin-rich meals
Carbohydrates Mannitol, Sorbitol Art i f icial sweeteners
Polysaccharides Viscosifiers
Pharmaceuticals Sterols Steroid hormones
Chlorella extract Antibiotics
Toxins Anti-parasitic
Table 5. Valuable products from microalgae.
Source Product Reference
Anabaena flos-aquae Protein Molton et al., 1980
Chlorella sp.
Phaeodactxlum tricornutum
Dunaliella tertiolecta
Dunaliella tertiolecta
Asteromonas 9racilis
Microphytes
Chlam~domonas agloeformis
Porphxridium cruentum
Bacillariophyceae d~atoms)
Botrxococcus braunii
Phaeocxstis p ~ i
Scenedesmus acutus
Spirulina
Dunaliella, Spirulina
Cyanobacteria
Spirulina
Lipids Molton et al. , 1980
Sansregr-6t~T986
Starch (insoluble)Williams et al., 1978
Glycerol Dubinskye-t-aT., 1978
Ben-Amotz~ta-a-T., 1982
Agar-Agar Bonin, 1983
Carrageenan
Polysaccharides Moltonet al., 1980
Gudin et--a.l~,Ig83
Chitan Gudin ~a l . , 1983
Hydrocarbons Gudin et-aT., 1983
Polyacrylates Gudin~aT., 1983
Pigments Partali e_t_ta___~., 1985
Phycocyanin Tel-or et al., 1980
g-carotene Tel-or e t aT., 1980
Chlorophyll a
Phycobiliprotein Curtin, 1985
Enzymes Tel-or et al. , 1980
Ferredoxin
Ferredoxin-NADP reductase
Cytochromes
Ribulose bi-phosphate
Carboxylase
Although the ini t ial investments appear high, even prohibit ive, the cost of
producing glycerol from Dunaliella could be competitive with that of
petrochemical methods (Ben-Amotz and Avron, 1980). The method is subject to
patent applications and pi lot-scale production has been undertaken in Israel
where long-term productivities of 4.5 g of glycerol/m2.day in 3.5 M NaCl have
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7 44 J D E L A N O U E a n d N D E P A U W
been measured (Ben-Amotz et al . , 1982). An intracellular glycerol content of up
to 4.4 molar in cell has been reported for D. v ir idis (Williams e al._.~., 1978) and
7 molar with D. salina (Borowitzka, 1981). This is about 50% of the dry weight.
Cell concentrations of above l g/L can, however, be di f f i cul t to obtain since
optimal growth conditions do not necessarily maximize glycerol production
(Ben-Amotz e_~_ta___~., 1982). Dunaliella exhibi ts the added advantage of having no
rigid cell wal l, thereby fac i l i tat ing extraction. By bacterial fermentation, the
algal-glycerol mixtures can be converted into neutral solvents (Nakas e a___~.,
1986).
I t is, however, the B-carotene-producing capacity of Dunaliella bardawil
(Ben-Amotz and Avron, 1980, 1983) which has stimulated a large commercial
investment, i.e. 2.5 mi ll ion by Koor Foods of Israel, over the last 6 years
(Weiner, 1985). About 5 to 9% of the dry biomass may be B-carotene. The same
incentive has been triggering large companies like Roche Products and
Wesfarmers to sponsor a sc ientific team in Austral ia to develop an industr ial
production unit for B-carotene from Dunaliella (Borowitzka e_tt al~, 1984,
1986a,b). In the USSR, attention has also been paid to the mass production of
Dunaliella (Massyuk, 1966). Within the group of the carotenoid pigments,
xanthophylls are also particularly useful in pigmenting chicken skin, tissue
and egg yolks (Benemann and Weissman, 19@4). Other pigments derived from
microalgae and receiving attention from industrial researchers are the
phycocyanins (blue pigments) making up the photosynthetic apparatus of
Spirulina and valuable as dyes in food or cosmetics (Ciferri, 1983; Sasson,
1983) or even in the jeans industry (Bionov, 1986, personnal commun.). Even
i f the production cost is high ( 15/mg) production under ar t i f i c ia l li gh t is
retained (Curtin, 1985). Other potential products from microalgae are phytol
and a wide range of sterols. Phytol is a suitable precursor for the synthesis
of several vitamins (A, E, K, K ) and B-carotene while sterols can be used as
substrates for the synthesis of steroid hormones (Borowitzka, 1988a).
