Environmental Engineering Reference
In-Depth Information
Table 12.1 Summary of the major processes in the nitrogen cycle, the organisms responsible for
the different processes, and the environmental conditions necessary for each process
Necessary environmental
condition
Process
Organism(s) responsible
Nitrogen fixation
[N
2
(gas)
Cyanobacteria, nitrogen-fixing
bacteria
Absence of O
2
; light
for cyanobacteria
!
reduced N]
Denitrification
NO
3
!
N
2
ð
gas
Þ
Denitrifying bacteria
Absence of O
2
Ammonification
½
Heterotrophic organisms
Presence of O
2
NH
4
N
org
!
Nitrogen assimilation
½NO
3
!
N
org
;
NH
4
!
N
org
Large phytoplankton/diatoms
for
NO
3
uptake; small
phytoplankton for
NH
4
uptake
Light
Nitrification
½NH
4
! NO
3
Bacteria
Presence of O
2
Annamox
½
Bacteria
Absence of O
2
NH
4
;
NO
2
!
N
2
ð
gas
Þ
a microzone of very low oxygen, thus allowing nitrogen fixation to proceed. Other,
smaller cyanobacteria have unusual biochemical adaptations that allow them to fix
N
2
as well, despite living in oxygen-saturated water.
Recently a new nitrogen transformation, the annamox pathway, has been
described in which anaerobic bacteria oxidize ammonium and nitrite directly to
gaseous nitrogen, providing a second means by which nitrogen is “lost” from the
nitrogen cycle [
10
]. This pathway has been found to be quantitatively important in
regions such as the Peruvian and Arabian Sea oxygen-minimum zones [
11
,
12
].
Because 30-50% of global nitrogen “losses” occur in these types of regions,
elucidation of this process, its oceanographic controls, and the absolute rates,
have important implications for the global nitrogen cycle.
The Biogeochemical Cycle of Iron
The understanding of the role of iron in the ocean has undergone a dramatic revision
in the past few decades. Until recently data on absolute iron concentrations were
seriously compromised by the difficulty of obtaining samples without contamina-
tion. As the collection and sampling aspects were greatly improved, the ability to
quantify concentrations of iron in the oxygenated waters of the ocean decreased
dramatically. Coincident with increased realization and acceptance of the vanish-
ingly low concentrations of iron was the hypothesis that iron could, and does,
regulate phytoplankton growth and productivity over large areas of the ocean
[
13
]. Indeed, the hypothesis appeared to explain a number of oceanic features
that were only partially explained. For example, large areas of the ocean, such as
