Chemistry Reference
In-Depth Information
Figure 5.
Pulse chase experiments for uptake of
14
C in the imperforate benthic foraminiferan,
A.
hemprichii
. Graphics and experimental details as in Figure 4. Uptake for calcification and photosynthesis
start without delay and both rates are similar suggesting 1:1 stoichiometry and self-enhancement of both
processes according to Reaction (1) in text. No carbon pool is detected for this species.
While the existence of the inorganic carbon pool is clearly demonstrated, its nature
and the concentration of carbon in this pool are still a mystery. The pool size in
A.
lobifera
according to ter Kuile and Erez (1988) is roughly 1
g C/mg foraminiferan.
Given the diameter, weight and volume relations of ter Kuile and Erez (1984), the
concentration of carbon in the pool is ~190 mM. This is a minimum estimate because this
calculation assumes that the volume is all fluid. Even if we assume a very effective
carbon concentration mechanism (CCM) in foraminifera similar to that known for algae
(e.g., Raven and Johnston 1991), it is difficult to imagine concentration by a factor of 100
relative to seawater. If the diameter of the pseudopodial network of
A. lobifera
were
increased roughly by a factor of four then the internal concentration will be around that of
seawater. Alternatively the pool may not be in solution but in some solid unstable phase
(see similar discussion on Ca below).
Calcium pool.
An internal calcium pool, serving for calcification, was first observed
by Anderson and Faber (1984) in the planktonic foraminiferan
G. Sacculifer.
They
observed a gap (of 40%) between the actual CaCO
3
deposited and the incorporation of the
radiotracer
45
Ca into the skeleton in short incubation of 24 hrs. This gap diminished after 48
and 72 hrs of incubation with the radiotracer. In a similar study conducted in our laboratory
on
A. lobifera,
we also observed a large gap between the incorporation of
45
Ca into the
skeletons of and the actual calcification (measured as CaCO
3
weight added) during the
experiment (Table 1). The data showed that for small fast-growing specimens there was a
good agreement between the two methods, while for the older and larger specimens the
radiotracer underestimated the actual calcification by a factor up to 9. Obviously these data
cannot be explained by isotopic discrimination against the radiotracer because the mass
difference between
40
Ca and
45
Ca is only five mass units and such fractionations are not
known in nature. The most likely explanation for these observations is that suggested by
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