Environmental Engineering Reference
In-Depth Information
significance of nitrogenase gene expression and activity has yet to be determined
in anoxic water columns.
Keywords:
Nitrogen fixation,
nif
, anaerobes, biogeochemical cycles, marine nitrogen cycle,
evolution, anoxic environments.
1. INTRODUCTION
Nitrogen fixation is the primary pathway for introduction of fixed nitrogen
into the biosphere and comprises a central process in the global nitrogen cycle.
Microbially mediated N
2
fixation accounts for an annual input of
1.5x10
13
molNyr
−
1
globally, with approximately half of this annual N
2
fixation in
the oceans. Oceanic N
2
fixation has received increasing attention in the past
two decades as basin-scale nitrogen budgets provide conflicting estimates of
the magnitude of N
2
fixation. In addition, previously unrecognized N
2
-fixing
microorganisms have recently been discovered and the contributions of these
poorly characterized microorganisms to ocean nitrogen budgets are not yet
known.
Nitrogen fixation in the water columns of oxic ocean basins has received
considerable attention, but to date, little research has focused on N
2
fixation in
anoxic ocean basins. Anoxic waters have played a pivotal role in the evolution
of N
2
fixation on Earth as the oceans were anoxic over large periods of the
Earth's history. N
2
fixation is an energetically expensive reaction that is highly
regulated. The characteristics of the enzyme and the chemical reaction are
important for understanding how and why N
2
fixation evolved in anoxic oceans,
and why it could occur in anoxic water columns. In this chapter, we review the
properties of N
2
fixation, what is known about N
2
fixation in the oceans, and
examine N
2
fixation in anoxic marine environments, including anoxic basins.
∼
2. THE BIOCHEMISTRY AND MOLECULAR
BIOLOGY OF NITROGEN FIXATION
2.1 Chemical Reaction
Biological N
2
fixation is the enzymatic reduction of N
2
to ammonia (NH
3
).
Enzymes that catalyze this reaction are called nitrogenases. The reduction of
one molecule of N
2
requires 8 low potential electrons to form 2 molecules
of NH
3
and one molecule of hydrogen (H
2
). N
2
fixation is an energetically
expensive reaction requiring approximately 16 ATP molecules per molecule
of N
2
reduced corresponding to 2 ATP per electron [29]. The low potential
electrons are provided by oxidation of ferredoxin or flavodoxin :
8
H
+
+
8
e
−
+
N
2
+
16
MgAT P
→
2
NH
3
+
H
2
+
16
MgADP
+
16
P
i
