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Dr. Sailabala Padhi, M.Phil, Ph.D., D.Sc.Director, Centre for Environmental Studies

From the Director’s Desk...Dissemination of information on various issues related to environment of theState is the main objective of establishment of our ENVIS Centre. We havediscussed on various issues in our earlier publications. In this issue we havefocused on one of the important topic "CO2 Sequestration Technologies usingAlgae for Sustainable Climate Change".I hope this issue of Newsletter will be useful for various planners, decision makers,scientists, environmentalists, researchers, academicians and other stake holders.

Intoduction

Global warming is caused mainly by theemission of carbon dioxide (CO2), with thermalpower plants being responsible for about 7 per centof global CO2 emissions. The use of fossil fuels tomeet our energy requirement has resulted inadverse effects on the climate, over dependenceon foreign oil and economic uncertainties. Theincrease in the surface temperature due to globalwarming, causing catastrophic effects, compels thescientist community to tag it as an issue of priorconcern. The most of the environmental groupurging for individual, group or community actionsagainst global warming, through switching toalternate energy to hinder the speed of warmingup. These methodologies remain virtual. To mitigatethese harmful effects, biological alternatives ofcapture CO2 are being investigated. The use ofcarbon sequestration by micro algae is a major toolfor reducing atmospheric concentrations of CO2.

Atmosphere is loaded with around 90Mt(million tonees) of heat trapping substances everyday that slowly wrap the earth with an artificialgreenhouse gaseous screen. Most anthropogeniccarbon dioxide (CO2) emissions result from thecombustion of fossil fuels for energy production.Flue gases from power plant are the main sourcesof CO2 emissions which lead to Global warming.

To tackle climate change effects CO2 level shouldnot be allowed to get much higher than 550ppm:the current level is 380 pm. CO2 emissions isexpected to increase at an annual rate of 3 per cent.Consensus within the scientific and most of thepolitical community is that emission of greenhousegases is detrimental to the environment and resultsin worse air quality and alteration of globalbiological systems. The potential effects of globalwarming on India vary from the submergence oflow-lying islands and coastal lands to the meltingof glaciers in the Indian Himalayas, threateningthe volumetric flow rate of many of the mostimportant rivers of India and South Asia. In India,such effects are projected to impact millions of lives.As a result of ongoing climate change, the climateof India has become increasingly volatile over thepast several decades; this trend is expected tocontinue. Hence there is a urgent need to solve theproblem of CO2.

During the 5 billion years of its existence,earth has endured all sorts of vagaries of natureand supported the process of evolution of life eversince the first organism appeared about 3.5 millionyears ago. Life consistently modified theenvironment and in quest for more comforts, overexploited the nature. Rapid industrialization

CO2 Sequestration Technologies usingAlgae for Sustainable Climate Change

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CO2 using solar energy with efficiency 10 timesgreater than that of the terrestrial plants withnumerous additional technological advantages.Algae are more efficient at utilizing sunlight thanterrestrial plants [1], consume harmful pollutants,have minimal resource requirements and do notcompete with food or agriculture for preciousresources [2]. India has a unique opportunity foralgae production because it contains the basicresources needed to grow algae in abundantquantities: India produces over 170 million metrictons of CO2 annually; contains abundant salinewater; receives abundant sunlight; and has aimpressive knowledge base and technical expertisewithin the energy industry.

Algae have higher growth rates thanterrestrial plants, allowing a large quantity ofbiomass to be produced in a shorter amount of timein a smaller area. Algae growth rates of 10 to 50 gm-2 d-1 (grams of algal mass per square meter perday) have been published in the literature [3].Compared to terrestrial plants such as corn andsoy, algae have shorter harvest times because theycan double their mass every 24 hours.

These short harvest times allow for much moreefficient and rapid production of algae comparedto corn or soy crops. To illustrate the landrequirements for bio fuel, Crop production, yieldsof different oil producing crops can be examined,as shown in Table 1. Compared to terrestrial crops,algae utilize solar energy more efficiently andbecause they are not limited to one growth cycleper year, they can be harvested much more often.

