Calcification and photosynthesis in montipora sp. (cnidaria) and corallina officinalis (rhodophyceae)

Abstract

Over half of the world's calcification is carried out by algae or by organisms which harbour them, such as coccol ithoph ores, foraminiferans, coralline seaweeds and reef-building corals. Calcification acts as a sink for inorganic carbon and although rather little is known about the precise mechanisms of biological CaC03 formation, the process as a whole is thought to be under threat from atmosphericC02 rise. This study examined the response of a reef-building coral, Montipora digitata and a coralline seaweed, Corallina officinalis to the main factors which influence calcification, namely light, dissolved inorganic carbon (DIC), pH, nitrate and calcium. In contrast to the commonly held view, this study demonstrates that both photosynthesis and calcification were carbon limited in seawater. Since the degree of stimulation by DIC in the light was different for each process, and dark calcification also increased with added DIC, it is clear that photosynthesis and calcification are only loosely coupled. Simultaneous pH measurements were made on the surface of the epithelium and at the site of calcification in the coral Galaxea fascicularis using pH microelectrodes, and demonstrated for the first time that pH at the site of calcification is not a simple response to seawater pH. 2 In this study, nitrate inhibition of calcification was shown to be more powerful in the dark than in the light, indicating that daylength may be a more significant factor in coral biology than previously realised. The currently-accepted hypothesis that biological calcification rates are a simple function of seawater CaC03saturation state was tested experimentally. Results from both Corallina officinalis and Montipora digitata reveal that: a) calcification is far more responsive to changes in inorganic carbon than to calcium concentrations; and b) when [C03 2-] is kept constant, increases in [HC03-1 cause dramatic increases in calcification rates, even at reduced pH. All of these data suggest that calcification in M. digitata and C. officinalis is a strongly biologically controlled process, influenced principally by the seawater bicarbonate concentration and pH, but strongly mediated by light and combined nitrogen.