Biology Reference
In-Depth Information
Bacteriological nitrifi cation is the most practical method for the removal
of ammonia from closed aquaculture systems and it is commonly achieved
by setting of a sand and gravel bio-fi lter through which water is allowed
to circulate.
The ammonia oxidizers are placed under four genera:
Nitrosomonas
.
Ellipsoidal or short rods;
Nitrosococcus
.
Spherical cells;
Nitrosolobus
.
Lobate and pleomorphic cells;
and
Nitrosospira
.
Spiral-shaped cells.
The nitrite oxidizers belong to only one genus:
Nitrobacter
.
Short rods.
However,
Nitrosomonas
and
Nitrobacter
are the most common nitrifi ers
commonly encountered in the aquaculture system.
There are also some heterotrophic nitrifying bacteria (i.e.,
Pseudomonas,
Bacillus,
Alcaligenes,
etc.) that produce only low levels of nitrite and nitrate
and often use organic sources of nitrogen rather than ammonia or nitrite
(Focht and Verstraete, 1977).
Nitrifers in contaminated cultures have been demonstrated to nitrify
more effi ciently. Nitrifi cation not only produces nitrate but also alters the
pH slightly towards the acidic range, facilitating the availability of soluble
materials (Ayyappan and Mishra, 2003). The vast majority of aquaculture
ponds accumulate nitrate, as they do not contain a denitrifying fi lter.
Denitrifying fi lters helps to convert nitrate into nitrogen. It creates an
anaerobic region where anaerobic bacteria can grow and reduce nitrate to
nitrogen gas.
NO
3
-
NO
2
-
NO N
2
O N
2
Nitrogen (N
2
) gas diffuses into the air and does not pose a toxicity threat
to culture animals. However, ammonia and nitrite released by certain
denitrifying bacteria can be toxic to fi sh, shrimp, and other aquaculture
organisms.
At National Centre for Aquatic Animal Health, Cochin University of
Science and Technology, Cochin, India, four nitrifying bacterial consortia,
which effi ciently oxidize ammonia to nitrate, have been developed.
Their effectiveness has been demonstrated under different conditions in
hatcheries. They have been integrated into two reactor systems, namely,
In-Situ
Stringed Bed Suspended Bioreactors and
Ex-Situ
Packed Bed
Bioreactors in immobilized form (Patent No. 828/DEL/2000).
Microbial Remediation 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 gets oxidized to sulphate. Sulphate is highly
