Seasonal variation in dinitrogen fixation and oxygen fluxes associated with two dominant zooxanthellate soft corals from the northern Red Sea

Abstract

Dinitrogen (N2) fixation by specialized prokaryotes (diazotrophs) represents an important source of bioavailable nitrogen (N) in the ocean. In coral reefs, several substrates and organisms are associated with diazotrophs, but potential N2 fixation activity by zooxanthellate soft corals has not yet been investigated. Such soft corals may contribute importantly to the input of new N into the reef ecosystem, as they can cover substantial benthic areas in today's coral reefs. Therefore, this study investigated N2 fixation of 2 dominant zooxanthellate soft coral groups (Sarcophyton sp. and Xeniidae) in a northern Red Sea fringing reef during all 4 seasons of 1 yr. This was supplemented by respirometry incubations and in situ monitoring of key environmental parameters. Findings revealed detectable N2 fixation for both soft corals during all seasons. Annual N2 fixation by Sarcophyton sp. was 3 times higher than that by Xeniidae, but both soft corals exhibited similar seasonal patterns. N2 fixation significantly increased during summer, when water temperature and light intensity were highest and inorganic nutrient availability was lowest. Coral respiration also peaked during summer and was positively correlated to N2 fixation, while photosynthesis revealed maximum rates during the nutrient-enriched spring season. Given the importance of N for reproduction and growth, N2 fixation may be a key component of soft coral nutrition during summer, when inorganic nutrient availability in the water column is lowest and likely not sufficient to sustain the high metabolic demand of soft corals.