Geoscience Reference
In-Depth Information
the ecosystem functions and biogeochemical roles
of corals.
( Inagaki
et al.
2006), cold-eddies, and upwelling sys-
tems which have lower pH and higher
p
CO
2
levels
than ambient water and are thus suitable for study-
ing the effects of ocean acidii cation. Nevertheless,
the experimental set-up is challenging due to fac-
tors such as advection or migration of microorgan-
isms. The data interpretation is also challenging,
given the temporal and spatial variability in
pH/
p
CO
2
and hence the uncertainty on the dose-
response relationship.
In order to study the effects of ocean acidii cation,
seawater pH must be lowered while maintaining
total alkalinity constant (Gattuso and Lavigne 2009).
Bubbling seawater with gas mixtures is often used
to reach the desired level of
p
CO
2
. However, this
approach can be a problem when addressing the
question of the effects of ocean acidii cation on the
microbial ecosystem, since bubbling can promote
aggregation of organic matter via surface aggrega-
tion and by increasing turbulence (Kepkay 1994),
which is known to affect microbial processes
(Kepkay and Johnson 1989). Thus, bubbling should
be avoided whenever possible or kept as short as
possible. If bubbling is used, it must be carried out
in the same way in all treatments, including the
control. Addition of acid avoids bubbling but has
other disadvantages; for example it does not mimic
the natural process as it decreases total alkalinity
(see Gattuso and Lavigne 2009 ).
5.3 Ocean acidii cation: approaches
and evidence
Table 5.1 summarizes the studies and reports on the
response of microorganisms to ocean acidii cation
and provides details on parameters such as experi-
mental set-ups, type of manipulations, and
p
CO
2
and pH levels used. The focus is mainly on
in situ
observations and on studies which tested parame-
ters of the carbonate chemistry within the range of
values projected for this century as a result of ocean
acidii cation driven by the uptake of CO
2
through
the air-sea interface. Unless mentioned otherwise,
pH values are reported on the total scale. If
p
CO
2
levels were not reported in publications, they were
calculated from pH and other ancillary data with
the R software package 'seacarb' as described by
Nisumaa
et al.
(2010). Data collected at
p
CO
2
and
pH levels that are not environmentally relevant are
nevertheless reported when no other data are avail-
able on a particular process. Four types of responses
are considered: (1) positive, when ocean acidii ca-
tion stimulates the parameter or process; (2) nega-
tive, when ocean acidii cation decreases the
parameter or process; (3) change, when ocean acidi-
i cation affects the community composition
(whether it is a positive or negative effect cannot be
determined); and (4) neutral, when ocean acidii ca-
tion has no signii cant effect.
5.3.2 Direct versus indirect effects of ocean
acidii cation
Heterotrophic marine microorganisms can be
affected either directly by the chemical changes
generated by ocean acidii cation or indirectly
through effects on other levels of the community.
The direct effects of elevated
p
CO
2
on the physiology
and metabolism of microorganisms can be studied
using isolates. Microbial species have growth
optima at different pH values and many physiolog-
ical functions such as hydrolytic enzyme activities
are dependent on pH (Yamada and Suzumura
2010). Therefore, it is critical that the internal pH
(pH
i
) is controlled, which is achieved by a pH
homeostatic system (Booth 1985). Studies at pH
and
p
CO
2
levels relevant to ocean acidii cation
have not been performed yet. The only results
5.3.1 Approaches to study the effect
of ocean acidii cation on microbes
The effect of ocean acidii cation on heterotrophic
microorganisms has been investigated using per-
turbation experiments with isolates of natural com-
munities over short periods of time (days to weeks).
So far, only a few bacterial and viral isolates have
been tested, and to the best of our knowledge, no
study has been performed on heterotrophic
protistan isolates. Open-water CO
2
fertilization
experiments still seem unrealistic, but it is poten-
tially rewarding to take advantage of systems natu-
rally enriched with CO
2
such as shallow-water CO
2
vents (Hall-Spencer
et al.
2008 ),
deep-sea
vents