Selection of microalgal strainsBiodiversity of microalgae lipid properties

Microalgae comprise several groups ofunicellular, colonial or filamentous, photosyntheticor heterotrophic micro organisms containingchlorophyll and other pigments. Microalgae cangrow autotrophicalfy or heterotrophically, with awide range of tolerance to different temperature,salinity, pH and nutrient availabilities (Hu et al 2008:

accentuated the problem and polluted even the airwe breathe. Today, more than 80 per cent of theworld’s energy requirement is being met byburning the fossil fuels. This is leading to increasedemission of greenhouse gases comprising mainlyof carbon dioxide. Concentration of carbon dioxidehas risen from 220 ppm during the pre-industrialization era, to the current level of about350 ppm, resulting in 0.7 per cent rise in the globaltemperature over the last 140 years (Gribbin, 1981;Pearman, 1981). The CO2 concentration is steadilyincreasing at the rate of about 1.5 ppm per year(Siegenthaler, 1990).

The direct manifestation of the greenhouseeffect is warming up of the earth surface andmelting of the snow cap at the North Pole. It isestimated that about 54,000 km2 of snow isvanishing every year leading to rise in the sea leveland shift of the weather belts. This is fundamentallydisrupting our livelihoods and socio-economicsystems. There does not seem to be an escape fromthis inevitable consequence of modernizationexcept to adapt and try to sequester the effect atleast to some extent The situation can be saved byreducing the production of CO2 and harnessing thecarbon utilization attribute of cholorophyll bearingmembers of the biosphere specially the microalgae.The planktonic microalgae serve as efficientcontraptions converting the atmospheric CO2 intocarbohydrates. This photosynthetic fixation ofcarbon dioxide is the major solar energy trappingsystem which sustains the life. Planktonic algae,since they abound all aquatic habitats includingthe sea, are the major contributors to the totalcarbon fixed through photosynthesis.

Characteristics of Micro Algae

Algae are simple unicellular organisms thatproduce carbohydrates, proteins and lipids as aresult of photosynthesis. Sunlight, water, nutrientsand arable land are the major requirements forgrowing algae. Micro algae have the ability to fix

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Brennan and Owende 2010). More than 40,000microalgal species have been classified asprokaryotes (cyanobacteria) and several eukaryotesincluding green algae, diatoms, yellow-green algae,golden algae, red algae, brown algae, dinoflagellatesand others (Hu et al 2008; Packer 2009).

Many different classes of lipids can beproduced in microalgal cells. Based on chemicalstructures and polarity, these lipids are divided intopolar and neutral lipids. In most cases, polar lipidsfunction as membrane structure components,which commonly include phospholipids (e.g.phosphatidylinositol, phosphatidylcholine andphosphatidylethanolamine) and grycolipids (e.g.monogalactosyldiacylglycerol and digalactosy-ldiacylglycerol). Neutral lipids include tri-, di- andmono-acylglycerols, waxes and isoprenoid-typelipids (e.g. carotenoids), among whichtriacylgrycerols (TAGs) are frequently found to beaccumulated as energy storage under various stressconditions (Roessler 1988: Bigogno et al 2002:Mansour et al. 2003: Basova 2005: Khozin-Goldberg and Cohen 2006). Although almost alltypes of microalgal lipids can be extracted, onlyTAGs are easily transesterified into biodiesel bytraditional methods. Analysis of thousands ofmicroalgal species have shown tremendousdifference in lipid content among different strains,ranging from 1% to approximately 85% of dry cellweight (DCW) (Spolaore et al. 2006: Chisti 2007:Li et al. 2008). Microalgae produce a wide varietyof fatty acids with chain length from C10 to C24(Hu et al 2008), depending on species or strains.For example, the filamentous cyanobacteriumTrichodesmium erythraeum can synthesize C10fatty acid accounting for almost 50% of total fattyacids (Parker et al 1967): whereas the dinoflagellateCrypthecodinium cohnii can producedocosahexaenoic acid (C22:6003, DHA) as high as30-50% of total fatty acids (De Swaaf et al. 1999).Moreover, for any one microalgal strain, the lipidcontent, lipid class and fatty acid compositionfluctuate under different culture conditions (Emdadi