Algae as a source of edible oi ls have begun to receive attention, in
spite of fierce competition among tradit ional vegetal oils and a slowly
expanding market . Other non-food markets may be suitable for algal oi ls
because of the ir chemical diversity and thei r high l ipid content, which can be
comprised between l and 85% on a dry weight basis (Shifrin, 1984) depending
upon the species and the culture conditions. For example, Phaeodact~lum
tricornutum is a marine diatom that contains some highly unsaturated long-
chain acids, some of which could be used as substitutes of drying oils in paints
and lacquers (Molton e_~_tal__~., 1980). These authors made some economic projections
on a process incorporating manure hydrolysis and algal culture for
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MICROALG AL BIOTEC HNOLOG Y 7 5
oil and feed protein production and calculatea a conservative simple return on
investment of 21%.
If microalgae are used for oil production, the production cost based on
very conservative figures for capital and operating costs would be within the
range of 0.25-2.00/gai ( 0.07-0.75/L), which is quite encouraging (Shifrin,
1984). Certainly one of the factors which has aroused interest is the enormous
productivity possible - up to 25 and more tons/ha.yr requiring however a huge
capital investment. The possibi lity of coupling production to sewage
treatment may satisfy economic requirements (Dubinsky e_t_t al_~., 1978; Shelef e_t_t
al__~., 1978). The oi l content of many microalgae is in the range of 40-70%
(Ratledge and Boulton, 1985) and includes a large proportion of neutral lipids
with fatty acids in the C12-22 range (Shifrin, 1984). Species of Chlorella
appear to offer promise as a source of edible oils (Ratledge and Boulton, 1985)
while Botr~ococcus braunii excretes oils (unsaponifiable lipids) and carotenoids
which may have various industrial uses (Gudin et al., 1983; Wolf, 1986). Much
research remains to be done both on the production of the oi ls by the algae in
question, and on their recovery and chemical characterization (Benemann and
Weissman, 1984).
Another area of potential development for microalgae is the extraction
of pharmaceuticals (Table 6). Despite some di fficulties, such as relative
inaccessibility and lack of characterization, lower product-yield than usual
sources, complexity of purification, the search for pharmaceuticals from algae
and marine organisms is under way and some very interesting biologically
active products have been found (Wright, 1984; Borowitzka, 1988a; Cohen,
1986). For example, a lipid antioxidant function may be responsible for the
purported action of B-carotene as an anti-cancer agent (Burton and Ingold,
1984). Extracts from Chlorella and Scenedesmus have been shown to have in vitro
antibacterial activity (Reichelt and Borowitzka, 1984) and stimulate the growth
and yield of yeasts and other microorganisms (Fingerhut e_t_t al~, 1984). This
property finds application in the fermentation industry to improve the growth of
lactic acid bacteria (Soong, 1980). Extracts of Scenedesmus and Spirulina could
also serve as a replacement for serum (Kumamoto, 1984). Certain compounds
extracted from Porpt~vridium could play a role in commercial abalone culture,
inducing the settlement and metamorphosis of the larvae (Morse and Morse, 1984).
Other valuable compounds of interest for the food industry which may be
produced from microalgae include other osmoregulatory substances such as
sorbitol, mannitol, and cyclohexanetetrol, bio-emulsifiers and various low-
molecular weight metabolites, e.g. amino acids. The latter may be released
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74G J DE LA NOUE an d N DE PAU W
Table 6. Pharmaceuticals rom microalgae.
Source Product Reference
Amphiridiumcarterae 8-2 blocker
(dinoflagellate}
Nitzchia ovalis (diatom)
Dunaliella tertiolecta
Cyanobacteria
(blue-green algae)
Rivularia firma
Scxtonema hofmani
Lyngbya majuscula
Lyngbya lutea
Gomphosphaeria aponina
Spirulina
Dinoflagellates
Ptxchodiscus brevis
Microphytes
Antibacterial
Antioedema
Bronchodilator
Polysynaptic
Blocker
Anticonvulsant
Hypotensive
Antiinflammatory
Analgesic
Anaesthetics
Antiallergic
Cyanobacterin (antibiotic)
Malyngolide (antibiot ic)
Antineoplastic
Antiamphetamine
Aponin (lysis of dinofl.)
