Inorganic nutrient availability affects organic matter fluxes and metabolic activity in the soft coral genus Xenia

Summary The release of organic matter (OM) by scleractinian corals represents a key physiological process that importantly contributes to coral reef ecosystem functioning, and that is affected by inorganic nutrient availability. While OM fluxes have been studied for several dominant reef taxa, no information is available for soft corals, one of the major benthic groups in tropical reef environments. Thus, this study investigates OM fluxes along with other key physiological parameters (i.e. photosynthesis, respiration and chlorophyll a tissue content) in the common soft coral genus Xenia after a 4-week exposure period to elevated ammonium (N, 20.0 µM), phosphate (P, 2.0 µM) and combined inorganic nutrient enrichment treatment (NP). Corals maintained without nutrient enrichment served as non-treated controls and revealed constant uptake rates for particulate organic carbon (POC) (-0.32 ± 0.16 mg POC m-2 coral surface area h-1), particulate nitrogen (PN) (-0.05 ± 0.02 mg PN m-2 h-1) and dissolved organic carbon (DOC) (-4.8 ± 2.1 mg DOC m-2 h-1). While DOC uptake significantly increased in the N treatment, POC flux was not affected. The P treatment significantly enhanced PN release as well as photosynthesis and respiration rates suggesting an influence by autotrophic carbon acquisition of zooxanthellae endosymbionts on OM fluxes by the coral host. Our physiological findings confirm the significant effect of inorganic nutrient availability on OM fluxes and key metabolic processes for the soft coral Xenia, and provide first clues on OM cycles initiated by soft corals in reef environments exposed to ambient and elevated inorganic nutrient concentrations.