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In-Depth Information
uptake mechanisms and CO
2
sensitivity in phyto-
plankton, as well as information on the direct effects
of pH on various species. Early work established
upper and lower pH limits for a variety of phyto-
plankton species, as summarized by Hinga (2002),
but these studies examined pH gradients much
larger than those expected to occur naturally over
the next century, and seawater carbonate chemistry
was often poorly constrained. More recent work has
focused on taxonomic differences in carbon uptake
in marine phytoplankton (e.g. Rost
et al.
2003 ). As
discussed above (Section 6.3.1), phytoplankton pos-
sess physiological/biochemical carbon concentrat-
ing mechanisms that impose energy and mineral
resource demands on cells. Taxonomic differences in
the sensitivity to ocean acidii cation could thus
result from differences in carbon uptake among spe-
cies (Raven and Johnston 1991). In this respect, per-
haps the most fundamental distinction among
phytoplankton groups is the difference in the sub-
strate specii city factor of RubisCO. The poor cata-
lytic properties of this enzyme (low CO
2
afi nity,
competitive O
2
inhibition, and slow catalytic rate)
determine an intrinsic sensitivity to increased CO
2
concentrations in the absence of any physiological
carbon-concentrating mechanism (CCM).
Meta-analysis of existing biochemical data dem-
onstrates that there is a statistically signii cant dif-
ference among phytoplankton groups in the CO
2
afi nity of RubisCO, and an apparent trend of
improved CO
2
afi nity over the course of phyto-
plankton evolution as atmospheric CO
2
has
decreased and O
2
increased over the past 500 Myr
(Tortell 2000). Differences in RubisCO specii city
appear to be related to the intrinsic capacity of vari-
ous phytoplankton to concentrate inorganic carbon
intracellularly. For example, diatoms which have
high-afi nity RubisCO tend to have much lower cel-
lular carbon concentration factors than other taxo-
nomic groups such as cyanobacteria whose RubisCO
has a low selectivity for CO
2
binding over O
2
( Tortell
2000). The implications of such differences for the
CO
2
sensitivity of diatoms relative to other phyto-
plankton groups remain unclear. It has been sug-
gested, however, that the rapid growth of diatoms
may be attributable to the relatively low energetic
costs of a CCM in cells with more efi cient isoforms
of RubisCO (Tortell 2000). Conversely, cyanobacte-
ria, which have low-afi nity RubisCO (half satura-
tion constant for CO
2
> 200 μmol l
-1
), appear to
invest heavily in cellular carbon accumulation.
This may explain the large apparent CO
2
stimula-
tion of productivity and nitrogen i xation observed
in
Trichodesmium
(see Table 6.3 ) as energetic
resources become reallocated from carbon uptake
to other biochemical and physiological processes
( Levitan
et al.
2010). These hypotheses should be
examined experimentally in order to gain a more
mechanistic understanding of how ocean acidii ca-
tion could inl uence phytoplankton species compo-
sition in the oceans. Such a process-based
understanding is critical for predicting ecosystem-
level responses.
Our discussion thus far has focused on the bot-
tom-up effects structuring phytoplankton assem-
blage composition. Yet grazers can play an
important role in this process through, for exam-
ple, size-selective grazing. An excellent example
of this occurs in iron-limited open-ocean waters
where iron availability controls the biomass of
large phytoplankton, while rapid grazing by micro-
zooplankton maintains a low standing crop of
small phytoplankton (Landry
et al.
1997 ). Under
various ocean acidii cation scenarios, changes in
relative grazing pressure across both the classic
food chain and the microbial loop could thus act to
inl uence the relative abundance of certain phyto-
plankton species (see Chapter 5). Moreover,
changes in zooplankton assimilation efi ciencies
and excretion rates could strongly inl uence nutri-
ent remineralization. As discussed below, very lit-
tle information is currently available to examine
these potential ecological feedbacks.
6.3.7 Zooplankton growth, reproduction,
and grazing
Relative to work on phytoplankton, much less
information is available about the effects of ocean
acidii cation on zooplankton. This rel ects, in part,
the greater experimental complexity inherent in
working with these organisms. Nonetheless, there
has been a signii cant effort in recent years to begin
documenting the potential CO
2
/pH sensitivity of
various zooplankton. Much of this work has focused
on groups including pteropods and foraminifera
