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In-Depth Information
continuous culture experiment (Rose
et al.
2009 ),
whereas the other experiment did not detect
any signii cant effect in a mesocosm experiment
( Suffrian
et al.
2008 ). Rose
et al.
( 2009 ) suggested that
the grazing rates were inl uenced by a change in
phytoplankton community composition rather than
by physiological effects. Ocean acidii cation could
also change the food quality and thus affect grazing.
For example, it is known that the formation of i la-
mentous cells, i lamentous cell colonies, or aggrega-
tion of cells prevents or reduces grazing (Jürgens
and Güde 1994). Since Takeuchi
et al.
( 1997 ) have
reported on morphological changes of microorgan-
isms due to increased
p
CO
2
, it is possible that the
changes in grazing rates are due to changes in the
appearance of morphologies resistant to grazing.
are key parameters for assessing the efi ciency of
energy transfer to higher trophic levels via the
microbial loop.
In on-board experiments under increased
p
CO
2
,
the production of TEP increased as a function of
CO
2
uptake (Engel 2002). In a mesocosm experi-
ment which induced a phytoplankton bloom domi-
nated by
E.
huxleyi
, the TEP production per cell was
highest at 710 μatm, lowest at 190 μatm, and inter-
mediate at 410 μatm, whereas the total TEP concen-
tration and production were not affected by the
highest and lowest
p
CO
2
levels (Engel
et al.
2004 ).
Egge
et al.
( 2009 ) did not i nd a signii cant effect of
p
CO
2
(350, 700, and 1050 μatm) on TEP concentra-
tion in a subsequent mesocosm study. It was con-
cluded that the increase in TEP concentration (when
observed) was due to the increased production of
TEP precursors. In a batch culture study, an increase
in TEP concentration and production was observed
in the absence of cells (Mari 2008). This supports the
idea that the observed increase is due to a modii ca-
tion of the TEP structure linked to an alteration of
aggregation processes, rather than to increased pro-
duction. Overall, a negative effect of ocean acidii -
cation on TEP formation has never been detected.
An increased TEP aggregation often results in
higher abundances and production of bacteria by
attracting these cells to the microbial hot spots
( Simon
et al.
2002). Therefore, increased TEP aggre-
gation could result in higher bacterial abundance
and production.
5.3.6 Effect of ocean acidii cation on microbial
nutrient cycling and organic matter dynamics
Nitrii cation is the biological oxidation of ammonia
into nitrite followed by the oxidation of nitrite to
nitrate. Huesemann
et al.
( 2002 ) investigated the
effects of CO
2
-induced pH changes on marine nitri-
i cation in the context of deep-sea CO
2
disposal.
They found that the rate of nitrii cation drops dras-
tically with decreasing pH. Relative to the rates at
pH 8 (presumably on the NBS scale), nitrii cation
decreased by ~50% at pH 7 and by more than 90%
at pH 6.5, while it was completely inhibited at pH 6.
Despite the fact that this study was not aimed at
projected ocean acidii cation conditions of the
future ocean, it indicates a potentially large sensi-
tivity of nitrii cation to changes in
p
CO
2
.
No signii cant effects of ocean acidii cation were
found on the concentrations of chromophoric
DOM (cDOM) and DOC (Engel
et al.
2004 ;
Rochelle-Newall
et al.
2004 ; Grossart
et al.
2006 ;
Schulz
et al.
2008). However, it must be noted that
the fraction of readily bioavailable DOC is small.
Thus, effects of
p
CO
2
on the bioavailability of
DOM might not be detectable with the available
or applied methods. Data on the effect of ocean
acidii cation on the processing of organic matter
by bacteria and the heterotrophic microbial food
web are not available. For example, there has been
no study of the effect of ocean acidii cation on
bacterial respiration and growth efi ciency which
5.4 Implications
5.4.1 Potential implications for microbial food
webs and biogeochemical cycles
Since most effects of elevated
p
CO
2
on micro-
organisms are poorly known and evidence is some-
times contradictory, the implications are highly
speculative and must be viewed with caution.
Among the effects of ocean acidii cation are changes
in the composition of viral and microbial communi-
ties at elevated
p
CO
2
. Changes in diversity, i.e.
changes in the relative abundance of species, can also
mean changes in the specii c functions of single spe-
cies. Since changes in microbial diversity can be linked
to changes in ecosystem functions (Bell
et al.
2005 ),
