PhDMost global calcification is carried out by organisms which are also
photosynthetic. In this study, the coccolithophore Emiliania huxleyi (Lohmann)
Hay and Mohler and two species of hermatypic coral were used to:
examine the effect of dissolved inorganic carbon (DIC) and light on
photosynthesis and calcification; and
determine the extent to which these two processes interact.
A novel method of producing coccolith-less (non-calcifying) cells from
calcifying cells of the same strain of E huxley! was developed thus allowing
photosynthesis and calcification to be studied separately. The kinetics of
photosynthesis in both types of cell, and of calcification in coccolith-bearing cells,
were shown to be biphasic with respect to DIC concentration. The hiatus in all
three cases was located at 1 mM DIC. This unusual pattern was shown to be
the product of two carbon uptake mechanisms: an anion exchanger working at all
DIC concentrations and an external carbonic anhydrase active only at low DIC
concentrations.
In contrast to the commonly-held view, this study demonstrated that
calcification did not promote photosynthesis in E. huxleyi. Nevertheless, there
was clearly strong biological control of calcification in this alga since DIC uptake
was mediated by an anion transporter and a dehydroxylating enzyme.
This work also showed that in E huxleyi, DIC addition enhanced
photosynthesis at both limiting and saturating photon flux densities and that
bicarbonate affected photochemical processes directly. Photosystem II activity
was stimulated and non-photochemical quenching was reduced, possibly
protecting the photosynthetic apparatus from damage by light.
In the two corals; Porites porites and Acropora sp., strong biological
control of calcium carbonate precipitation was also evident. Again, calcification
did not stimulate photosynthesis. Calcification rates of Acropora sp. were
monitored in the dark and although these were lower than in the light, they still
increased dramatically with bicarbonate addition. This showed that high
concentrations of the bicarbonate ion can compensate for the lack of light.
Hence, it seems that in hermatypic corals, light-dependence of calcification may
be facultative and not obligate. It is therefore clear from the results of this study
that calcification and photosynthesis are not as closely coupled as has been
previously thought.
In neither E. huxleyi, nor in the hermatypic corals, were photosynthetic and
calcification rates saturated at the present ambient DIC concentration of
seawater