BIOFLOCS IN AQUAFARMING

Pradeep KumarM.Venkatasamy

Fisheries College and Research Institute,Tamil Nadu Veterinary and Animal Sciences University,

Thoothukudi 628 008.

Intensification of aquaculture industry may enhance the chances of disease

outbreak and environmental pollution. The increased level of intensification requires

more supplementary protein feed which in turn contributes to nitrogen pollution in

culture and surrounding systems. Decomposition of dead algae, uneaten feed, fish

excreta and other organic matters release excess nitrogen in the form of ammonia and

nitrite and amplify the nitrogen toxicity level, which affects the growth and molting of

aquatic organisms and in extreme cases cause mass mortalities which ultimately result in

reduction in production. To overcome this problem, there are so many techniques such as

use of biofilter, frequent water exchange and green water culture system that can be used

to remove excessive nitrogen, but none of these techniques has proved technically

feasible and economically viable. So there is a need to use such a technique, which is

economical, ecofriendly as well as user friendly to remove or optimize the level of such

toxic compounds in the pond water. One such option is, the biofloc based culture system.

‘Biofloc’ is the retention of waste for conversion of small protein molecules in

the form of floating mass, which consists of a wide variety of beneficial nitrifying

bacteria, fungi, protozoans, rotifers, brown and green microalgae, grazing micro

invertebrates and detritus. They are continuously mixed and suspended, treating and

bioconverting via autotrophic, heterotrophic and filter feeding or grazing both dissolved

and particulate wastes into microbial biomass. It is possible to convert particulate wastes

and dissolved wastes into beneficial form of microbiofloc through bioconversion process.

Formation

All microorganisms are omnipresent, but it is valid only for open continuous

system and in closed environments inoculation may be necessary. Aquaculture system,

being an open system, there may be no need of such inoculation. The biodiversity is

determined by the influx of the arrival of new species. Microbial community makes its

own niche and ecosystem engineering occurs by the inhabitants themselves. As there

may be plenty of wastes present in the culture system such as fish excreta, unconsumed

feed and other organic and inorganic matter, there must be the growth of omnipresent

microbes, but the success of floc formation depends upon optimizing the amount of

carbonaceous material to be added. Research showed that the excess addition of the

carbon source enhance the development of poly hydroxy alkanoate (PHA) for

accumulating microorganisms. PHAs play a greater role in microbial balance in the guts

of hosts. Bacteria and other microorganisms require carbohydrates for their growth. As

protein is the major component of new cell material, nitrogen is also required as an

essential element. Thus microbial utilization of carbohydrates or any other low nitrogen

feed in accompanied by the immobilization of inorganic nitrogen. This is the basic

principle carried out by the microbial community which lives in floc structure because

there is less chance of wash out i.e., avoidance of wash out. Second most probable

reason behind it is that it will be very difficult for them to get food when they live singly

(size 1m), so in floc they can enjoy much food. Third reason may be due to

prevention of predation i.e., lower predation by natural enemies because in floc they can

only grazed from the edge.

Constituents

Bioflocs are heterogenous mixtures, which constitute floc forming

bacteria, filamentous microalgae, some predating microorganisms like protozoea and

rotifer and non living components, as stated earlier. The composition depends upon

environmental factors such as C/N-ratio, predation, light, temperature etc. The

concentrations of floc vary from a few to 40g/dm3, which constitute 10 – 90% living

things with C/N-ratio 10. Extra cellular polymeric substance (EPS), storage polymers

(PHA, glycogen and polyphosphate) are the special components in biofloc.

Role

The deterioration of water quality due to unconsumed feed, faecal matter of

cultured organisms or the presence of other organic matters in ponds is nullified because

the floc microbes act as conditioner for water, which always control excess nitrogen by

feeding with carbohydrates and through the subsequent uptake of nitrogen from the water

for the synthesis of microbial protein. Hence biofloc based aquaculture system also offers

potential to use as zero exchange recirculation aquaculture system.

In most of farming practices farmers are using feeds comprising protein

(18 -50%), lipid (10-25%), carbohydrates (15-20%), ash (<0.5%) and trace amount of

vitamins. The bacterial protein and single cell protein synthesized by the heterotrophic

bacterial population in the biofloc are utilized directly as a food source by the cultured

organism, thus lowering the demand for supplemental feed protein. In biofloc, 10-90% is

living things, which serve as feed for cultured organism simulating natural environment

and hence there is an increment in production. So biofloc based aquaculture system can

be compared with estuarine ecosystem which is the world’s most productive system

because these flocs are essentially the same as the suspended detritus and planktonic

organisms of nutrient rich estuaries.

Probiotic bacteria in the microbial floc continuously surround the stocked fish or

shrimp and provide natural disease prevention and control. The presence of calcium in

biofloc protects the cultured species against heavy metal toxicity. The bacteria such as

Alcaligenes eutrophus, Azotobactor vinelandii, Pseudomonas oleovorans and others of

biofloc synthesize and accumulate PHA granules. Such granules are synthesized under

condition of nutrient stress i.e., when an essential nutrient like nitrogen is limited in

presence of excess carbon source. These PHA are polymers of beta hydroxy short chain

fatty acids and if degraded in gut, they could have antibacterial activity similar to short

chain fatty acids or organic acids. The break down of PHA inside the gastro intestinal

tract can be carried out via enzymatic and chemical hydrolysis. Thus it acts as a

biocontrol agent against pathogenic disease.

And so, in the present scenario of aquafarming with the emerging viral problems

and the high input cost, the biofloc technology based culture system appears to be an

answer for sustainable production with lower cost, by containing and converting the

waste material of the system into microbial protein rich food.

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