RELEASE OF DISSOLVED ORGANIC-CARBON AND NITROGEN BY THE ZOOXANTHELLATE CORAL GALAXEA-FASCICULARIS

Corals are known to release large amounts of particulate and dissolved organic carbon (POC and DOG) and nitrogen (PON and DON). Production o f POC and PON in the form of mucus has been relatively well studied, b ut very few data are available on the release of DOC and DON by corals . In order to investigate several aspects of carbon and nutrient cycli ng 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 f ed with artemia or supplied with nitrogen- and phosphorus-enriched sea water. We measured DOC and DON fluxes from corals using the high tempe rature catalytic oxidation method and DOC release as C-14-photosynthat e using a radioisotope technique. Corals released significant amounts of dissolved organic matter (DOM). Two large release peaks were observ ed in mid-morning and mid-afternoon. DOC concentrations increased from ca 100 mu M (background level) to 300-1700 mu M, depending on the siz e of the colony and the trophic status. DON concentrations also increa sed 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 artemi a-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 rele ased more DOC than unfed colonies, but tended to conserve organic nitr ogen, suggesting that heterotrophic nutrition may serve corals as a so urce of new nutrients. Calculations of the carbon balance for the unfe d colonies showed that DOC release represents ca 14% of the net daily photosynthetically fixed carbon. Following each peak in release, conce ntrations of DOM fell back to routine background levels. The role of f ree-living, epibiotic and/or intracellular bacteria in the uptake of D OM was therefore investigated. Colonies were labelled with C-14-bicarb onate and the subsequent release of C-14-DOM was followed in filtered seawater treated with and without prokaryotic inhibitors. No subsequen t uptake of C-14-DOM was observed in the presence of inhibitors, sugge sting that bacteria may play an important role in DOM uptake. This pro cess may lead to tight nutrient recycling within coral colonies and ma y enable corals to thrive in oligotrophic waters.