Environmental Engineering Reference
In-Depth Information
Figure 1.
Phylogenetic tree of N
2
fixing microorganisms based on approximately 2000
nifH
sequences in GenBank [142]. Sequences that cluster in Cluster III, which contains nitrogenases
from many anaerobes, have been detected in some aquatic environments, but are particularly
abundant in Mono Lake which had an anoxic water column [123]. The sequences in the tree
CB= Chesapeake Bay, GN= Guerrero
Negro microbial mat, ML= Mono Lake, SP=
Spartina
marsh sediments.
indicate Cluster III nitrogenases that have been
detected.
Fe
8
S
8
clusters (actually formed from two Fe
4
S
4
clusters) [53]. Component II is
a homodimer (α
2
) containing an Fe
4
S
4
cluster. Component II serves to reduce
Component I through 8 successive reductions per molecule of N
2
converted to
NH
3
. The
nifH
gene encodes component II.
Although all three nitrogenases reduce N
2
, the characteristics of the reaction
and interaction with substrates as well as sensitivity to environmental condi-
tions such as oxygen and temperature differ. The alternative nitrogenases are
more sensitive to oxygen, have a lower ratio of H
2
production per N
2
reduced,
