Effects of ocean acidification on pelagic organisms and ecosystems - Ocean Acidification - page 115

Moreover, species shifts may change the abundance
of biogeochemically important functional classes
such as calcii ers or groups which produce large
amounts of climate-active gases including dimethyl-
sulphide (DMS; see Chapter 11). In experiments
conducted in the equatorial Pacii c, Tortell et al.
(2002) observed a shift from diatoms to Phaeocystis
pouchettii under low-CO 2 conditions. This taxo-
nomic shift is biogeochemically signii cant since
Phaeocystis is a prolii c producer of DMS (Stefels
and Vanboekel 1993). DMS levels appear to have
been high during the Last Glacial Maximum
( Legrand et al. 1991), and it has been hypothesized
that these high DMS values may have resulted from
a CO 2 -dependent enhancement of Phaeocystis
growth during this time (Tortell et al. 2002 ).
Conversely, if high CO 2 favours the growth of large
diatoms relative to coccolithophores or Phaeocystis ,
oceanic DMS concentrations could be expected to
decrease in the future. In contrast to these expecta-
tions, recent mesocosm experiments have demon-
strated increased DMS production under high-CO 2
conditions (Chapter 11). Other bottle experiments
have demonstrated CO 2 -dependent shifts in dis-
solved and particulate DMSP, the precursor mole-
cule to DMS. Given the complexity of the oceanic
DMS cycle (Stefels et al. 2007 ), CO 2 -dependent
effects on the production of this gas could result
from either direct physiological effects on phyto-
plankton or indirect ecological effects involving
bacteria, viruses, and/or grazers. Fully understand-
ing the CO 2 sensitivity of the DMS cycle will thus
require a much deeper ecosystem-level understand-
ing than is currently available.
The focus of this section has been on phytoplank-
ton-dependent processes since these are best studied
in the context of ocean acidii cation. However,
changes in phytoplankton community structure
have ecological implications for higher trophic lev-
els, which can in turn inl uence biogeochemical
feedbacks. In particular, CO 2 -dependent shifts from
small to large phytoplankton species would inl u-
ence the composition of zooplankton communities,
and thereby affect the degree of coupling between
producers and consumers (see Section 6.2.2). This
would affect the partitioning of carbon between its
various dissolved and particulate phases in the
upper ocean, and thus affect the marine carbon

cycle. Understanding the nature and quantitative
signii cance of such multitrophic-level effects should
be considered as a priority for future studies.

6.6 Critical information gaps

Our present knowledge of the pH/CO 2 sensitivities
of marine organisms is almost entirely based on
short-term perturbation experiments, neglecting
the possibility of evolutionary adaptation. With
generation times of about 1 day, unicellular algae
and bacteria will go through tens of thousands of
generations as p CO 2 increases to projected maxi-
mum levels, which may be sufi cient for adaptive
processes to become relevant when environmental
change occurs over decadal timescales or longer.
Also, little is currently known regarding effects
from multiple and interacting stressors, such as sea-
surface warming, enhanced stratii cation, and
changes in nutrient availability and speciation.
Moreover, there is a complete lack of information
on the transfer of responses from the organism to
the community and ecosystem level and the replace-
ment of sensitive species by those tolerant to ocean
acidii cation.
Given the lack of fundamental information on the
biological impacts of ocean acidii cation on organ-
isms and communities, experimental manipulations
will continue to play an important role in informing
our understanding of this topic. Laboratory studies
have the advantage of isolating individual variables
under well-controlled conditions, thus facilitating
the interpretation of cause and effect. However,
such studies may be limited in the extent to which
they can be extrapolated to natural plankton assem-
blages in the oceans. One of the main problems is
the inherent biological variability observed among
individual species, or even within different strains
of the same species. Phylogenetic analysis has
demonstrated very high microdiversity and unique
ecologically distinct phytoplankton clones (Iglesias-
Rodríguez et al. 2006 ; Rynearson et al. 2006 ).
Genomics, transcriptomics, proteomics, and the
expression of specii c marker genes for crucial func-
tions are promising methods for addressing this
issue. Even in the case where individual isolates
of a given species show strong and consistent res-
ponses to ocean acidii cation (e.g. Trichodesmium 's