and Berland 1989: Peeler et al 1989: Reitan et al 1994:Khozin-Goldberg and Cohen 2006).

Screening of oleaginous microalgae

Due to the variation and diversity of microalgal

lipids, selection of oleaginous microalgal strains suitable

for biodiesel production will require screening large

number of microalgal strains. The first large-scale

collection and screening of oleaginous algae dates back to

1978, when the Aquatic Species Program (ASP) was

launched by U.S. National Renewable Energy Laboratory

(NREL) for production of biodiesel from high lipid-content

algae. With 8 years of effort, over 3,000 strains were

collected and eventually around 300 species were

identified as oil-rich algae (Sheehan et al 1998). Recently,

some studies on screening of oleaginous microalgae were

reported, focusing on optimizing culture conditions to

increase lipid productivity and evaluation of the potential

for biodiesel production (Gouveia et al 2009: Rodolfi et al

2009: Li et al 2010 a,b). The routine procedures for screening

of microalgae involve sampling from the field or an algae

collection library, isolation and purification, microalgal

identification and maintenance, and evaluation of

potential for lipid production, all of which have been well

reviewed (Mutanda et al. 2011).

The main indexes determining the potential of

microalgal strains as biodiesel feedstock are growth rate,

lipid content, and lipid productivity. Table 1 shows both

lipid content and productivity of some microalgal species,

indicating that suitable oleaginous strains have the

potential with no less than 20% (w/w) lipid content and

40 mg 1-1 day-1 lipid productivity. Further adaptation to

local environment, genetic improvement and alteration of

cultivation method might enhance lipid productivities of

microalgae. For example, Chlorella protothecoides grown

heterotrophicalry genetic improvement remains a distant

goal because methods formulation, selection, and

recombination are not developed for most strains.had a

55% lipid content compared with 14.5% of autotrophicalry-

cells (Miao and Wu 2006). For the present, however, genetic

improvement remains a distant goal because methods

formulation, selection, and recombination are not

developed for most strains.

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Table 1: Some microalgal species with relatively high lipid content and productivities (Gouveia et al.2009; Li et al. 2007; Mata et al. 2010)

Free fatty acid content and compositioninfluence the quantity and quality of synthesizedbiodiesel, especially when some microalgal strainsare capable of producing polyunsaturated fattyacids. Free fatty acids cause the occurrence ofsaponification during the esterification of lipids,while excess unsaturated fatty acids may lead totar formation induced by cross linking of fatty acidchains (Bruton et al 2009), and low cetanenumbers (Ramos et al 2009). Moreover, increasedcontent of polyunsaturated methyl esters causesdecreased oxidation stability of biodiesel (Ramoset al 2009). Other performances factors need to betaken into account when one strain is chosen forfurther evaluation, including the ability of nutrientremoval of wastewater effluent, CO2 capture fromindustrial sources (Gonzales et al 1997: Iwasaki etal 1998: Lee and Lee 2001), environmentaltolerance of high salinity, extreme pH,temperatures and high light intensity, productionof by-products with commercial values, and rapidgrowth in photoreactors. The best microalgalspecies or strains for biodiesel production willsatisfy all these requirements.