Serum replacer
Toxins (saxitoxin)
Brevetoxin
Cosmetics
Baker et al., 1985
Maksimova et al., 1984
Baker e_~tal__.~., 1985
Norton and Wells, 1982
Norton and Wells, 1982
Wood et al. , 1982
Cardillo et al., 1981
Wright, 1984
Baker e_t_tal~, 1985
Lem and Glick, 1985
Lem and Glick, 19~5
Curtin, 1985
Wright, 1984
Bonin, 1983
from immobilized cyanobacteria (Synechocystis) by osmotic shock (Reed et al.,
1986). I t has long been known that some microalgae, such as Spirulina, are
among the richest known sources for vitamins, especially Bl2 (Lem and Glick,
1985). The proportion of an alga s vitamin production excreted to the medium
may be quite high and could be enhanced by manipulating the growth conditions
(Borowitzka, 1988a). Although the studies done with immobilized microalgae
or cyanobacteria are recent (see Rao and Hall, 1984) progress has been recently
made with carrageenan for Scenedesmus (Chevalier and de la NoUe, 1985a),
calcium alginate (Martinez e_~_t al., 1987) or chitosan (Proulx and de la NoUe,
1988) for Phormidium., or polyurethane for Porph~ridium (Gudin e_.t_t al___:., 1983).
One interesting possibi lity, at least on theoretical grounds, is that of the
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M I C R O A L G A L B I O T E C H N O L O G Y 7 7
co-immobilization of algae with bacteria, the former producing oxygen for the
latter which produce the product of interest for the food industry (Chevalier and
de la No~e, 1986).
PROCESSING OF MICROALGAL BIOMASSES
As previously shown, microalgal biomasses can be used as such in animal
feeding, aquaculture or for human consumption, despite low digestibi l i ty in
some cases or deficiency in some amino acids, such as sulphur-containing ones.
In some cases, however, processing of the biomasses might be desirable or
necessary either to improve the nut ri tional qualit y or to allow the
conservation of the biomass for later use. I f one wishes to extract specific
substances or added value products, additional processing steps are obviously
required.
Processin9 Methods and their Effect on the Safet~ and Nutr it ional Value of
Algal Biomasses
Sometimes, i t is possible to obtain strains of microalgae which possess
characteristics that simplify thei r ut il izat ion. For example, a strain of
Chlorella vulgaris (CCAP no 211-la) lacks the resistant cell wall component
sporopollenin (Strain et al~_., 1984). This strain has a relat ively high
digest ibi l i ty (81.7 ) without any treatment and , therefore, no expensive
processing to increase it s digestibi l i ty is required. In most cases, however,
i t is necessary to break cell walls either by applying physical, physico-
chemical or enzymatic treatments. Breaking cell walls is not an easy matter.
For example, the breaking pressure (by gas decompression technique) is 95
atmospheres for Chlam,vdomonas as compared to only 30 atmospheres for cultured
cells of carrot in suspension (Carpita, 1985).
Drying the biomasses is one of the most common treatments applied but
degradation may account for some 30 of the to tal production cost (Lin, 1985).
The choice of technique used for dehydrating algae affects the appearance, the
texture, the nutr it ional value and the digest ibi l i ty of the fina l product.
Spray-drying appears to be more useful than freeze-drying since toxic
substances are more effectively destroyed (Lin, 1985). The resulting powder
is harder, an important characteristic for the production of hard tablets.
Although these processes are expensive, thei r overal l evaluation shows that
they are superior to drum-drying (Lin, 1985). For Spirulina, thermal
treatment has been reported to be effect ive for detoxi fication (Voronkova e
a___~., 1983).
