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the main carrier phase for POC to the deep ocean.
Following their line of thought, CaCO
3
would pro-
vide POC with excess density (ballasting) thereby
increasing its sinking speed. It is also hypothesized
that the association between CaCO
3
and POC might
protect the latter from bacterial degradation. If a
control of POC l uxes by CaCO
3
is assumed, then a
decrease in CaCO
3
production would imply less
ballasting of POC l uxes, resulting in a decrease of
its penetration depth. Particulate organic carbon
would be remineralized at shallower depth and the
overall efi ciency of the biological pump would
decrease resulting in a positive feedback to rising
atmospheric CO
2
.
Barker
et al.
( 2003 ) were the i rst to address the
combined calcii cation and ballast feedback. Their
box model sensitivity study coni rms that the bal-
last effect counteracts the negative feedback of
reduced calcii cation and, depending on the pene-
tration depth of particle l uxes, might overcome it
completely. In line with these results, Heinze (2004)
reported a positive feedback attributed to a decrease
in ballasting of POC l uxes which counteracts the
small excess uptake of CO
2
in response to a decrease
in CaCO
3
production. Taking into account climate
change does not modify the picture. By the year
3000, the combined effect of ballasting and reduced
calcii cation yields a negative feedback to atmos-
pheric CO
2
of 50 ppmv compared to 125 ppmv for
the calcii cation feedback only (Hofmann and
Schellnhuber 2009). This study projects a strong
decrease in meridional overturning circulation in
response to climate change, leading to a decrease
in ventilation of intermediate water masses. A
decrease in penetration depth of POC due to the
ballast effect and remineralization of POC at shal-
lower depths will increase the oxygen demand.
Physical and biogeochemical processes combine to
draw O
2
levels down and promote an extension of
oxygen minimum zones. Oxygen minimum zones
are sites of intense denitrii cation, a suboxic meta-
bolic pathway yielding N
2
O, a potent greenhouse
gas. An increase of the ocean source of N
2
O would
correspond to a positive feedback on the earth's
radiative balance. This example illustrates the
potential for cascading effects of ocean acidii ca-
tion running across multiple biogeochemical proc-
esses and cycles.
12.3
The marine nitrogen cycle
Ocean acidii cation affects the marine nitrogen cycle
in a myriad of ways. On the one hand, this is a con-
sequence of many biologically mediated transfor-
mations of nitrogen-involving pH-dependent redox
reactions ( Fig. 12.3 ; Gruber 2008 ). On the other
hand, many of these transformations are mediated
by autotrophic organisms that require CO
2
for their
growth, so that these organisms may become stimu-
lated by the higher availability of dissolved CO
2
resulting from the uptake of anthropogenic CO
2
from the atmosphere (e.g. Rost
et al.
2008 ). Given
the intricate and tight connection of the marine
nitrogen cycle with those of carbon, phosphorus,
oxygen, and many other important biogeochemical
elements, any alteration of the marine nitrogen
cycle will invariably impact upon the cycles of these
other elements, possibly leading to feedbacks to the
earth system. While we have just begun to quantita-
tively understand the impact of ocean acidii cation
on certain isolated processes of the marine nitrogen
cycle (e.g. the recent review by Hutchins
et al.
2009 ),
such as N
2
i xation or nitrii cation, our knowledge
of how these changes interact with each other and
affect the other biogeochemical cycles is very poor.
These interactions and their potential effects are
addressed below, but the discussion and conclu-
sions remain somewhat speculative.
12.3.1 Nitrogen i xation
Nitrogen i xation, the conversion of biologically una-
vailable N
2
into organic forms of nitrogen, plays a
central role in the marine nitrogen cycle, as it resup-
plies a substantial fraction of the nitrogen that is lost
from the biologically available i xed nitrogen pool by
denitrii cation. This process is undertaken by photo-
autotrophic organisms, of which the cyanobacterium
Trichodesmium
is the best known and studied (Capone
et al.
1997). With the advent of molecular and genetic
tools (Jenkins and Zehr 2008), the number of species
known to be able to i x N
2
is rapidly increasing (e.g.
Zehr
et al.
2001 ; Montoya
et al.
2004 ), and the geo-
graphical areas where they have been reported to
exist is expanding (Moisander
et al.
2010 ).
Acidii cation experiments with
Trichodesmium
cultures have so far yielded a consistent positive