Determination of microalgal lipidcontent

Screening of oleaginous microalgal strainsrequires frequent measurement of lipid content, acrucial parameter of microalgal lipid properties.The conventional method used for determinationof lipid content involves a complicated lipidextraction with solvent, separation, concentrationand gravimetric determination (Bligh and Dyer1959). Although this method has high accuracy,its major limitations are time-corouming, laborintensive and requirement of large amounts ofmicroalgal biomass (no less than 10-15 mg wet cellweight), thus not suitable for large-scale screeningof microalgal strains. Chromatographic methodswith internal standards can provide informationon both fatty acid quantity and profile in a singleanalysis, and was a commonly employed methodin several fields (Carvalho and Malcata 2005). Oddchain fatty acids have often been considered asuseful internal standard since they are structurallysimilar to the fatty acids being analyzed and donot interfere with the chromatographic analysis.The fatty acid content determined by internalstandard method can be used for quantitativeanalysis of neutral lipids that are suitable for

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biodiesel production because in oil-accumulatingmicroalgae TAGs comprise a major proportion ofthe total lipids. Although internal standard methodrequires the extraction and derivatizing of fattyacids, it may be applicable to quantitative analysisof lipid content of microalgae when it is performedin batch mode.

Some in situ measurement methods havebeen evaluated and used in determination ofmicroalgal lipid content, including Nile Red (NR)staining, time-domain nuclear magnetic resonance(TD-NMR), colorimetric quantification and others.NR is a red phenoxazine dye that can effectivelydetect neutral and polar lipids within microalgaefrom certain classes (Lee et al 1998: ELsey et al2007). Selecting adequate excitation and emissionwavelengths can yield the greatest sensitivity of thismethod for different species (Elsey et al 2007).Improved NR staining can also be used as a highthroughput technique for screening of green algalspecies, many of which have thick and rigid cellwall (Chen et al 2009). Although some factors mayhave impact on its accuracy, NR staining is still a

rapid and efficient method in preliminary screeningof microalgal strains with high content of neutrallipid. Another lipophilic fluorescent dye BODIPY505/515 has recently been used for detecting lipidswithin viable microalgal cells (Cooper et al 2010).Screening of oil-rich microalgal strains can beoperated with a micromanipulator system and flowcytometry. TD-NMR is a fast and convenientmethod to quantify lipid of C.protothecoides (Gaoet al. 2008), and its application for other microalgalspecies needs further investigation. Wawrik andHarriman (2010) developed a simple colorimetricmethod for quantification of algal lipid from smallcultures. Algal lipids are saponified into fatty acidsand reacted with a copper reagent. The resultingcopper soaps are then colorimetrically measuredby addition of substrate to produce a coloredproduct. However, fatty acids of chain length withless than 12 carbons can not be detected, leadingto underestimation of total lipid content in somealgal species. The advantages and limitations ofthese methods are summarized in Table 2. Basedon the analysis of these methods, measurements

Table 2: Advantages and limitations of lipid determination for microalgae

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of neutral lipid contents using fluorescent dyes (e.g.,Nile Red or BODIPY) have been most effective atthis time. It is advisable that in situ measurementmethods and chromatographic method can beconsidered for high-throughput preliminaryscreening of naturally occurred or geneticallymodified microalgal strains, while traditionalgravimetric method can be used for finalexamination of resulting candidate oleaginousstrains. The maximum accuracy of this method willdepend on effective cell disruption and extractionof microalgal lipids.

Algae cultivation methods and designconsideration

Algae are typically found growing in ponds,waterways or other locations that receive sunlight,water and CO2 Manmade production of algaetends to mimic the natural environments to achieveoptimal growth conditions. Growth depends onmany factors and can be optimized fortemperature, sunlight utilization [12,13] pHcontrol, fluid mechanics and more. The methodbehind biofixation is capturing the CO2 and NOxfrom power plant smokestacks and feeding the CO2to an algae system where up to 50 per cent ofharmful emissions from the smokestack will bedevoured. At present there are two commonmethods or algae based carbon sequestration: openponds and closed photo bioreactors.