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748 J DE LA NOUE an d N DE PAU W
With respect to true protein content only, drum-drying, deamination,
irradiation and autoclaving appear o be equivalent in the case of Ooc~stis (Lee
e a.__~., 1982). Rats fed with autoclaved algae showed a lower Net Protein Ratio
(NPR) and a lower protein digestibility than rats fed algae treated by other
methods (Becker, 1980; Lee e_~_ al., 1982). Improved digestibi lity after
gamma-irradiation may be expected since this treatment brings about cellulose
depolymerization (Campbell e_~_ a___~., 1986). Studies done in vivo with rats fed
Chlorella have shown that treatment processes give different results. For
dried cells, digestibility is 60 , and rises to 73 after heating to lO0°C for
30 minutes and to 80 for broken cells (Fujiwara-Arasaki, 1984). Considering
the high cost of most processing techniques, i t appears that the most
economical may be to evaporate water from algae by sun-drying on sand beds
(Lincoln and Koopman, 1986; Becker and Venkataraman, 1982; Richmond and
Becker, 1986). It is clear that the choice of the technique is a matter of
balance between cost, intended use and changes in the characteristics of the
processed biomass. In the case of algae intended for human consumption,
concern has been expressed that their color might hinder consumer acceptance.
It has been shown that decolorization may be achieved by photolysis
(15 000 lux, fluorescent lamp, lO h). Moreover, this treatment resulted in
the removal of the unpleasant odour of the blue-green alga used, Anabaena
flos-aquae (Choi and Markakis, 1981). Enzymatic treatments are st i l l not
widely used for microalgae. By analogy with what has been done with marine
macrophytes (Fujiwara-Arasaki, 1984), one can expect that treatment with
pepsin, pancreatin and pronase might lead to increased digest ibi lity of
alkali-soluble proteins.
Another aspect of biomass treatment is that of supplementation.
Microalgae are known to be deficient in sulphur-containing amino acids
(Becker, 1978b; Ben-Amotz and Avron, 1980; Saxena et al., 1983). Supplemen-
tation of sun dried Spirulina platensis biomasses with 0.2 methionine has
been shown to significantly improve the biological value and the net protein
utilization with the same digestibility (Narasimha e_t_t al~, 1982). It would
also be possible to improve the nutritional value of S__~ platensis by
compensating for the low content in methionine through the use of mutants that
have higher intracellular pools of this amino acid (Ciferri, 1983). It is
suggested that with appropriate genet ic enhancement, microalgae producing
desirable amino acids in sufficiently high concentrations could be produced
(Borowitzka, 1988a).
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MICRO LG L BIOTECHNOLOGY
Preparation of ProteinConcentrates
7 9
Algal biomasses are generally high in protein content, values up to 60
or more being reported for Spirulina for example (Santillan, 1982; Ciferri and
Tiboni, 1985; Bourges, 1986). This has prompted some investigators to explore
the possibility of preparing protein concentrates that exhibit good functional
properties such as water and fat absorption, emulsification and foaming
capacities, and foam stabi lity. In general, these functional properties for
Spirulina protein concentrate are better than or similar to those of soybean
meal, except for foam stabi lity (Anasuya Devi and Venkataraman, 1984).
Preparation of protein concentrates from blue-green algae such as Anabaena
flos-aquae has been thought to be a means of overcoming its low digestibility
due to cell walls, its unattractive color and its strange flavor (Choi and
Markakis, 1981). Treating the biomass with 3N HCI at 95°C for lO minutes and
neutralizing afterwards has been found to be an effective way of obtaining a
solution containing 80 of the cell nitrogen (Choi and Markakis, 1981). It is
likely that enz~nnic treatments could be applied to such concentrates to
prepare partial ly hydrolyzed fractions. Unless such fractions yield highly
priced products, i t is unlikely that algal biomasses may constitute a
competitive protein source. However, as the price of traditional sources such
as fish or soybean meal continues to rise, microalgal SCP used as a protein
supplement may become increasingly attractive especially when produced as a
by-product of wastewater treatments (Aaronson and Dubinsky, 1982; Pouliot and
de la NoUe, 1985). Protein isolated from algae shows a much higher
digest ibi lity than entire cells (Choi and Markakis, 1981), a result that is
not surprising. In vi tro digestion of Chlorella proteins with trypsin, for
example, gave digest ibi lities of 45 for dried cells, 69 for frozen cel ls,
71 for broken cells and 86 for extracted proteins (Ishii e al., 1974;
Mitsuda et al., 1977a, b).
