Release of hydrogen peroxide and antioxidants by the coral Stylophora pistillata to its external milieu

Hydrogen peroxide (H2O2), a common reactive oxygen species, plays multiple roles in coral health and dis- ease. Elevated H2O2 production by the symbiotic algae dur- ing stress may result in symbiosis breakdown and bleaching of the coral. We have recently reported that various Red Sea corals release H2O2 and antioxidants to their external mi- lieu, and can influence the H 2O2 dynamics in the reef. Here, we present a laboratory characterization of H2O2 and an- tioxidant activity release kinetics by intact, non-stressed Sty- lophora pistillata. Experimenting with bleached and non- bleached corals and different stirring speeds, we explored the sources and modes of H2O2 and antioxidant release. Since H2O2 is produced and degraded simultaneously, we devel- oped a methodology for resolving the actual H2O2 concen- trations released by the corals. H2O2 and antioxidant activity steadily increased in the wa- ter surrounding the coral over short periods of 1-2 h. Over longer periods of 5-7 h, the antioxidant activity kept increas- ing with time, while H2O2 concentrations were stabilized at 1 µM by 1-3 h, and then gradually declined. Solving for H2O2 release, corals were found to release H2O2 at increas- ing rates over 2-4 h, and then to slow down and stop by 5-7 h. Stirring was shown to induce the release of H2O2, possibly since the flow reduces the thickness of the diffusive bound- ary layer of the coral, and thus increases H2O2 mass flux. Antioxidant activity was released at similar rates by bleached and non-bleached corals, suggesting that the antioxidants did not originate from the symbiotic algae. H2O2, however, was not released from bleached corals, implying that the symbi- otic algae are the source of the released H2O2. The observed flow-induced H 2O2 release may aid corals in removing some of the internal H2O2 produced by their symbiotic algae, and