Environmental Engineering Reference
In-Depth Information
DIVERSITY, DISTRIBUTION AND
BIOGEOCHEMICAL SIGNIFICANCE OF
NITROGEN-FIXING MICROORGANISMS IN
ANOXIC AND SUBOXIC OCEAN ENVIRONMENTS
Jonathan P. Zehr
1
, Matthew J. Church
2
, and Pia H. Moisander
3
1
University of California Santa Cruz, Ocean Sciences Department, 1156 High Street, Santa
Cruz, CA 95064, USA
2
University of Hawaii, School of Ocean and Earth Science and Technology, Honolulu, HI 96822,
USA
3
NASA Ames Research Center, Exobiology Branch, Moffett Field, CA 94035, USA
Abstract
Nitrogen fixation, the reduction of atmospheric dinitrogen (N
2
) to biologically
available ammonium, has been important in the balance of biologically available
nitrogen since early in the evolution of life on Earth. The nitrogen fixation reaction
requires ATP and reductant and also reduces H
+
to H
2
. Nitrogenase is composed
of two multi-subunit metalloproteins. There are at least three evolutionarily
related nitrogenase gene families that require different metals (Mo, V or Fe)
in the cofactor for the protein component (Component I) that contains the active
site for N
2
reduction. The requirements for metals probably played an important
role in the evolution of nitrogenases, as the oceans progressed from anoxic waters
containing relatively high concentrations of reduced Fe, to low-Fe oxic waters.
The nitrogenase genes are distributed widely throughout prokaryotic taxa, and
thus, nitrogen fixation is found in
facultative
microorganisms, with equally diverse physiological strategies. Many
of these
organisms can be found at oxic-anoxic interfaces and in anoxic
waters.
Nitrogenase expression and activity is regulated in the presence of both NH
4
+
and
O
2
, which are biogeochemically important variables in anoxic ecosystems. It has
generally been assumed that N
2
fixation is unimportant in anoxic waters since
ammonium concentrations tend to be high in these environments. N
2
fixation
is known to be quantitatively important in oxygenated surface waters of the
oceans and in oxic waters overlying anoxic basins, with much of this N
2
fixation
catalyzed by cyanobacteria. N
2
fixation occurs in marine anoxic habitats such
as salt marshes and in sediments, but has not been reported in anoxic water
columns. N
2
fixation studies in hypersaline Mono Lake indicate that N
2
fixing
microorganisms do express nitrogenase in anoxic waters, but in general, the
diverse anaerobic, aerobic, and
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