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efi cient CCMs. These differences are likely to alter
competitive relationships among phytoplankton
groups and result in shifts in plankton species com-
position as the oceanic CO 2 concentration continues
to rise. The level of coni dence of this stimulating
effect is medium and its magnitude must be better
constrained, especially at community level under
in situ conditions.
a small number of species tested so far, it is too early
to assess the global signii cance of a stimulating
effect of rising CO 2 on oceanic nitrogen i xation.
15.2.2.4 Some species or strains are tolerant
It is known with a very high level of coni dence that
some species or strains are tolerant to ocean acidii ca-
tion. Tolerance to ocean acidii cation in the range pro-
jected for the next century varies greatly among phyla
and even species and is largely determined by the
mechanisms and capacities of acid-base regulation.
In metazoans, this is linked to the metabolic rate and,
in turn, to the transport capacities for oxygen and
CO 2 . Several groups of animals (e.g. mammals, i shes,
and some molluscs) with a high capacity for oxygen
and CO 2 transport and exchange appear to be toler-
ant of lower pH, at least over short periods such as
those of intense activity. In contrast, most marine
invertebrate taxa have less developed gas exchange
and acid-base regulatory capacities, and appear to
have a lower tolerance to ocean acidii cation.
Tolerance levels are also likely to be lower in early life
stages, e.g. during egg and larval development.
In autotrophic organisms CO 2 /pH sensitivity is
linked to the carbon acquisition mechanisms sup-
plying inorganic carbon to photosynthesis and, in
the case of calcareous autotrophs, also to calcii ca-
tion. Differences in CO 2 /pH sensitivity exist, for
example, between diatoms, coccolithophores, and
cyanobacteria, partly related to the efi ciency of their
CO 2 concentrating mechanisms. Different tolerances
are also observed between species of the same group,
or even strains of the same species, as seen for exam-
ple in coccolithophores. To what extent these differ-
ences will affect overall competitive i tness and lead
to the replacement of CO 2 /pH-sensitive species by
tolerant ones is currently unknown.
15.2.2.3
Ocean acidii cation will stimulate nitrogen
i xation
The i xation of atmospheric nitrogen gas (N 2 ) into
ammonium (NH 4
+ ), a form readily available to the
biota, is carried out by a small group of diazotrophic
cyanobacteria. This process represents a major input
of 'new' nitrogen to oligotrophic marine ecosystems.
Because large parts of the global ocean are nitrogen-
limited, nitrogen i xation plays a key role in deter-
mining primary production of the world's oceans. It
is an energetically costly process which also requires
the synthesis of a complex iron-rich enzyme.
Additionally, cyanobacteria have to invest heavily
in concentrating CO 2 at the site of carboxylation due
to the low afi nity of their RubisCO. Increasing sur-
face-ocean CO 2 concentrations may therefore be
benei cial for diazotrophic cyanobacteria. If the
energy saved in the carbon acquisition process is
reallocated to other processes, elevated CO 2 could
stimulate nitrogen i xation (see Chapter 6 ). Enhanced
nitrogen i xation in response to elevated CO 2 was
indeed observed in the abundant i lamentous diazo-
troph Trichodesmium sp. as well as the unicellular
species Crocosphaera watsonii under iron-replete con-
ditions . No stimulation was observed for Nodularia
spumigena , a heterocystous diazotrophic cyanobac-
terium common in the Baltic Sea.
The level of coni dence that ocean acidii cation
will stimulate nitrogen i xation is medium because,
considering the potential phylogenetic and meta-
bolic diversity of marine nitrogen i xers, it is cur-
rently difi cult to determine the representativeness
for the global ocean of laboratory results on two of
the three species investigated so far. Moreover,
because of the high demand for iron and energy, the
ecological niche of diazotrophs is restricted to iron-
rich, nitrogen-poor waters in areas, or during peri-
ods of, high solar irradiance. With little information
on the synergistic effects of CO 2 , light, and iron, and
15.2.2.5 Some taxonomic groups will be able to adapt
Changes in environmental conditions generate two
types of responses (Bradshaw and Holzapfel 2006):
phenotypic plasticity, the ability of individuals to
modify their behaviour, morphology, or physiology,
and genetic (evolutionary) changes. Phenotypic
plasticity is well known, but genetic changes have
also been observed in populations of animals as
diverse as birds, squirrels, and mosquitoes in
response to the environmental changes that have
 
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