Geoscience Reference
In-Depth Information
abundant form of inorganic carbon in seawater, then transforming this
bicarbonate into CO
2
using the enzyme carbonic anhydrase. The CO
2
concentrating mechanisms (CCMs) generally have an energy cost, and
an increase in the concentration of CO
2
in seawater could be an energy
gain for cells, and therefore beneficial for growth. Positive effects of
acidification have been observed for several species of diatoms (e.g.
[WU 10]) and dinoflagellates (e.g. [ROS 06]). The results are not
clear-cut for coccolithophores for which the response of
photosynthesis is either non-existent, or slightly positive (e.g.
[GAO 09, MUL 10]). However, research carried out on the diazotroph
(nitrogen gas fixing) algae
Trichodesmium
sp.
shows an increase in
the growth of these algae at elevated pCO
2
levels (e.g. [HUT 07]).
How can such diverse responses to ocean acidification be explained?
Two, non-mutually exclusive hypotheses have been proposed. On the
one hand, the efficiency of CCMs differs from one species to another,
and on the other hand, the affinity of the RUBISCO for the CO
2
is also
variable between species.
Finally, we must highlight the fact that an increase in
phytoplanktonic primary production would not have only positive
effects. For example, the frequency and intensity of harmful algal
blooms has increased dramatically in recent years in response to
eutrophication [HAL 93]. The effects of an enrichment in CO
2
on the
dynamics and toxicity of these species remain relatively unknown
[FU 10].
The fact that certain species seem to benefit from elevated CO
2
whereas others do not or could change the composition of communities,
with consequences on primary production, the nutritional value of
organic matter for higher trophic levels and the export of organic
matter to the deep ocean (biological pump). Unfortunately, few
studies have been carried out on planktonic communities. Some have
highlighted, at low pH values, an increase in the relative abundance of
diatoms (particularly large ones) at the expense of other species such
as haptophytes and nanoflagellates [TOR 08]. Other works do not
show an increase in the relative abundance of diatoms [FEN 09,
HAR 07]. Perturbation experiments carried out in mesocosms have