COMPETITION BETWEENDIFFERENT INDUSTRIES FORTHE USE OF ALGAL BIOMASSES
If commercial scale production of microalgal biomasses is to be
ini t ial ly limited to existing installations, e.g. sewage treatment fac il i ties
or to their extension as tertiary treatment systems, i t is conceivable that
supply may fall short of demand, given all the possible applications already
described. The price of the biomass produced will have to increase before the
number of production sites can increase substantially. This could arise
through other uses for microalgae to which biotechnology might contribute.
Possibili ties of low-to-medium value-added products include biosorption of
metals, with a wide range of applications (Darnall et al., 1986) ammonia
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M I C R O A L G A L B I O T E C H N O L O G Y 75
growth rate, capacity to grow at suboptimal temperatures, higher yields or higher
content of nutrit ionally important components. However, basic genetic research
has been slow to reach algae and studies on them are relatively few Lemieux et
a___~., 1985; Necas, 1985; Turmel e_t_t al__~., 1986; Chauvat e_t_t al_._~., 1987; Lee and Tan,
1987), since they do not appear to offer obvious advantages over the favourites,
i .e .E, coli , Bacillus sp. or Saccharomxces ei ther as a vehicle of expression for
genes of higher organisms or as a model for the genetics of higher eukaryotes.
However, the possibility of inserting new gene sequences in smal l linear DNA
demonstrated in Chlamxdomonas Turmel et al . , 1986) might open interesting
perspectives, even more promising than the use of plasmids G. Bellemare person.
comm. .
For the study of microalgal genetics the prokaryotic cyanobacteria offer
several advantages. Most can be grown on defined medium, with generation
times as short as f ive hours and can be handled and grown on,agar medium as
colonies. Transformation does occur, and to-date several shuttle vectors for
cyanobacteria can replicate stably in E. coli Baker e_t_t al__:., 1985; Lem and
Glick, 1985).
Algal pond culture may already be considered for a wider range of
products. Extension of the latest genetic techniques to the eukaryotic
microalga Dunaliel la is considered a high priority Baker e_t_t al___~., 1985).
Approaches to the genetics of this alga have been presented by Simon and
Latorella 1986). Use of genetic alterations in cyanobacteria for maximizing
H production have been recently demonstrated Spiller and Shanmugam, 1986).
Spirul ina, a tropical species with optimum growth temperature around 30°C, has
been manipulated genetical ly and a strain has been developed by Japanese
workers which grows at 4°C Anon., 1984). This might open vast possibi li ties
for culture systems under temperate or even cold cl imatic conditions.
Strain-specific differences in the chemical composition with respect to
l ight intensity have been demonstrated for Phaeodactxlum ricornutum Terry e
al._~., 1983). Genetic studies are also required in order to transfer the
capacity for the production of algal polysaccharides to faster growing
bacteria or to transfer t raits such as pesticide or herbicide resistance into
nitrogen- fi xing blue-green algae in rice paddies Erickson et al . , 1984)o
Genes coding for the production of polysaccharides can be amplified by
inclusion of smal l extrachromosomal elements and genetic manipulations,
including transfer to bacteria, according to one report Weiner e al_._~., 1985).
Other properties of algae to be exploited, pending genetic characterization
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7 5 2 J DE LA NOUE and N DE PAUW
Gallagher, 1986), include the abi l i ty of certain species such as Chlorella to
grow under extreme conditions of sa lini ty and acioity and the super- sensi ti vi ty
of some strains of C_.~. saccharophila to the toxic heavy metal cadmium Kessler,
1985). These are only a few examples of possible interesting applications of
genetic manipulations with microalgae. A much more complete treatment of the
subject wi ll be found in the recent review contribution of Craig et al . 1988).
ACKNOWLEDGMENTS
The authors wish to thank deeply D. Proulx and S. Davids for thei r help in
preparing this paper, D. Ni Eidhin for revising i t and G. Gagnon for the typing.
Financial help was provided by FCAR-Equipe grant, NSERC and the minist~re des
Relations Internationales du Quebec.
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