Open ponds

The infrastructures of open ponds orraceways are not expensive and easy to operate.To minimize the cost, microalgal cells in openponds must utilize sunlight and C02 in theatmosphere. However, the quality and quantity ofnatural sunlight are affected by dairy and seasonalfluctuation (Grobbelaar et al 1996: Chisti 2008). Formost open ponds or raceways, shallow waterdepths of 0.2-0.3 m are generally used to providesufficient sunlight for most microalgal cells. Themixing of nutrients and water flow is mechanicallyachieved by paddle wheels and guided by baffles

in recirculation channels (Grobbelaar 1994:Sheehan et al 1998: Chisti 2007), which avoidsettlement of microalgal cells and boost gasexchange. The biomass productivities of openponds or raceways are determined to a certainextent by the areas of the ponds due to the restrictedwater depth. This not only increases the costs ofland use and harvesting of cells (Borowteka 1999:Hase et al 2000: Scott et al 2010), but also makes itdifficult to control temperature on extremely hotor cold days.

Microbial contamination is inevitable becauseopen ponds or raceways cannot be sterilized orkept under axenic conditions (Packer 2009). Underhighly selective conditions, open ponds are suitablefor mass culture of some microalgal species suchas native species or those that tolerate high salinityor pH. For example, Dunaliella salina candominate under high salt conditions in an openpond system An alternative is to provide largequantities of inoculum or starter cultures for openponds by smaller photobioreactors, whichguarantees the dominance of selected microalgalstrains (Singh et al. 2011). Although open pondsor raceways have some advantages compared tophotobioreactors, the main drawback is theirrelatively low productivity.

Enclosed photobioreactors

Enclosed photobioreactors have receivedmuch attention because of high biomassproductivity and easier control of cultureconditions. There are various types ofphotoreactorsdesigned for different purposes (for reviews seeCarvalho et al 2006: Eriksen 2008: Ugwu et al2008). Flat-plate, tubular photobioreactors withlarge ilkirnination surface area are suitable foroutdoor mass cultures of microalgae (Ugwu et al2008), whereas column bioreactors have theadvantages of efficient mixing, high volumetric gastransfer rate and flexible control of growthconditions (Eriksen 2008). Tubular photobioreactors are often considered the most suitable

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for commercial large-scale production of microalgalbiomass, which partly relies on the multiplicationof bioreactor unit (Janssen et al 2003: Chisti 2008).Tn indoor closed photobioreactors, illuminationcan be from natural solar light or metal halidelamps; whereas outdoor photobioreactors usenatural sunlight or solar collection devices (AcienFernandez et al 1997: Greenwell et al 2010).Sunlight is considered only for large-scale biodieselproduction purpose. To ensure optimumphotosynthetic efficiency for each of the cells in anoutdoor photobioreactor, a mechanical pump orairlift pump is used to mix the cultures (Chisti 2007).

The productivities ofphotobioreactors areaffected by the supplies of light and C02, changesof temperature, pH, dissolved 02 levels of cultures(Molina Grima et al. 2001: Chisti 2008), and theperformance of selected microalgal strains. Biomassmeasurements or growth rate evaluations arecritical in assessing the potential of photobioreactorsand the performance of the algal strains.Microalgal biomass productivity can be evaluatedin photobioreactors based on areal productivity(productivity per unit of occupied land area orillurninated reactor surface per unit of time),volumetric productivity, photosynthetic efficiencyor biomass yield. Table 3 summarizes the maximalbiomass productivities measured in three types ofbioreactors, in which most microalgal strains canreach productivities of 10-30 gm-2 day-1 or 0.2-3.0g 1-1 day-1. The maximal biomass concentrationranges from 1 to 7 g1-1 . In some cases, the maximalbiomass productivity was reached at relatively lowbiomass concentration (Cuaresma et al 2009;Meiser et al 2004). Microalgal growth rate can beaffected by the average illumination, lightsaturation constant and biomass concentration.Maximal biomass productivity can be achievedunder optimal culture conditions. To meet transportfuel needs, large scale production of biodieselrequires producing sufficient biomass that must beprovided by appropriate culture systems. Biomassproductivity data and the volume of mass cultures

