Biotechnology and Bioinformatics (1+1)

6.1.5.3. Bioremediation of Hydrogen Sulphide

Sulphur is of some interest in aquaculture because of its importance in anoxic sediments.

• In aerobic conditions, organic sulphur decomposes to sulphide, which in turn get oxidized to sulphate.
• Sulphate is highly soluble in water and so gradually disperses from sediments.
• Sulphide oxidation is mediated by micro organisms in the sediment, though it can occur by purely chemical processes. Under anaerobic conditions, sulphate may be used in place of oxygen in microbial metabolism. This process leads to the production of hydrogen sulphide gas. The H₂S is produced by a series of microbially mediated reductions.

SO₄ ^2- + 4H₂ + 2H H₂S + 4H₂O

Organic loading can stimulate H₂S production and reduction in the diversity of benthic fauna.

• H₂S is soluble in water and has been suggested as the cause of gill damage and other ailments in fish.
• Unionised H₂S is extremely toxic to fish at concentrations that may occur in natural waters as well as in aquaculture farms. Bioassays of several species of fish suggest that any detectable concentration of H₂S should be considered detrimental to fish production.

The photosynthetic benthic bacteria that break H₂S at pond bottom have been widely used in aquaculture to maintain a favourable environment.

• They are purple and green sulphur bacteria that grow at the anaerobic portion of the sediment-water interface. Photosynthetic purple non-sulphur bacteria can decompose organic matter, H₂S, NO₂ and harmful wastes of ponds.
• The green and purple sulphur bacteria split H₂S to utilize the wavelength of light not absorbed by the overlying phytoplankton. The purple and green sulphur bacteria obtain reducing electrons from H₂S at a lower energy cost than H₂O splitting photoautotrophs and thus require lower light intensities for carrying out photosynthesis. The general equation of this reaction is as follows.

CO₂ + 2H₂S → (CH₂O) +H₂O + 2S

Chromatiaceae and Chlorobiaceae are the two families of photosynthetic sulphur bacteria that favour anaerobic conditions for growth while utilizing solar energy and sulphide. Chromatiaceae contain sulphur particles in cells but Chlorobiaceae precipitate them out.

S+CO₂ + 3H₂S → (CH₂O) + H₂SO₄

CO₂ + NaS₂O₃ + 3H₂O → 2(CH₂O) + NaS₂O₄ + H₂SO₄

• The family Rhodospirillaceae is not of any use for H₂S removal as they mainly utilize organic material, such as lower fatty acid, as source of hydrogen. But they can be used as efficient mineralizers at pond bottom as they grow in both aerobic and anaerobic conditions as heterotrophic bacteria even in the dark without utilizing solar energy.

Rhodospirillaceae

Rhodospirillum, Rhodopseudomonas, Rhodomicrobium

Chromatiaceae

Chromatium,Thiocystis, Thiosarcina, Thiospirillum, Thiocapsa, Lamprocystis, Thiodictyon, Thiopedia, Amoebobacter, Ectothiorhodospira.

Chlorobiaceae

Chlorobium, Prosthecochloris, Chloropseudomonas, Pelodictyon, Clathrochloris.

For bioremediation of H₂S toxicity, the bacterium that belongs to Chromatiaceae and Chlorobiaceae can be mass cultured and can be applied as pond probiotic. Being autotrophic and photosynthetic, mass culture is less expensive and the cultured organisms can be adsorbed on to the sand grains and applied so that they may reach the pond bottom to enrich the hypolimnion and ameliorate H₂S toxicity.