Geoscience Reference
In-Depth Information
NO
-
V
?
IV
NO
-
III
?
II
Nitrification
N
2
O
I
?
N
2
0
Assimilation
NH
+
Organic nitrogen
-III
Ammonification
Figure 12.3
The marine nitrogen cycle highlighting the reactions that have been shown to be sensitive to changes in pH and the oceanic carbonate
system. + and - indicate reactions stimulated or inhibited by ocean acidii cation. ?: effect of ocean acidii cation is unknown. anammox = anaerobic
ammonium oxidation. Adapted from Gruber ( 2008 ).
response of this organism to elevated CO
2
and low-
ered pH (e.g. Barcelos e Ramos
et al.
2007 ; Hutchins
et al.
2007 ; Levitan
et al.
2007 ; Kranz
et al.
2009 ). As
summarized by Hutchins
et al.
( 2009 ), the mean
enhancement of the rate of N
2
i xation at elevated
CO
2
levels (750-1000 μatm) from the six reported
experiments with isolated cultures amounts to
about 40 ± 20%. The only experiment with natural
populations of
Trichodesmium
collected from the
Gulf of Mexico also yielded increased i xation rates,
although of smaller amplitude, i.e. between 6 and
41% (Hutchins
et al
. 2009). The enhanced rate of N
2
i xation tends to be accompanied by enhanced rates
of carbon i xation, so that the mean C:N ratio of
these organisms remained relatively constant
(Barcelos e Ramos
et al.
2007). This suggests that the
primary mechanism responsible for the enhanced
rate of N
2
i xation is the CO
2
-induced increase in
photosynthesis, providing additional energy for the
i xation of N
2
.
Experiments with non-
Trichodesmium
N
2
i xers
are rare, in large part due to the fact that the major-
ity of the identii ed species have not been cultured
yet (Carpenter and Capone 2008). In the case of
Crocosphaera watsonii
, an enhancement was found
that is very similar to that identii ed for
Trichodesmium
( Fu
et al
. 2008 ). In contrast,
Nodularia
spumigena
exhibited almost no response to ocean
acidii cation (Czerny
et al.
2009 ). Thus, with the
exception of one species, the majority of experi-
ments point toward an enhancement of pelagic
marine N
2
i xation in response to ocean acidii ca-
tion. However, given the fact that the majority of
the experiments were performed with the same
