Environmental Microbiology of Aquatic and Waste Systems

4.1 Taxonomy of Microorganisms in Aquatic Environments 71

effluents that they readily convert the ammonium
compounds therein into nitrates. The nitrates can accu­
mulate in groundwater, and may ultimately enter drinking
water. Regulations in many countries control the amount
of nitrate in drinking water, because in the anaerobic
conditions of the animal alimentary canal, nitrates can be
reduced to highly reactive nitrites by microorganisms.
Nitrites are absorbed from the gut and bind to hemoglobin,
reducing its oxygen­carrying capacity. In young babies,
this can lead to a respiratory illness known as blue baby
syndrome. Nitrites can also react with amino compounds,
forming nitrosamines which are highly carcinogenic.

Denitrification
Denitrification is the conversion of nitrate to gaseous
compounds (nitric oxide, nitrous oxide, and N 2 ) by
microorganisms. Denitrification goes through some
combination of the following intermediate forms:

The denitrification process can be expressed in
terms of electron transfer thus:

Denitrification is brought by a large number of

different bacteria which are mainly heterotrophic.

They complete the nitrogen cycle by returning N 2 to

the atmosphere. Denitrification occurs under special

conditions in both soil and aquatic conditions, includ­

ing marine environments. Denitrification occurs when

oxygen supply is low such as in ground water, wet­

lands in seafloors, and other poorly aerated parts of

aquatic systems. The conditions which encourage den­

itrification are those in which there is a supply of oxi­

dizable organic matter, and absence of oxygen and the

availability of reducible nitrogen sources. Under such

conditions, the terminal electron acceptor for the deni­

trifying bacteria is not oxygen but the nitrogen com­

pounds given in the formula above. The organisms

prefer nitrates and the other compounds in the equa­

tion, in the order they occur in the equation above and

ending with nitrous oxide. When the terminal electron

acceptor is an inorganic compound such as those in the

formula above, the condition is also termed respiration

as is also the case with oxygen.

A mixture of gaseous nitrogen products is often

produced because of the stepwise use of nitrate, nitrite,

nitric oxide, and nitrous oxide as electron acceptors in

anaerobic respiration. The commonest denitrifying

bacteria include several species of Pseudomonas,

Alkaligenes, Bacillus,, and Paracoccus denitrificans.

Autotrophic denitrifiers (e.g., Thiobacillus denitrifi­

cans) have also been identified. In general, however,

several species of bacteria are involved in the com­

plete reduction of nitrate to molecular nitrogen, and

more than one enzymatic pathway have also been

identified.

Anammox

In some organisms, direct reduction from nitrate to

ammonium (also known as dissimilatory nitrate

reduction to ammonium or DNRA) may also occur;

although, this is less common than denitrification.

Anammox, an abbreviation for ANaerobic AMMonium

OXidation, is a globally important microbial process

of the nitrogen cycle. It takes place in many natural

environments.

The bacteria mediating this process were identified

only 20 years ago. They belong to the bacterial phylum

Planctomycetes, of which Planctomyces and Pirellula

are the best known genera. Four genera of anammox

bacteria have been identified: Brocadia, Kuenenia,

Anammoxoglobus, Jettenia (all freshwater species),

and Scalindua (marine species).

NO 32 NO NO N O 2 N 2

nitrate nitrite nitric oxide nitrous oxide gas

−−→→ → → ↑

2NO 10e 12H N 6H O. 3 22

−− ++ + →+

Fig. 4.14 The enzymes of nitrogen fixation (Reproduced from
Berg et al. 2002. With permission)
Note: Ferrodoxins are a group of red­brown proteins contain­
ing iron and sulfur, which act as electron carriers during photo­
synthesis, nitrogen fixation, or oxidation­reduction reactions in
green plants, algae, and anaerobic bacteria.
Nitrogenase is a two­protein complex. One component, nitro-
genase reductase is an iron­containing protein that accepts
electrons from ferredoxin, a strong reductant, and then delivers
them to the other component, nitrogenase, which contains Iron
(Fe) and molybdenum (Mo).
The overall reaction in nitrogen fixation via nitrogenase is:
8H+ + N 2 + 8e­ + 16ATP + 16H 2 O →
2NH 3 + H 2 + 16ADP + 16Pi + 16H+