allow an estimate of the potential annual biomassyield of photobioreactors. Culture volume is limitedby the configuration and scalability ofphotobioreactors. Tubular reactors can be scaled-up by increasing the length and diameter of thetubes, or the number of units. However, an optimalconfiguration of tubular reactors was found to bea length of 80 m and diameter of 6 cm (MolinaGrima et al. 2001: Chisti 2007), and thus the culturevolume can only be scaled to 315 rrr on one hectareof land. The largest flat panel reactor wascomposed of five individual units of 200 1 and hada total capacity of 1,000 1 (Richmond and Zhang2001). Maximizing the number of this type ofreactor units allows an attainable scale of 900 m3

per hectare of land. There are limited examples oflarge-scale applications of vertical column reactors.Chini Zitelli et al (2006) described an outdoorannular column reactor with a height of 2 m anddiameter of 0.5 m, and an optimal arrangementallowed 0.81 columns per square meter.Extrapolation of these data indicates a maximumculture volume of 972 m3 on one hectare of land.Without considering the cost, at the maximumbiomass productivity of 3.8 g1-1 day-1, and ifaverage oil content is valued at 40% of the biomassby dry weight, the maximum annual oil yield indifferent photobioreactors can be estimated to 143-443 tons per hectare of land (300 days of productivegrowing season per year). Therefore, to replace thecurrent total U.S. transportation fuel needs (about0.76 billion m3 per year), production of microalgalbiodiesel would require land of approximately 1.5-4.6 M hectares. Although these estimates areapproximate, they illustrate the potential of long-term research and development to improve theprospects for microalgal biodiesel productioaHowever, Rodolfi et al (2009) estimated an annualyield of 20-30 tons of microalgal oil per hectare onthe local areas, according to the biomass and lipidproductivities of Nannochloropsis sp. based on atwo-phase (a nutrient sufficient phase followed bya nitrogen deprived phase) cultivation process with

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1101 Green Wall Panel photobioreactors. Such lowestimated yield relative to the theoretical valuewould be mainly attributed to the low biomassproductivity (on average 0.3 g1-1 day-1). Therefore,narrowing this gap of microalgal oil yields wouldrequire a major advance in engineeringimprovements of photobioreactors and choosing

suitable strains to significantly enhance the biomassproductivity. Notably, scaling-up ofphotobioreactors, especially column reactors in aneconomical way would also present greatchallenges. With present technologies, closedphotobioreactors have been considered to producestarter cultures (inocuk) or high-value products.

a24 h continuous illuminationbThe areal productivity was calculated based on ground area, and others are based on illuminated reactor surface

Table 3 Maximum biomass densities (Bd), areal and volumetric productivities (Parea and Pvolume), specific growth rate( ) of selected microalgal strains photoautotrophically grown in different types of enclosed photobioreactors.

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In contrast to open ponds, photobioreactorshave the advantages of low contamination, highproductivity, minimal evaporation, attainable highmieroalgae densities or biomass concentrations,reduced CO2 losses and better control over cultureconditions (Merchuk et al. 2000: Richmond 2004).The major drawbacks of photobioreactors are thehigh costs of construction, operating andmaintenance. Although these can be partiallycompensated by higher productivity, they still limitthe cost-effective production of microalgal biomasson a scale required for biodiesel production. Hybridalgae production system comprising photobioreactors and open ponds may be a promisingway. Sufficient contaminant-free inocula can beproduced by photobioreactors, followed by transferto open ponds or raceways to attain the biomassneeded for biodiesel production (Greenwell et al.2010).

