Abstract. Hydrogen peroxide (H₂O₂), a common reactive oxygen species, plays multiple roles in coral health and disease. Elevated H₂O₂ production by the symbiotic algae during 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 milieu, and can influence the H₂O₂ dynamics in the reef. Here, we present a laboratory characterization of H₂O₂ and antioxidant activity release kinetics by intact, non-stressed Stylophora 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 developed a methodology for resolving the actual H₂O₂ concentrations released by the corals.

H₂O₂ and antioxidant activity steadily increased in the water surrounding the coral over short periods of 1–2 h. Over longer periods of 5–7 h, the antioxidant activity kept increasing 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 increasing 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 boundary 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 symbiotic algae are the source of the released H₂O₂. The observed flow-induced H₂O₂ release may aid corals in removing some of the internal H₂O₂ produced by their symbiotic algae, and may possibly assist in preventing coral bleaching under conditions of elevated temperature and irradiance.