Environmental Engineering Reference
In-Depth Information
for heterocysts could only have occurred in localized environments under these
conditions.
Nitrogenase genes are widely distributed throughout the prokaryotic king-
dom, but are not uniformly distributed across closely related taxa. Since the
genes are so highly conserved (very similar in DNA and amino acid sequence
in very different microorganisms), it has been argued that nitrogenase genes
either evolved early, or were laterally (horizontally) transferred later after most
of the major lineages had diverged [99, 136]. Most phylogenetic analyses,
including those based on recent analyses of large datasets conclude that the
nitrogenase genes evolved early, prior to the separation of the Archaea from
the Bacteria [34, 76, 105, 136, 138]. Gene duplications and lateral gene trans-
fer must also have been involved in order to explain the distribution of the
different nitrogenase gene families [105]. Regardless of how early nitrogenase
evolved, the genes must have evolved when at least the deep oceans were anoxic
(Fig. 2).
Although it is difficult to speculate about the early evolution of metabolic
pathways [84], the sequence of events that shaped the Earth's chemistry and led
to nitrogen limitation of productivity in the oceans paralleled the evolution of
N 2 fixation, autotrophy and photosynthesis. Nitrogenase had to have evolved
early [105] prior to the diversification of the cyanobacteria, and prior to the
oxygenation of the atmosphere. The first nitrogenase protein may have been
the Fe nitrogenase, which could have evolved in a Fe rich ocean, that had low
concentrations of Mo and V [105]. The Mo and V enzymes would then have
evolved from duplication of the Fe nitrogenase more recently, perhaps after ox-
idation of the surface ocean, or when localized oxidized environments allowed
MoO 4 and VO 4 to remain in solution. Since the formation of these oxidized
environments presumably followed the diversification of life and evolution of
oxygenic photosynthesis, the Mo and V nitrogenases would have had to have
been horizontally transferred in order to give rise to the current distribution of
the Mo and V enzymes throughout the prokaryotic tree [105]. Alternatively,
the Mo and V nitrogenases could have evolved near hydrothermal vents, where
metal concentrations might have been high enough to select for these metal-
loenzymes.
The fixation of N 2 in anoxic water columns is likely to be important in the
evolution of life on earth, as well as in the modern day ocean basins. The
nitrogenase enzymes and N 2 fixation have undoubtedly been shaped by the
conditions of the early oceans, including anoxia. Coincident with changes in
oxygen concentrations were changes in metal and ion chemistry of importance
to N 2 fixation.
Search WWH ::




Custom Search