Geoscience Reference
In-Depth Information
reported
E. huxleyi
to double its calcii cation in
response to
p
CO
2
increasing from 280 to 750 μatm.
However, the difference can be explained by a size
difference in the cells incubated in the different CO
2
treatments. Cells grown at high CO
2
had an initial
biomass two to three times greater than low CO
2
-
grown cells (possibly due to differences in pre-
cultures). Due to the difference in cell size, however,
a comparison between CO
2
treatments on a per cell
basis is meaningless, but can only be done on a per
biomass basis. In fact, when normalized to algal
biomass, the trends in calcii cation and primary
production with increasing
p
CO
2
disappear (see
Riebesell
et al.
2008a). A re-analysis of the same
strain of
E. huxleyi
(NZEH) used by Iglesias-
Rodriguez
et al.
( 2008 ) by Hoppe
et al.
(pers. comm.)
revealed no effect on growth rate and a moderate
decrease in calcii cation with increasing
p
CO
2
.
In a study by Shi
et al.
( 2009 ) both growth rate
(cell division rate) and the cellular POC and PIC
content of
E. huxleyi
(strain NZEH) was higher at
pH
T
7.8 compared with pH
T
8.1, yielding higher
rates of organic carbon production and calcii cation
at elevated
p
CO
2
. The PIC:POC ratio was slightly
lower in cultures maintained at lower pH levels. As
discussed above, increased carbon cell quota and
cell size are frequently observed in coccolithophores
at elevated
p
CO
2
. However, the results reported by
Shi
et al.
(2009) differ from all other studies on coc-
colithophores in showing an increased cell division
rate at elevated
p
CO
2
. The signii cance of this i nd-
ing is difi cult to assess, partly because that study
was based on only two
p
CO
2
levels and was
observed in only two out of the three CO
2
manipu-
lation approaches, with the opposite trend when
carbonate chemistry manipulation was done
through CO
2
bubbling. Using the same strain of
E. huxleyi
, Hoppe
et al.
(pers. comm.) found no
effect on growth rate.
In summary, although much of the work on
CO
2
/pH sensitivity focuses on coccolithophores,
evidence currently available suggests that ocean
acidii cation will cause a decline in CaCO
3
produc-
tion in most planktonic calcii ers. It is currently
unknown whether a decreased calcii cation rate will
affect the competitive i tness of calcifying organisms
relative to their non-calcifying competitors and to
what extent CO
2
-sensitive calcifying organisms will
6.3.4 Calcii cation
CaCO
3
is one of the most common building materials
used in the formation of skeletons, shells, and other
protective structures in the marine biota. Organisms
exploit the supersaturation with respect to CaCO
3
in
the surface ocean, which prevents crystallized CaCO
3
from dissolving. Calcii cation, the precipitation of
CaCO
3
, is facilitated by high pH and high carbonate
ion (CO
3
2-
) concentration (see Box 1.1 in Chapter 1). In
calcifying organisms these conditions are achieved at
the site of calcii cation through energy-consuming
ion transport processes (Mackinder
et al.
2010 ). With
ocean acidii cation causing a decrease in pH and
[CO
3
2-
], the energetic cost of calcii cation is thought to
increase. The extra energy needed to compensate for
these changes in seawater carbonate chemistry
depends on the specii c pathways employed in
CaCO
3
precipitation, the details of which are cur-
rently poorly understood but which are likely to dif-
fer between taxonomic groups.
Most planktonic calcifying organisms tested so
far show a decrease in calcii cation in response to
elevated CO
2
/reduced pH ( Table 6.4 ), such as
foraminifera, pteropods, and planktonic larvae of
echinoderms. A wide range of responses to ocean
acidii cation was obtained for coccolithophores
( Table 6.4 ). Whereas calcii cation in
E. huxleyi
,
G.
oceanica
, and
Calcidiscus quadriperforatus
decr-
eases to varying degrees with increasing
p
CO
2
,
Calcidiscus leptoporus
shows an optimum curve with
reduced calcii cation at
p
CO
2
levels below and
above present conditions and
Coccolithus pelagicus/
braarudii
appears to be insensitive to elevated
p
CO
2
.
In a comparison of different strains of
E. huxleyi
Langer
et al.
(2009) observed either no change or a
decrease in calcii cation rate with increasing
p
CO
2
.
In all studies on coccolithophores the ratio of CaCO
3
to organic matter production (PIC:POC; PIC being
particulate inorganic carbon and POC particulate
organic carbon) decreases or remains unchanged
with elevated
p
CO
2
. No interacting effects of
p
CO
2
and temperature were observed on calcite produc-
tion, coccolith morphology, or on coccosphere size
by De Bodt
et al.
( 2010 ).
Two recent studies on coccolithophores appear to
suggest a stimulating affect of ocean acidii cation
on calcii cation. Iglesias-Rodriguez
et al.
( 2008 )
