Page 183 - Algae

166 Algae: Anatomy, Biochemistry, and Biotechnology

include hydrogen azide, nitrous oxide, acetylene, and hydrogen cyanide. The non-specificity of this
enzyme and the alternative nitrogenases containing other metal co-factors implicate the role of
varying environmental pressures on the evolutionary history of nitrogenase that could have selected
for different functions of the ancestral enzyme. The nitrogenase enzyme system is extremely O 2
sensitive, because oxygen not only affects the protein structure but also inhibits the synthesis of
nitrogenase in many diazothrops. The repression is both transient (lasting only a few hours) and
permanent. The reactions occur while N 2 is bound to the nitrogenase enzyme complex. The
Fe-protein is first reduced by electrons donated by ferredoxin. Then the reduced Fe-protein
binds ATP and reduces the MoFe-protein, which donates electrons to N 2 , producing diimide
(HN55NH). In two further cycles of this process (each requiring electrons donated by ferredoxin)
HN55NH is reduced to imide (H 2 N22NH 2 ), and this in turn is reduced to 2NH 3 . Depending on the
type of microorganism, the reduced ferredoxin that supplies electrons for this process is generated
by fermentation or photosynthesis and respiration.
As already stated, nitrogenase is highly sensitive to molecular oxygen (in vitro it is irreversibly
inhibited by exposure to O 2 ). Therefore, during the course of planetary evolution, cyanobacteria
have co-evolved with the changing oxidation state of the ocean and atmosphere to accommodate
the machinery of oxygenic photosynthesis and oxygen-sensitive N 2 fixation within the same cell
or colony of cells. As nitrogen fixation occurs in a varied metabolic context in both anaerobic
and aerobic environments, strategies have followed a very complex pattern of biochemical and
physiological mechanisms for segregation that can be simplified in some spatial and/or temporal
separation of the two pathways.
Nitrogenase is an ancient enzyme that almost certainly arose in the Archean ocean before the
oxidation of the atmosphere by oxygenic photoautotrophs. An attractive hypothesis of the develop-
ment of biological nitrogen fixation is that it arouse in response to changes in atmospheric compo-
sition that resulted in the reduction in the production of abiotically fixed nitrogen. On the early
Earth, concentrations of CO 2 in the atmosphere were high, because of the oxidation of CO produced
by impacts of extraterrestrial bodies and only slow removal of CO 2 by weathering (the continents
were smaller at this time, meaning that a smaller area of minerals was exposed for weathering).
With these CO 2 conditions, the initial production rate of NO was estimated to be about
21
3 10 11 gyr . Atmospheric CO 2 levels declined with time, however, as the impact rate
dropped and the continents grew. A rise in atmospheric CH 4 produced by methanogenic,
methane-generating, bacteria may have warmed the Archaean Earth and speeded the removal of
CO 2 by silicate weathering. As this happened, the production rate of NO by lightning dropped to
9
below 3 10 gyr 21 because of the reduced availability of oxygen atoms from the splitting of
CO 2 and H 2 O. The resulting crisis in the availability of fixed nitrogen for organisms triggered
the evolution of biological nitrogen fixation about 2.2 billion years ago. Under the prevailing
anaerobic conditions of that period in Earth’s history anaerobic heterotrophs, such as Clostridium,
developed. With the evolution of cyanobacteria and the subsequent generation of molecular
oxygen, oxygen-protective mechanisms would be essential. A semitemporal separation of nitrogen
fixation and oxygenic photosynthesis combined with spatial heterogeneity was the first oxygen-
protective mechanism developed by marine cyanobacteria such as Trichodesmium sp. and Katagny-
mene sp. A full temporal separation, in which nitrogen is only fixed at night, then developed in
unicellular cyanobateria diazotrophs and in some non-heterocystous filamentous diazotrophs
(e.g., Oscillatoria limosa and Plectonema boryanum). Finally, in yet other filamentous organisms,
complete segregation of N 2 fixation and photosynthesis was achieved with the cellular differen-
tiation and evolution of heterocystous cyanobacteria (e.g., Nostoc and Anabaena).
The non-heterocystous filamentous cyanobacteria Trichodesmium sp. and Katagnymene sp.,
unlike all other non-heterocystous species fix nitrogen only during the day. Nitrogenase is compar-
timentalized in 15–20% of the cells in Trichodesmium sp., and 7% of the cells in Katagnymene sp.
often arranged consecutively along the trichome, but active photosynthetic components (PSI, PSII,
RuBisCo, and carboxysomes) are found in all cells, even those harboring nitrogenase. A combined