Geoscience Reference
In-Depth Information
2003), may not provide the best simulation of the
ecological dynamics observed in many subpolar
regimes, which are characterized by strong non-
steady-state behaviour.
to increased CO
2
levels than those with less efi cient
CCMs (Burkhardt
et al.
2001 ; Rost
et al.
2003 ). Because
the sensitivity to carbon enrichment differs widely
among taxa, rising CO
2
levels will alter competitive
relationships and result in shifts of plankton species
composition (see below; Rost
et al.
2008 ).
Stimulating effects of elevated CO
2
on photosyn-
thesis and carbon i xation have been observed in a
variety of phytoplankton taxa (Table 6.1), including
diatoms, coccolithophores, cyanobacteria, and dino-
l agellates. No effect of elevated
p
CO
2
on organic
matter production was observed in the two coccol-
ithophore species
Coccolithus pelagicus
and
Calci-
discus leptoporus
( Langer
et al.
2006 ). The only study
showing decreasing organic matter production at
elevated
p
CO
2
in
Emiliania huxleyi
was conducted in
chemostats under nitrate limitation (Sciandra
et al.
2003). Modest increases (~10%) in growth and pro-
ductivity in response to elevated
pCO
2
were also
observed in natural phytoplankton assemblages
( Hein and Sand-Jensen 1997 ; Tortell
et al.
2002 ,
2008). In a mesocosm study the net drawdown of
inorganic carbon by primary production was 27%
and 39% higher at 700 μatm and 1050 μatm com-
pared with 350 μatm (Riebesell
et al.
2007 ; Bellerby
et al.
2008), whereas increases in net community
production of only 8% and 15% were estimated
based on day-time
14
C primary production meas-
urements from the same mesocosm experiment
( Egge
et al.
2009). In this study no CO
2
/pH effect
was observed on community respiration rate.
In summary, investigations on both individual
species and natural assemblages of phytoplankton
reveal CO
2
fertilization of photosynthetic carbon
i xation and rates of production of organic matter.
While few individual phytoplankton species appear
to be insensitive to ocean acidii cation, studies on
natural phytoplankton assemblages consistently
show increased carbon i xation in response to ele-
vated
p
CO
2
. The magnitude of this stimulation is,
however, relatively small compared with the stimu-
lation of plankton growth by iron enrichment in
iron-limited waters.
6.3 Direct effects of ocean acidii cation
on planktonic organisms
6.3.1
Photosynthesis and carbon i xation
In the oceans, photosynthesis, the formation of
organic matter using energy from sunlight, is car-
ried out chiel y by microscopic phytoplankton. For
this purpose, these single-cell organisms must
acquire, from surface seawater, inorganic carbon
and a suite of major and trace nutrients, including
nitrogen, phosphorus, and trace metals such as iron.
CO
2
rather than the much more abundant bicarbo-
nate ion, HCO
3
-
, is the substrate used in the 'carbon
i xation' step of photosynthesis that is catalyzed by
the enzyme ribulose-1,5-bisphosphate carboxylase
oxygenase (RubisCO). This enzyme has an intrinsi-
cally low afi nity for CO
2
, achieving half saturation
of carbon i xation at CO
2
concentrations well
above those present in seawater (Badger
et al.
1998 ).
To overcome RubisCO's low afi nity, CO
2
must
be concentrated at the site of i xation, an energy-
consuming process. Because CO
2
diffuses readily
through biological membranes and leaks out of the
cell it is expected that an increase in the CO
2
concen-
tration of surface seawater will reduce CO
2
leakage,
which in some phytoplankton groups facilitates
photosynthesis and can lead to an increase in pri-
mary production (i.e. the rate of synthesis of organic
matter per unit time and unit area of the ocean).
This theoretical expectation is supported by both
laboratory and i eld experiments (see below).
The extent to which phytoplankton may respond
to increased CO
2
(decreased pH) is likely to depend,
to a signii cant extent, on the physiological mecha-
nisms of inorganic carbon uptake and intracellular
assimilation. Primary producers in the marine realm
encompass phylogenetically very diverse groups
( Falkowski
et al.
2004), from prokaryotes to
angiosperms, differing widely in their photosyn-
thetic apparatus and carbon enrichment systems
( Giordano
et al.
2005). Species with effective carbon-
concentrating mechanisms (CCMs) are less sensitive
6.3.2
The cell division rate
No consistent response to ocean acidii cation has
been obtained with respect to phytoplankton cell
