Effects of ocean acidification on the diversity and activity of heterotrophic marine microorganisms - Ocean Acidification - page 91

available were obtained in the context of CO 2 dis-
posal at pH and p CO 2 levels well outside the range
of values expected in the next decades (Takeuchi
et al. 1997 ; Labare et al. 2010 ). A Vibrio sp. isolated
from the deep sea and grown at a pH of 5.2
responded with a change in morphology and tem-
porary growth inhibition. At a pH of 6 the growth
recovered. As pH is usually around 8 in natural
marine systems (see Chapter 1), this suggests a
strong capacity for pH homeostasis or high tolerance
to low pH i . If the direct effects differ between spe-
cies, this could change microbial diversity, and thus
associated ecosystem functions. Direct effects on
viruses seem unlikely as marine phage isolates
remain quite stable and infective over a range of pH
much wider than that found in marine systems
( Børsheim 1993 ).
Indirect effects of ocean acidii cation are likely to
occur in complex food webs. For example, a change
in p CO 2 could affect phytoplankton community
composition and metabolism, e.g. by inl uencing
primary production (see Chapter 6), leading to
changes in the quantity and quality of DOM
released, which in turn would inl uence bacterial
growth. Indirect effects could also occur in zooxan-
thellate corals. For example, if rates of primary pro-
duction are inl uenced by changes in p CO 2 , this
could also inl uence metabolism and mucus pro-
duction and thus the associated microorganisms.
The studies discussed in the following sections of
this chapter were not designed to separate direct
from indirect effects; they rather report net effects.

occurrence of bacterial groups often found in
diseased corals such as Bacteroidetes and Fusobacteria .
Larsen et al. (2008) investigated, in a mesocosm
study, the effect of elevated p CO 2 on the viral com-
munity composition using l ow cytometry, and
showed that the 'high-l uorescence viruses' were
stimulated and that no other viral subgroups were
altered. Viral groups identii ed by l ow cytometry
and nucleic acid staining belong to different 'spe-
cies' or types of viruses. Using molecular markers,
Larsen et al. (2008) could also show that the abun-
dance of EhV, a virus infecting Emiliania huxleyi ,
and of an unidentii ed virus decreased at elevated
p CO 2 (1050 μatm). This indicates changes in the
viral community composition. Suffrian et al. ( 2008 )
did not i nd any effect of elevated p CO 2 (700 and
1150 versus 350 μatm) on the composition of micro-
zooplankton (mainly dinol agellates and ciliates) in
a mesocosm study. Overall, there is no consistent
effect of ocean acidii cation on microbial biodiver-
sity and community composition.

5.3.4 Effect of ocean acidii cation on
prokaryotic abundance, production,
and enzyme activity

No or only a small effect of elevated p CO 2 levels
was found on bacterial abundance in mesocosm
studies (Rochelle-Newall et al. 2004 ; Grossart et al.
2006 ; Allgaier et al. 2008 ; Paulino et al. 2008 ; Yamada
et al. 2008 ). Grossart et al. (2006) found that the total
prokaryotic production and the cell-specii c total
and attached production were enhanced under ele-
vated p CO 2 (750 μatm). In contrast, Allgaier et al.
(2008) did not detect a signii cant difference in bac-
terial production at p CO 2 levels of 350, 700, and
1050 μatm. However, they found that regression
lines between bacterial production and the C:N
ratio of suspended matter varied depending on
p CO 2 . This suggests an indirect effect via modii ca-
tion of the quality of the organic matter rather than
a direct effect on bacterial production. Overall, to
date no study has shown a clear negative effect of
elevated p CO 2 on prokaryotic production at pH
levels relevant in the context of ocean acidii cation.
Thus, the few data collected so far suggest a neutral
or positive effect on total and cell-specii c prokaryo-
tic production.

5.3.3 Effect of ocean acidii cation on microbial
diversity and community composition

Only a few studies have been performed on the
effects of ocean acidii cation on microbial diversity
( Table 5.1 ). Allgaier et al. (2008) found, in a meso-
cosm study and using genetic i ngerprints (DGGE),
that p CO 2 induced changes in the composition of
the free-living community but not in the attached
community. Vega Thurber et al. ( 2009 ) used a
metagenomic analysis to study the effect of strong
decreases in pH NBS from 8.1 to 6.7 on bacteria
associated with the zooxanthellate coral Porites com-
pressa . Corals stressed by a change in pH showed strong
shifts in bacterial community composition and the