Biotechnology and Bioinformatics (1+1)

6.1.5.5 Bioremediation of aquaculture effluent using microbial mat

The conventional bioremediation technologies applied to remove the pollutant nutrients are impractical for sensitive areas, gene rally costly to operate for developing countries, often lead to secondary pollution and to incomplete utilization of natural resources.

• Bioremediation using microbial mats is the latest concept.
• Microbial mats are laminated heterotrophic and autotrophic vertically stratified communities typically dominated by cyanobacteria, eukaryotic micro algae like diatoms, anoxygenic phototrophic bacteria and sulphate reducing bacteria.
• The microbial communities convert the organic pollutants into the non – harmful products on a useful time scale.

Bioremediation of shrimp ( Litopenaeus vannamei) culture effluent

The treatment concept relies on the immobilization of natural marine microbial consortium on glass wool to mitigate the levels of dissolved nitrogen from a shrimp culture effluent.

The treatment via construct ed microbial mats is a technically feasible method for simultaneously reducing effluent nutrient loading (especially nitrate and ammonia) and for reducing organic loading (especially BOD₅) of shrimp culture effluents.

Michel and Garcia (2003) developed an efficient ex-situ bioremediation method for shrimp (L. vannamei) culture effluent using constructed microbial mat.

Collection and construction of microbial mat

• The natural microbial mats were collected from the fish / shrimp culture ponds.
• The collectors were used for collecting microbial mat in semi- intensive pond using substrate of polyester membrane.
• The collected mats were kept vertically suspended in the water column at different representative sites in the pond.
• After 35 days, the fully colonized collectors were recovered and placed in properly sealed polyethylene plastic bags in an oxygen rich atmosphere at 27^oC and transferred to the laboratory.
• The species components of the different microbial mats were analyzed. The microbial mat was constructed by different immobilization methods like glass wool, plastic mesh sheet and low density polyester support. The constructed mats were inoculated in a two different prototype bioreactors .

Inoculation of microbial mat

The constructed mats of three different immobilized supports were suspended in these two different prototype bioreactors, where in aquaculture effluent waste water was inoculated.

• The initial water quality parameters such as dissolve oxygen, ammonia nitrogen (NH_{3 –} N), Nitrite nitrogen (NO₂-N), Nitrate nitrogen (NO₃ –N) and Phosphate phosphorus (PO₄ –P) can be analyzed.
• After 35 days of inoculation, the final water quality parameters such as dissolved oxygen, ammonia nitrogen (NH_{3 –} N), Nitrite nitrogen (NO₂-N), Nitrate nitrogen (NO₃ –N) and Phosphate phosphorus (PO₄ –P) can be analyzed.
• The concentration of ammonia nitrogen, nitrate nitrogen and BOD₅ will be decreased significantly in the microbial mat treatment when compared to the control. Nitrifying activities in the reactors will be increased significantly through 20 days.
• The photoautotrophic (top of the mat) and heterotrophic community will be dominated by diatoms (Nitzchia sp. and Navicula sp.) and filamentous forms of cyanobacteria (Microcoleus chthonoplastes, Spirulina sp., Oscillatoria sp., Schizothrix sp., Calothrix sp. and Phormidium sp.) as well as green algae comprised by Chlorella sp. and Dunaliella sp.
• Mixed microbial mats containing filamentous cyanobacteria (Oscillatoria sp.) as the dominant species have been shown to remove nitrogenous compounds and other toxic chemicals from polluted sites.
• The efficiencies of 97% of ammonia nitrogen and 95% of nitrate nitrogen removal will be observed over a 20 days period of culture.

Microbial mats are able to degrade a variety of organic compounds, including those found in fish wastes. Mats spontaneously form in estuarine and fresh water environments and can be easily cultured for various specialized uses. Microbial mats are rich in nitrogen and can possibly be used as feed for young tilapia and thus if tilapia are fed microbial mats there is the possibility of a complete recycling of all nitrogen and carbon.

Advantages associated with the constructed microbial mat

1. The isolated microorganisms required for the construction of the mats attach effectively to the polyester carrier

2. The wastewater and faeces of the shrimp are suitable substrates for the development of nitrification in the continuous and self-sufficient growth of indigenous microorganisms in the mats

3. Additional oxygen can be supplied to the water column by the photosynthetic components of the mats for the degradation of the organic matter and the nitrification process;

4. The use of the constructed microbial mats represents an effective and economical in situ model of environmentally clean land-based shrimp production and represents an additional source of nutrition for L. vannamei. The advantages of this support over other immobilization supports (e.g., polymeric matrices, bead) include a lower cost, easy operation, and durability. The water exchange can be reduced without sacrificing growth or survival and, consequently, the total amount of nutrients and BOD discharged into adjacent water bodies may be minimized.

Bioremediation using aquatic plants

· Seaweed (Gracilaria fisheri), is capable of assimilating NH₃, NO₂, NO₃ and PO₄ from shrimp-farming effluents.

· Other seaweed, such as, red seaweed (Gracilaria salicornia), green seaweed (Caulerpa macrophysa), and brown seaweed (Sargassum polycystum), also assimilate waste nitrogen (NH₃ and NO₃-) from shrimp pond effluent efficiently.

· The maximum absorption rates of all seaweeds were found within the first 24 h of experiments with 1 g/l stocking density. C. macrophysa has higher growth rate as well as higher efficiency than the other two species

· The Red algae Gracilaria lameneiformis has high nutrient bioremediation efficiency and assimilative capacity, and its co – culture with fish could be an effective measure to reduce nutrient loading in coastal fish culture.

· Besides seaweed, fresh water aquatic plants such as coontail (Ceratophyllum demersum) and Duck weed (Lemna sp.) can efficiently assimilate ammonia nitrogen.