Energy Budgets for the Caribbean Reef Coral Porites porites (Pallas)

The aim of this study was to extend the contemporary energy budget methodology for algal-cnidarian symbioses and apply it to the tropical reef coral Porites porites growing in two different environments. It was hoped that the measured energy inputs to P. porites would be equalled by the measured energy requiring processes, without a surplus or deficit of energy being determined by subtraction, and that P. porites would show a differential derivation, allocation and utilisation of energy in the two environments. Porites porites was studied at 10 m depth at both the West Fore Reef (WFR) and Columbus Park (CP) regions of Discovery Bay, Jamaica. The transmission of surface photosynthetically active radiation (PAR) to lu m depth, the temperature, sedimentation rates and nutrient levels were recorded at both sites. The CP site received less PAR, had a greater sedimentation rate and slightly enhanced nitrate concentrations compared to the WFR site. The energy budgets were calculated from measurements of photosynthesis, zooxanthellae and host respiration, zooxanthellae and host growth, colony reproduction and mucus loss. These budgets show that P. porites at both sites can be autotrophic in terms of energy on an 'ideal' day. No evidence was obtained for habitual zooplankton capture at either site. Photoadaptation at the CP site ensured that the input of photosynthetically fixed energy (PFE) was greater than at the WFR site. On an 'ideal' day at both sites 21% of the PFE was used for zooxanthellae respiration and less than 0.5% for colony reproduction whilst 26.3% (WFR site) or 6.6% (CP site) was used for host respiration and 7.1% (WFR site) or 4.3% (CP site) for zooxanthellae and host tissue growth. The loss of soluble or particulate macromolecular mucus did not account for significant amounts of energy. More than 45% of the PFE remained unaccounted for at both sites on an ideal day. A similar autotrophic status and an apparent surplus of PFE can be maintained on nearly every day of the year at both sites. In order to account for the apparent surplus energy from P. porites the colony respiration rate was compared during the day and night, the formation and energy content of mucus tunics were measured and lipid reserves determined. The colony respiration rate (representing the colony energy expenditure) increased after exposure to light and this was a function of the host tissue alone. The effect was greater at the CP site. Calcification and nutrient uptake mechanisms, and in particular the active transport of ions involved in these processes may account for the increased energy expenditure. The between-site differences in the magnitude of the elevated daytime respiration rate may represent a hitherto unrecognised cost of inhabiting the CP site. Lipid accounted for more than 18% of the dry tissue content of P. porites at both sites and supported the concept of a lipid based energy economy in corals. The daily surplus of energy can be stored as lipid in P. porites, although lipid was not metabolised during 5 days of darkness. It is suggested that lipid reserves may be important at the CP site where they could be lost as mucus during sediment shedding. Circumstantial evidence suggests that P. porites can utilise heterotrophic sources of nutrition (50 mum in size). The hypothesis that energy was periodically lost as mucus tunics was shown to be incorrect. Although tunics have a low energy content they may have an important effect in decreasing the autotrophic input by decreasing colony photosynthesis. It is suggested that the loss of low molecular weight compounds may be an alternative route for surplus energy instead of mucus. The initial objectives of the study were only partly fulfilled. Although the energy budgets showed some between site differences they were still balanced by subtraction. However, studies of lipid reserves, the daytime respiration rate and mucus tunics show that these can partially account tor the apparent surplus of energy. Further studies of the significance and mechanisms of an elevated daytime respiration rate and the role of heterotrophic nutrition in scieractinian corals will probably make a major contribution to our further understanding of the nutritional relationships in algal-cnidarian symbioses