Release of dissolved organic carbon and nitrogen by the zooxanthellate coral Galaxea fascicularis

Overview & Synopsis

Corals are known to release large amounts of particulate and dissolved organic carbon (POC and DOG) and nitrogen (PON and DON). Production of POC and PON in the form of mucus has been relatively well studied, but very few data are available on the release of DOC and DON by corals. In order to investigate several aspects of carbon and nutrient cycling in corals, release of DOC and DON by fed and unfed colonies of the zooxanthellate coral Galaxea fascicularis (Linnaeus 1767) was measured in the laboratory under controlled conditions. Colonies were either fed with artemia or supplied with nitrogen- and phosphorus-enriched seawater. We measured DOC and DON fluxes from corals using the high temperature catalytic oxidation method and DOC release as C-14-photosynthate using a radioisotope technique. Corals released significant amounts of dissolved organic matter (DOM). Two large release peaks were observed in mid-morning and mid-afternoon. DOC concentrations increased from ca 100 mu M (background level) to 300-1700 mu M, depending on the size of the colony and the trophic status. DON concentrations also increased from 15 to 120 mu M Release rates varied from 2-3 mu mol DOC and 0.5-0.6 mu mol DON (mg protein)(-1) d(-1) for the unfed colonies to 13-25 mu mol DOC and 1-3 mu mol DON (mg protein)(-1) d(-1) for the artemia-fed colonies to 4-6 mu mol DOC and 0.2-1.3 mu mol DON (mg protein)(-1) d(-1) for the nutrient-enriched colonies. Fed corals therefore released more DOC than unfed colonies, but tended to conserve organic nitrogen, suggesting that heterotrophic nutrition may serve corals as a source of new nutrients. Calculations of the carbon balance for the unfed colonies showed that DOC release represents ca 14% of the net daily photosynthetically fixed carbon. Following each peak in release, concentrations of DOM fell back to routine background levels. The role of free-living, epibiotic and/or intracellular bacteria in the uptake of DOM was therefore investigated. Colonies were labelled with C-14-bicarbonate and the subsequent release of C-14-DOM was followed in filtered seawater treated with and without prokaryotic inhibitors. No subsequent uptake of C-14-DOM was observed in the presence of inhibitors, suggesting that bacteria may play an important role in DOM uptake. This process may lead to tight nutrient recycling within coral colonies and may enable corals to thrive in oligotrophic waters.