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early studies reviewed by Hinga (2002) used
extremely broad pH ranges with often poorly con-
strained carbonate chemistry, it is difi cult to inter-
pret them in the context of projected future ocean
acidii cation.
In summary, the rather uniform response in car-
bon i xation rate to elevated p CO 2 is not mirrored in
phytoplankton cell division rates. This is explained
by the l exible carbon quota in most phytoplankton
taxa and is consistent with the notion that cell
division is triggered by cell constituents other
than cellular carbon content. The observed differ-
ences in the CO 2 /pH sensitivity of cell division rate
between phytoplankton groups and species suggests
that ocean acidii cation has an impact on phyto-
plankton community composition and succession.
Czerny et al. (2009) speculate that structural and
regulatory differences between heterocystous and
non-heterocystous cyanobacteria may be a reason
for the different responses. It should be noted that
heterocystous cyanobacteria are largely coni ned to
freshwater and brackish water environments. Even
within marine environments, there appears to be a
high diversity of potentially nitrogen-i xing organ-
isms (Zehr et al. 1998) whose sensitivity to ocean
acidii cation is, at present, entirely unknown.
Experimental studies with these organisms are
hampered by the difi culty in culturing them under
controlled laboratory conditions.
While all studies to date, with the exception of
that of Czerny et al. (2009), suggest a stimulating
effect of ocean carbonation on nitrogen i xation
(Table 6.3), a note of caution should be applied when
extrapolating these i ndings to the global ocean.
Nitrogen i xers have a high demand for iron and
their ecological niche requires excess phosphate
after nitrate depletion. Whether CO 2 fertilization of
nitrogen i xation also occurs under iron- and
phosphorus-limited conditions is largely unknown.
In a comparison of iron-limited and iron-replete
Crocosphaera , only the iron-replete cultures responded
to elevated p CO 2 with increased nitrogen i xation
( Fu et al. 2008a). When exposing phosphorus-
depleted and phosphorus-replete cultures of
Trichodesmium to elevated p CO 2 Hutchins et al. ( 2007 )
found that CO 2 fertilization of nitrogen i xation still
persisted in severely phosphorus-limited cultures.
Clearly, more work needs to be done on the interact-
ing effects of p CO 2 with other nutrients and temper-
ature and on the CO 2 -sensitivity of natural diazotroph
communities before reliable predictions about the
impacts of ocean acidii cation on nitrogen i xation
can be made (see discussion below).
6.3.3 Nitrogen i xation
Recent studies with the dominant diazotrophic
cyanobacterium Trichodesmium revealed enhanced
rates of both carbon and nitrogen i xation at ele-
vated p CO 2 (Table 6.3). These laboratory results are
coni rmed by bioassay experiments with natural
populations of Trichodesmium showing similar
responses when exposed to high CO 2 ( Hutchins et
al. 2009). A stimulation of nitrogen i xation by ele-
vated CO 2 was also observed for the unicellular
cyanobacterium Crocosphaera under iron-replete
conditions (Fu et al. 2008a). The opposite trend, a
decrease in growth and nitrogen i xation rates at
elevated CO 2 , was observed in Nodularia spumigena ,
a heterocystous cyanobacterium regularly bloom-
ing in the open Baltic Sea during the summer
months (Czerny et al. 2009 ). While a mechanistic
understanding of the effects of ocean acidii cation
on diazotrophic nitrogen i xation is still lacking,
Table 6.3
Observed effects of ocean acidii cation on nitrogen i xation in planktonic organisms, comparing rates at pre-industrial/present-day p CO 2
levels with those at p CO 2 projected for the end of this century.
Species
Response
References
Trichodesmium erythraeum
Barcelos e Ramos et al . ( 2007 ), Hutchins et al . ( 2007 ), Levitan et al . ( 2007 ), Kranz et al . ( 2009 , 2010 )
Natural colonies of Trichodesmium
Preliminary data reported in Hutchins et al . ( 2009 )
Crocosphaera watsonii
«
Fu et al . 2008a
Nodularia spumigena
Czerny et al . 2009
¯
, enhanced; ¯ , slowed down; « , unaffected/inconclusive.
 
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