Geoscience Reference
In-Depth Information
strain of
Trichodesmium
, it is premature to draw
i rm conclusions. If it is nevertheless assumed that
all nitrogen i xers are stimulated by ocean acidii ca-
tion, the global rate of marine N
2
i xation may
increase by more than 50 Tg N yr
-1
by 2100. Relative
to present-day rates of the order of 120 ± 50 Tg
N yr
-1
(Gruber 2008), this represents a substantial
acceleration of the rate of input of i xed nitrogen
into the marine i xed nitrogen pool.
In nitrogen-limited regions of the ocean, such an
addition has the potential to substantially increase
marine productivity and thereby increase the export
of organic carbon from the surface to the interior.
This tends to enhance the uptake of CO
2
from the
atmosphere, i.e. this is a negative indirect group 2
type feedback ( Table 12.1 ).
Fundamentally, as is the case for photoautotrophic
organisms, one would expect a positive effect
though, i.e. enhanced i xation of CO
2
due to a
higher efi ciency of the CO
2
-i xing
enzyme
RubisCO (Rost
et al.
2008 ).
Using results from the highly limited experi-
ments, Hutchins
et al.
( 2009 ) estimated a global-
mean reduction of nitrii cation by about 10% for a
business-as-usual scenario. This would result in a
substantial shift of i xed nitrogen in the near-sur-
face waters from the nitrate pool to the ammonium
pool. However, the implications of this shift for
earth system feedbacks are likely to be small
because it does not directly alter the amount of
i xed nitrogen in the ocean, keeping the impact on
total production small. However, it is conceivable
that this will cause alterations of the phytoplank-
ton community structure, since not all phytoplank-
ton can take up nitrate and ammonium equally
well. For example, one could expect that species
that are highly adapted to low nitrate and high
ammonium concentrations, such as
Prochlorococcus
( Moore
et al.
2002 ), would benei t at the expense of
other phytoplankton that are more adapted to high
nitrate and low ammonium concentrations (e.g.
many diatoms).
A potentially larger effect may occur in shallow
coastal sediments that are overlain by waters with
relatively low nitrate concentrations. Here, the
reduction of nitrate may cause a substrate-driven
reduction in the rates of sedimentary denitrii ca-
tion (Blackford and Gilbert 2007). Given the fact
that sedimentary denitrii cation is the largest sink
for i xed nitrogen in the ocean (Gruber 2008), a
reduction in this sink would increase the pool of
i xed nitrogen in the ocean and hence probably
increase biological productivity. However, there
is little evidence that sedimentary denitrii cation
is limited by nitrate on a global scale (Middelburg
et al.
1996), so that this may remain a more local
effect.
In summary, changes in nitrii cation induced by
ocean acidii cation may lead to several indirect
group 2 feedbacks. The direction of these feed-
backs is unclear, but their magnitudes appear to
be small. Given the limited extent of the studies
undertaken so far, this conclusion remains
tentative.
12.3.2 Nitrii cation
Nitrii cation, the combined processes of the oxida-
tion of ammonium (NH
4
+
) to nitrite (NO
2
-
), and the
oxidation of nitrite to nitrate (NO
3
-
) is undertaken
by two distinct classes of chemoautotrophic organ-
isms which use the chemical energy released from
these two processes as their source of energy:
Nitrosomonas
is responsible for the i rst oxidation
step, i.e. the conversion from ammonium to nitrite,
while
Nitrobacter
oxidizes nitrite to nitrate.
Nitrii cation is inhibited by light, so it tends to be a
dominant process only in the aphotic zone.
However, a recent compilation by Yool
et al.
( 2007 )
shows that a substantial amount of nitrii cation also
occurs in the near-surface ocean.
Ocean acidii cation studies on marine nitrii ers
are not as abundant as those that have been under-
taken on marine N
2
i xers, but tend to show a nega-
tive response, i.e. reduced rates at lower pH and
elevated CO
2
( Huesemann
et al.
2002 ). This may be
due to two effects (Hutchins
et al.
2009 ). First, in
the case of ammonium oxidation by
Nitrosomonas
,
substrate limitation may be responsible. This is
because these organisms prefer NH
3,
which
becomes substantially less abundant relative to
NH
4
+
in a more acidii ed ocean (pK of NH
3
/NH
4
+
is
around 9.2). Second, both
Nitrosomonas
and
Nitrobacter
may be responding directly to the
increased concentration of dissolved CO
2
, which is
their substrate for the synthesis of organic matter.
