Biology Reference
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soluble in water and so gradually disperses from sediments. Sulphide
oxidation is mediated by microorganisms in the sediment: the predominant
microorganisms are Thiobacillus , Bacillus , Arthrobacter , Flavobacterium and
Pseudomonas (Boyd, 1995). Under anaerobic conditions, sulphate may be
used in place of oxygen in microbial metabolism. This process leads to
the production of hydrogen sulphide (H 2 S) gas. The H 2 S is produced by a
series of microbially mediated reductions (Boyd, 1995).
SO 4 2- + 4H 2 + 2H +
H 2 S + 4H 2 O (Djurle, 2003)
Organic loading can stimulate H 2 S production and reduction in the
diversity of benthic fauna (Mattson and Linden, 1983). H 2 S is soluble
in water and has been suggested as the cause of gill damage and other
ailments in fi sh (Beveridge, 1987). Bioassays of several species of fi sh
suggest that any detectable concentration of H 2 S should be considered
detrimental to fi sh production (Boyd, 1979). The photosynthetic benthic
bacteria that break H 2 S at the bottom of the pond have been widely used in
aquaculture to maintain a favourable environment (Boyd and Silapajarn,
2006). These bacteria contain bacterio-chlorophyll that absorb light (blue
to infrared spectrum, depending on the type of bacterio-chlorophyll) and
perform photosynthesis under anaerobic conditions (Haung, 2003). These
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, H 2 S, NO 2 and harmful wastes of ponds.
The green and purple sulphur bacteria split H 2 S to utilize the wavelength
of light not absorbed by the overlying phytoplankton. The purple and
green sulphur bacteria obtain reducing electrons from H 2 S at a lower
energy cost than H 2 O splitting photoautotrophs and thus require lower
light intensities for carrying out photosynthesis. The general equation of
this reaction is as follows:
CO 2 + 2H 2 S
(CH 2 O) +H 2 O + 2S
S+CO 2 + 3H 2 S
(CH2O) + H 2 SO 4
2(CH 2 O) + NaS 2 O 4 + H 2 SO 4
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. The family Rhodospirillaceae
is useful for H 2 S removal as they mainly utilize organic material, such as
lower fatty acid, as a source of hydrogen. But they can be used as effi cient
mineralizers at the bottom of the pond as they grow in both aerobic and
anaerobic conditions as heterotrophic bacteria even in the dark without
utilizing solar energy (Boyd and Silapajarn, 2006). Photosynthetic bacteria
of importance in aquaculture belong to the following three families
(Haung, 2003):
CO 2 + NaS 2 O 3 + 3H 2 O
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