70 4 Taxonomy, Physiology, and Ecology of Aquatic Microorganisms
The APBs produce a photosynthetic apparatus
similar to that of purple phototrophic bacteria.
However, this apparatus, in contrast to that of the
anaerobic photosynthetic bacteria is produced only
under aerobic conditions. In facultatively anaero
bic organisms, the photosynthetic apparatus is syn
thesized under conditions of oxygen shortage and
absence of light.
They are a phylogenetically diverse group inter
spersed predominantly throughout the aProteobac
teria, closely related to anoxygenic phototrophic
purple nonsulfur bacteria as well as chemotrophic
species. Recently, however, more and more APBs
have been placed in the bProteobacteria.Nitrogen Economy in Aquatic Systems
Nitrogen Fixation
Nitrogen is important in microorganisms for the manu
facture of proteins and nucleic acids, both of which are
essential for the continued existence of all living things.
Although the element is abundant in the atmosphere,
constituting about 80%, the ability of making atmo
spheric nitrogen available to living things is present
only in a few organisms. Biological nitrogen fixation
can be represented by the following equation, in which
2 moles of ammonia are produced from 1 mole of
nitrogen gas, at the expense of 16 moles of ATP and a
supply of electrons and protons (hydrogen ions):
This reaction is performed exclusively by prokaryotes
using a nitrogenase enzyme complex. This enzyme
consists of two proteins – an iron protein and a
molybdenumiron protein – as shown Fig. 4.14.
The reactions occur while N 2 is bound to the nitroge
nase enzyme complex. The Fe protein is first reduced by
electrons donated by ferredoxin. Then the reduced Fe pro
tein binds ATP and reduces the molybdenum–iron protein,
which donates electrons to N 2 , producing HN=NH. In two
further cycles of this process (each requiring electrons
donated by ferredoxin), HN=NH is reduced to H 2 N–NH 2 ,
and this in turn is reduced to 2NH 3. Depending on the type
of microorganism, the reduced ferredoxin, which supplies
electrons for this process, is generated by photosynthesis,
respiration, or fermentation (Anonymous 2010 e).
Nitrogen fixation may be done by bacteria living
symbiotically with higher plants such as Rhizobium
spp. and legumes or by freeliving organisms. In the
aquatic environment, the nitrogen fixers are freeliving
microorganisms including aerobic and anaerobic ones.
(Naqvi 2006 ). Among aerobes, nitrogen fixers include
all members of Azotobacter and Beijerinckia,, some
Klebsiella and some cyanobacteria. Under anaerobic
conditions, such as, occur in sediments or in the deeper
regions of water columns, the following organisms fix
nitrogen: Some Clostridium spp., Desulfovibrio, pur
ple sulfur bacteria, purple nonsulfur bacteria, and
green sulfur bacteria.
The nitrogenase enzyme complex is highly sensitive
to oxygen and it is inactivated when exposed to oxy
gen, because oxygen reacts with the iron component
of the proteins. Aerobic organisms including cyanobac
teria, which produce oxygen during photosynthesis,
combat the problem of nitrogenase inactivation by
different methods. Cyanobacteria for example have
special cells, heterocysts, where nitrogen fixation
occurs and in which nitrogenase is protected because
they contain only photosystem I whereas the other
cells have both photosystem I and photosystem II
(which generates oxygen when light energy is used to
split water to supply H 2 in photosynthesis.). For the
same reason, also, Azotobacter and Rhizobium pro
duce large amounts of extracellular polysaccharide,
which helps limit the diffusion of oxygen to the cells.
Furthermore, Rhizobium root nodules contain oxygen
scavenging molecules such as leghemoglobin, which
regulate the supply of oxygen to the nodule tissues inthe same way as hemoglobin regulates the supply of
oxygen to mammalian tissues.
Other microbial activities which participate in
regulating the nitrogen economy of aquatic systems
are nitrification and denitrification.Nitrification
Nitrification is the conversion of ammonium to nitrate by
the nitrifying bacteria. These bacteria are chemo
autothrophs which obtain energy by oxidizing ammo
nium, while using CO 2 as their source of carbon to
synthesize organic compounds. The nitrifying bacteria
are found in most soils and waters of moderate pH, but
are not active in highly acidic soils. They almost always
are found as mixedspecies communities or consortia.
Some of them – e.g., Nitrosomonas convert ammonium
to nitrite (NO 2 −) while others – e.g., Nitrobacter convert
nitrite to nitrate (NO 3 −). The nitrifying bacteria are so
numerous in waters rich in ammonium such as sewageN 2 8H 8e 16 ATP 2NH 32 H 16ADP 16 Pi.
+ ++ + = + + +−