Evidence of enhanced microbial activity in the interstitial space of branched corals: possible implications for coral metabolism

Abstract

Water samples from the interstitial space of 4 Indo-Pacific coral species (Acropora sp., Echinopora horrida, Psammocora digita and Pavona clavus) and a Mediterranean coral (Cladocora cespitosa) were analysed for NO 3 - +NO 2 - , NH 4 + , molybdate reactive phosphorus, bacterial and flagellate biomass and dissolved organic matter (DOM) and compared with ambient water concentrations. Higher values of NO 3 - +NO 2 - , bacterial and flagellate biomass were observed within the interstitial space of the corals. The lower DOM pool in the interstitium in combination with the high bacterial biomass suggests high bacterial activity and efficient substrate utilization, necessary to compensate for nanoflagellate predation. Since corals may be able to feed on bacteria, the high microbial biomass (bacterial and flagellate) may be utilized either directly as an additional heterotrophic food source, or indirectly in that microbes may act as attractants for microbe-feeding zooplankters, which in turn serve as food for the corals. The combined effect of reduced flow velocities between the coral branches and its associated fauna are probably the main factors in creating a specific environment more or less independent of the nutritive stage of the surrounding water.