Algal photosynthesis

Algae are a heterogeneous assemblage oftypologically different microorganisms with diversemorphology and varied physiology, inhabitingalmost all conceivable habitats. While exceptionsare not uncommon, majority of these plants arephotolithotrophs utilizing CO2 at the expense ofsolar energy with quite a few showing trophicindependence for nitrogen also. Being at the baseof the trophic pyramid, they are the maincontributors to the productivity of an aquaticecosystem. They have been estimated to fix about3.2 X 1010 mt of carbon dioxide, which constitutesabout 40 per cent of the total CO2 fixed annuallyon this planet.

Like the higher plants, algae need all themajor and minor elements essential for variousmetabolic processes (Gerloff et al., 1952; Allen andArnon, 1955; Goldman et al., 1972). The eukaryoticalgae have typical C3 type of photosynthesis similarto the higher plants. The radiant energy is trapped

mainly through non-cyclic photophosphorylation.Unlike photosynthetic bacteria, even theprokaryotic blue-green algae also possess oxygenicphotosynthesis. The main source of carbon foralgae is the atmospheric carbon dioxide but theycan readily utilize carbonates and bicarbonatesalso. Heterotrophic and chemotrophic nutrition hasalso been reported in a few cases under certain setof conditions.

Most of the information on inorganic carbon(Ci) assimilation and mechanism of photosynthesishas been generated from studies conducted withunicellular algae like, Chlorella, Chlamydomonas,Scenedesmus and Euglena and exhaustivelyreviewed by Fogg (1953), Rabinowitch (1956),Goldman et al. (1972), Aizawa and Miyachi (1984),Badger (1987), Spalding (1985), Miler et al. (1990)and Badger and Price (1992, 1994).

Morphology

Carbon dioxide level has been shown tomarkedly affect the morphology of microalgae.Nagh-Toth et al. (1992) reported that high CO2increased the size and perforations in thechloroplasts and enlarged pyrenoids in green algae.EM scanning of the cells grown in presence of CO2under anaerobic conditions revealed fewerchloroplast lamellae and larger pyrenosomes. Welldeveloped chloroplasts in green algae andcarboxysomes in cyanobacteria with electronicallydenser chloroplasts were reported by Turpin et al.(1984). Hanagata et al. (1992) reported change inthe morphology of the coenobia of Scenedesmuswhich grew into paired or solitary cells at 350 Cand 20% CO2 level. It is possible that suchmorphological changes enhance the tolerance tohigh CO2 levels and temperature. The contentionis supported by the reports of idso et al. (1989) onAzolla pinnata var. pinnata and Hanagata etal.(1992) in Chlorella. They reported enhancedtemperature tolerance induced by higher CO2levels.

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Microalgae as co2 sinks

Increase in the forest cover and primary

productivity of the oceans were suggested as

effective tools to combat the menace of increasing

CO2 level (Ritschard, 1992).The technical,

engineering, economical and environmental

aspects of using marine algae as ‘sinks’ for CO2are being extensively explored. Raising of large

‘microalgal farms’ is being considered for Ci

sequestering and food production. Laws and

Berning (1991 a,b) reported on various aspects of

maximizing the photosynthetic activity of marine

forms Tetraselmis suecica and Gracilaria tikvihae

to reduce the pollution caused by the emissions

from power plants. Koganel (1991) suggested that

tiny, genetically engineered photosynthetic

microorganisms may help in mopping up the excess

CO2 from the air.

Hanagata et al. (1992) found that at higher

CO2 levels, Scendesmus sp. K 34 and Chlorella sp.

K 35 displayed very long log phase. Takeuchi et al.

(1992) identified Oocystic sp. Which grew very well

at 20% CO2 . A marine alga Chlorococcum littorale

performed better at 300 C and pH 4 in presence of

20% CO2 (Kodama and Miyachi, 1991). Pesheva

at al. (1994) in Chlorococcum littorale, observed

extended log phase at 20% and higher CO2 levels.

