PHOTOSYNTHESIS AND CALCIFICATION IN THE COCCOLITHOPHORE EMILIANIA HUXLEYI AND TWO HERMATYPIC CORALS, PORITES PORITES AND ACROPORA SP.
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Most 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.
AuthorsHerfort, Lydie M. C. C.
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