Climate Change Effects on Marine Phytoplankton - Marine Ecology in a Changing World - page 72

Phytoplankton Responses to Hydroclimatic Changes
Ocean acidifi cation
Calcifi cation: Compared to the vast studies about warming effects on
marine phytoplankton growth and production rates, data are relatively
scarce on the responses to increased CO 2 and low pH (Berge et al. 2010).
This is in part related to the fact that the chemical reactions involved
in ocean acidifi cation are complex, and therefore diffi cult to reproduce
under laboratory conditions (Iglesias-Rodriguez et al. 2008). The general
mechanisms can be explained as follows: the decrease in carbonate ions
concentration (CO 3 2- ) linked to ocean acidifi cation causes higher solubility of
calcium carbonate (CaCO 3 ) (Caldeira and Wickett 2003), and thus aragonite,
the metastable form of CaCO 3 , becomes less available for organisms that
need it to build their skeletons, such as coccolithophores, corals, foraminifers
and mollusks (Orr et al. 2005, Hoegh-Guldberg et al. 2007). Coccolithophores
have received special attention because their calcite precipitation plays a
signifi cant role in alkalinity fl ux to the deep ocean (i.e., inorganic carbon
pump). For instance, malformation of CaCO 3 skeletons and reduced cell
size in response to high CO 2 levels have been reported in monocultures of
two marine dominant calcifi ed phytoplankton species: Emiliania huxleyi
and Gephyrocapsa oceanica (Riebesell et al. 2000). In agreement to this,
empirical studies using mesocosms have evidenced a decrease in E. huxleyi
calcifi cation and enhanced loss of organic carbon from the water column
when exposed to high CO 2 (Delille et al. 2005). However, other laboratory
experiences using the same coccolithophore species under elevated CO 2
showed an increase in calcifi cation and net primary production which agrees
with fi eld evidence based on geological records in the deep ocean over the
past two centuries of anthropogenic CO 2 rise (Iglesias-Rodriguez et al. 2008).
Likewise, a recent theoretical model provided by Irie et al. (2010) on the
growth schedule of coccolithophores forecasted how natural selection alters
phenotypes as ocean acidifi cation increase. Assuming that the formation of
exoskeleton is a defensive strategy to reduce the instantaneous mortality
rates, the model predicts that natural selection favors constructing more
heavily calcifi ed exoskeleton—and slows down the growth strategy—in
response to increased acidifi cation-driven costs. This raises fundamental
questions regarding the plasticity of phytoplankton species responses to
Global Ocean threats, and calls for an evolutionary perspective to assess
climate changes effects on phytoplankton.
The studies described above show a variety of responses to acidifi cation
by coccolithophore species. It is worth noting that the experiment design
can induce variability in the phytoplankton reactions to the dissociation of
carbonate species and pH. Manipulation of the pH levels by the addition