Kodama et al. (1993) identified a new species of a

unicellular green alga which could rapidly grow

at 60% CO2 . Singh (1961) reported that a number

of blue-green algae grew luxuriantly in alkaline/

saline soils. Forms like Microcoleus, Scyatonema,

Porphyrosiphon, Camptylonema, Cylindros

pemum, Anabaena, Nostoc and Aulosira, make

significant contributions to the fertility of these

problem soils. Kumar et al. (1991) confirmed the

observation and suggested that these algae acted

as efficient scavengers of the CO2 produced in the

soil.

Conclusions and Further Challenges

Both cyanobacteria and eukaryotic algaepossess mechanisms for actively acquiring Ci fromthe external medium. They also have very efficienttransport systems which enable them toconcentrate the Ci in the vicinity of primaryphotosynthetic carboxylating enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).This results in significant enhancement of thephotosynthetic affinity for Ci leading to improvedphotosynthetic efficiency. The CO2 concentratingmechanism depends on the membrane bound Citransport systems and a microenvironment withinthe cell where the accumulated Ci can be used toelevate CO2 at the site of Rubisco. This is achievedby the packaging of Rubisco and carbonicanhydrase (CA) into discrete structures known ascarboxysomes in blue-green algae and pyrenoidsin eukaryotic microalgae.

In these associations, the CA dehydrates anaccumulated HCO-3 pool resulting in the localizedelevation of CO2 around the active site of Rubisco.CA normally functions in three primary modes inphotosynthetic systems;

(i) to convert HCO-3 to CO2 for fixation byRubisco,

(ii) to convert CO2 to HCO-3 for fixation by PEP

carboxylase, and

(iii) to provide rapid equilibrium between CO2and HCO-3 to facilitate diffusion of CO2 .Higher levels of CO2 stimulate thecarboxylation reaction catalysed by Rubisco.This supports higher rates of photosynthesisleading to increased consumption of CO2.

To avoid feed back inhibition of the process,one has to look for genetically engineered algalstrains which are able to maintain higher rates ofphotosynthesis and rapidly consume thephotosynthates during prolonged incubation athigher levels of CO2 .

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Biological fixation of CO2 throughphotosynthesis seems to be the easily manageableand cheap way to combat the inevitable danger ofincreasing concentration of CO2 in the air.Microalgae have every attribute to act as sinks forCO2 and attempts to enhance the process of algalphotosynthesis will come to our rescue (Ritschard,1992). This will not only effectively scavenge theCO2 but also produce the much needed organicmatter and cheap protein.

In the endevour to reduce dependence onfossil fuels and cut carbon emissions to achieve aclean environment, humble algae appears to betaking a lead over the more-talked-about biodieselsource jatropha. Algae farming in less than 1 percent of India’s total land can make the country self-sufficient in liquid fuel. Hence algae farming foroil provides and excellent opportunity to absorbCO2 emissions from large industrial plants and

convert then to biofuel. While algae are a verypromising feedstock, many challenges inhibit theproduction of large amounts of algae in aneconomic and sustainable manner. Varying levelsof research is going on in labs but none havesucceeded in producing algae oil on a scalesufficient for meeting our transportationrequirements. The cultivation of microalgae forbiofuels in general and oil production still requiresrelatively long-term R&D, with current emphasison the R rather than the D. This is due in part tothe high costs o even simple algae productionsystems (e.g. open paddle wheel mixed, raceway-type ponds) and in even larger part due to theundeveloped nature of the algal mass culturetechnologies, from the selection of suitable algalstrains than can dominate in the ponds, to theirlow-cost harvesting and most importantly, to theachievement of the required high productivities ofbiomass with a high content oil.

Disclaimer :The views expressed by the writers do not necessarily reflect the views of either Centre for

Environmental Studies or The Editor.

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ENVIS EDITORIAL TEAM

Dr. Sailabala Padhi, M.Phil, Ph.D., D.Sc., DirectorPravat Mohan Dash, Programme OfficerPrashanta Ku. Nayak, Information Officer