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can also be gleaned from Falkowski &Raven (1997)
who provided data showing significant differences
in Rubisco efficiencies between different algal
groups. For this to have occurred during evolution
there must have been selection for Rubisco effi-
ciency, which means that the incorporation of CO
2
into the dark photosynthetic reactions was determi-
nate for species survival. This is an important quali-
fier to algal evolution that has often been missed
in previous discussions. It means that in the past,
CO
2
availability played a significant r
ˆ
le in algal
evolution; CO
2
concentration in the atmosphere
and oceans was a potential factor in the determi-
nation of species survivability in the algae. This is
very different from the r
ˆ
le played by CO
2
in the
ecology of the algae today, where algal growth is
far more affected by the availability of N, P and,
in a secondary way, Fe. The factors that affect
algal ecology today need not be the determining
factors in the evolution of algae in the past,
however (Strother 2008).
The acritarchs evolved as a group under
conditions of high CO
2
(aq) without the need for
evolving carbon concentration mechanism (CCM)s
(Strother 2008). Both coccolithophorids and diatoms
(groups that evolved later under conditions of lower
ambient pCO
2
) possess CCMs that clearly facilitate
their uptake of carbon, however. This is a second
example of CO
2
concentration in the environment
affecting the outcome of algal evolution through
natural selection. CCMs are used by algae to convert
HCO
3
2
into CO
2
at the active site of Rubisco, the
enzyme that fixes carbon. Photosynthesizers that
possess CCMs can utilize the bicarbonate ion
(HCO
3
2
) as an inorganic carbon source reducing
their dependence on the diffusion of CO
2
(aq) as
the sole source of carbon.
The phytoplankton of the mid-Palaeozoic
oceans, as represented by the organic-walled encyst-
ing forms, were susceptible to pCO
2
as a selective
factor in their evolution as there is a good corre-
lation with declining pCO
2
through the terrestriali-
zation (Pragian-Gzhelian) interval. Moreover, the
acritarchs lag the pCO
2
decline by about 10 Ma
(+5 Ma bin width). Today, atmospheric CO
2
mixes quite rapidly with the upper oceans; equili-
brium is reached in 3-6 years (Brovkin et al.
2002). The transfer of carbon into marine sediments
takes place at a much longer time scale of 10
5
yr
(Holmen 2000). It is this slower transfer that would
have driven pCO
2
down and, because atmosphere-
ocean mixing is 4-5 orders of magnitude faster
than the driving function, this would have subjected
the phytoplankton of the Devonian seas to a period
of declining CO
2
(aq) availability over many gener-
ations. This is sufficient time for Darwinian evol-
ution to take place at the population to species to
genus level. The quantitative effect of declining
genus and species-level taxa in the acritarch curve
is consistent with an evolutionary response to
declining dissolved CO
2
in the oceans.
The effects of declining dissolved CO
2
in the
oceans are more widespread than simply CO
2
avail-
ability because CO
2
(aq) is one of the chemical
species that determines ocean pH. It is therefore
likely that the pH of the oceans increased substan-
tially as carbon accumulated on land in terrestrial
ecosystems. The effects of changing pH on phyto-
plankton physiology and evolution are not well
known, but it is certainly likely to have changed
both the nitrogen speciation and the solubility of
potentially toxic cations (Royal Society 2005).
One key element in the proposal of Strother
(2008) and reiterated here is that the cause of the
acritarch decline was not catastrophic - it was gra-
dual, but progressively persistent. Both the species
and genus-level taxon richness curves (Figs 1 & 2)
show this trend quite clearly. The acritarchs show
high rates of extinction during both the later Silurian
and the later Devonian. The decline does not appear
to be restricted to just the Frasnian-Famennian
interval. The notion of a catastrophic acritarch
decline at the end of the Devonian is most probably
a residual effect caused by plotting data in Period/
System-level bins as seen in Tappan (1980) and
Strother (1996). The acritarchs, as a proxy for the
Palaeozoic phytoplankton, do not support the
notion of Frasnian/Famennian extinctions caused
by bolides, rapid sea-level fluctuations or other cat-
astrophic events as reviewed in McGhee (1996).
Conclusions
Two general themes emerge from the examination
of the links between changing pCO
2
and the evol-
ution of both marine and terrestrial photosynthetic
organisms during the terrestrialization interval of
the lower Palaeozoic. The first is a correspondence
between a step-wise increase in terrestrial biomass
accumulation as predicted by discrete phases in
plant evolution and the decline in pCO
2
as predicted
in the GEOCARB III model. The second is a lag
response between the decline of the acritarchs and
the same pCO
2
curve. These observations appear to
link the decline of the acritarchs to the rise of the ter-
restrial biota through the pCO
2
curve. The timing of
this linkage is entirely consistent with models of ter-
restrial carbon accumulation, based on the fossil
record that predicts pCO
2
changes in the atmos-
phere. The response of the acritarchs to declining
pCO
2
appears to be more speculative, but the evi-
dence of a correlation between pCO
2
and acritarch
diversity as seen in Figure 4 is quite compelling.
Although we have not discussed alternate proposals
for the acritarch decline in detail, some of these have
been recently reviewed by Strother (2008).
