Geoscience Reference
In-Depth Information
past bearing the i ngerprint of ocean acidii cation,
global warming, and expanding anoxia, led to dev-
astating changes in the abundance, diversity, and
evolution of calcifying organisms. The bottom line
from geological history is that the rate of the carbon
perturbation is the key to the response of calcifying
organisms (see Chapter 2 and above) as well as its
amplii cation by global warming and declining lev-
els of dissolved oxygen.
Despite the fact that some perturbation experi-
ments reported no effect or a positive effect of ocean
acidii cation on the rate of calcii cation of a few organ-
isms (reviewed in Chapters 6 and 7), meta-analyses
reveal an overall signii cant negative effect (Kroeker
e
t al.
2010). The mean effect is negative and signii cant
on corals, negative with a similar magnitude but non-
signii cant on calcifying algae, coccolithophores, and
molluscs, positive and signii cant on crustaceans, and
positive but non-signii cant on echinoderms. Whether
or not calcii cation decreases in response to elevated
CO
2
and lower Ω, the deposition of CaCO
3
is thermo-
dynamically less favourable under such conditions.
Some organisms may have the capacity to up-regu-
late their metabolism and calcii cation to compensate
for lower Ω. However, this would have energetic costs
that would divert energy from other essential proc-
esses, and thus would not be sustainable in the long
term. Full or partial compensation may be possible in
certain organisms if the additional energy demand
required to calcify under elevated CO
2
can be sup-
plied as food, nutrients, and/or light (for those organ-
isms dependent on photosynthesis).
Overall, calcii ers precipitating the less soluble arago-
nite and low-magnesian calcite are negatively affected,
whereas those precipitating the more soluble high-mag-
nesian calcite are not signii cantly affected. This may
indicate that biological control of calcii cation is more
important than the solubility of the mineral precipitated
(Kroeker
et al.
2010) but it seems that the analysis is
l awed (Anderson, pers. comm.). Interestingly, Kroeker
e
t al
. (2010) found that calcifying organisms are more
susceptible overall to ocean acidii cation than non-calci-
fying organisms across other processes.
Overall, the level of evidence that ocean acidii ca-
tion will adversely affect calcii cation is medium
and the coni dence level high. The remaining chal-
lenges are to estimate the energetic and physiologi-
cal trade-offs in all life stages, and to improve our
knowledge about the molecular and physiological
mechanisms involved in calcii cation in order to bet-
ter understand how the direction and magnitude of
the response to ocean acidii cation are controlled.
15.2.2.2 Ocean acidii cation will stimulate primary
production
In the oceans, photosynthesis is carried out prima-
rily by microscopic phytoplankton, and to a lesser
extent by macroalgae and seagrasses. Photosynthetic
organisms must acquire inorganic carbon and a
suite of major and trace nutrients from surface sea-
water. Dissolved CO
2
, rather than the much more
abundant bicarbonate ion, is the substrate used in
the 'carbon i xation' step of photosynthesis. The
enzyme responsible for carbon i xation, ribulose-1,
5-bisphosphate carboxylase oxygenase (RubisCO),
has a low substrate afi nity, achieving half-satura-
tion of carbon i xation at concentrations well above
those present in seawater. CO
2
must therefore be
concentrated at the site of i xation against a concen-
tration gradient and therefore with an energetic
cost. As CO
2
diffuses readily through biological
membranes, a large portion of the CO
2
transported
into photosynthetic organisms is lost again via leak-
age. It is conceivable, therefore, that an increase in
seawater CO
2
concentration reduces leakage and
lowers the cost of concentrating CO
2
, thereby stim-
ulating primary production.
Stimulating effects of elevated CO
2
on photosyn-
thesis and carbon i xation have indeed been
observed in a variety of phytoplankton taxa, includ-
ing diatoms, coccolithophores, cyanobacteria, and
dinol agellates. Modest increases of 10-30% in pho-
tosynthetic carbon i xation in response to elevated
CO
2
were also observed in bioassay studies with
oceanic plankton assemblages as well as in meso-
cosm experiments with coastal plankton communi-
ties. The extent to which phytoplankton respond to
increased CO
2
depends to a large extent on the
physiological mechanisms of inorganic carbon
uptake and intracellular assimilation. Marine pri-
mary producers encompass phylogenetically very
diverse groups, from prokaryotes to angiosperms,
differing widely in their photosynthetic apparatus
and carbon-concentrating mechanisms (CCMs).
Species with effective CCMs are likely to be less
sensitive to increased CO
2
levels than those lacking
