Estimating the daily contribution of carbon from zooxanthellae to coral animal respiration 1

An equation is derived which rigorously defines the photosynthesis : respiration ratio (P:R) for any alga : invertebrate symbiotic association and permits the computation of the fractional contribution of translocated algal carbon to the daily respiratory carbon requirement of the host animal. The equation is applied to two species of symbiotic reef corals, using 0, flux data from 24-h continuous measurements in situ. Given certain assumptions, the algae in the shallow-water Hawaiian reef corals Pocillopora danzicornis and Fungia scutaria can supply of the order of 63 and 69% of the daily respiratory carbon demand of their respective animal hosts. Symbiotic zooxanthellae in reef corals fix carbon dioxide by photosynthesis and translocate a substantial fraction of their reduced organic carbon to the host coral animal (Muscatine and Cernichiari 1969; Lewis and Smith 1971; Muscatine and Porter 1977). Translocated carbon is thought to be of energetic significance to the host since, in those corals studied, the daily carbon input from feeding on zooplankton does not meet the coral animal carbon requirements for respiration (Johannes et al. 1970; Johannes and Tepley 1974; Porter 1974). In addition, some symbiotic corals can live and grow under l This work was supported in part by grants from the National Science Foundation (PCM-33780 to L.M. and GA-27941 to L.R.M.) and from Edwin Pauley. conditions in the laboratory where particulate organic carbon is excluded (Yonge 1930; Johannes 1974), implying that such corals can satisfy all of their energy requirements from other sources including translocation. However, the quantitative significance of translocated carbon has not yet been determined for any algaeinvertebrate association. As a point of departure, we asked the question: how much of the daily carbon requirement for respiration of the animal host is supplied by the algae through translocation? Some investigators have tried to answer this question by using the photosynthesis : respiration (P:R) ratios of symbiotic corals. Typically, daily P:R ratios > 1.00 have been interpreted to mean that the coral is self-supporting with respect to carbon. There are several