Factors affecting the cycling of dimethylsulfide and dimethylsulfoniopropionate in coral reef waters of the Great Barrier Reef

Abstract

Environmental context. Levels of atmospheric dimethylsulfide (DMS) and its oxidation products are reputed to affect the microphysics of clouds and the amount of incoming solar radiation to the ocean. Studies of DMS and its precursor compound dimethylsulfoniopropionate (DMSP) at two inshore fringing coral reefs in the Great Barrier Reef highlight pronounced seasonal, diurnal and tidal variation of these compounds, with dissolved DMS and DMSP significantly correlated with sea surface temperatures (SSTs) up to 30°C. During a coral bleaching episode at one of the reef sites, dissolved DMS concentrations decreased when SSTs exceeded 30°C, a result replicated in chamber experiments with staghorn coral. The results raise interesting questions on the role of these organosulfur substances in corals and whether DMS emissions from coral reefs could have an effect on regional climate in the Great Barrier Reef. Abstract. A study of dissolved dimethylsulfide (DMSw), dissolved and particulate dimethylsulfoniopropionate (DMSPd, DMSPp), and atmospheric dimethylsulfide (DMSa) was carried out at two inshore fringing coral reefs (Orpheus Island and Magnetic Island) in the Great Barrier Reef (GBR) to investigate the variation of these organosulfur substances with season, sea surface temperature, tides, and time of day. Highest concentrations of these organosulfur substances occurred in the summer months at both reefs, with lowest concentrations occurring during winter, suggesting a biological source of these compounds from the reef flats. At the Orpheus Island reef, where more measurements were made, DMSw and DMSPd were significantly correlated with tidal height during the flooding tide over the reef (r = 0.37, P < 0.05; r = 0.58, P < 0.01 respectively), and elevated DMSw and DMSa concentrations generally occurred in the daylight hours, possibly reflecting photosynthetic production of DMSw from the reef flats. Chamber experiments with the staghorn coral Acropora formosa confirmed that corals produce DMSw in the day. DMSw (r = 0.43, P < 0.001) and DMSPd (r = 0.59, P < 0.001) were significantly positively correlated with sea surface temperatures (SST) at the Orpheus Island reef. During severe coral bleaching at the eutrophic Magnetic Island reef in the summer, DMSw concentrations decreased at SSTs greater than 30°C, suggesting that reef production of DMSw decreases during elevated SSTs. This was later confirmed in chamber experiments with Acropora formosa, which showed that when this coral was exposed to temperatures at its bleaching threshold (31°C), decreased production of DMSw occurred. These results suggest that DMS and DMSP in coral zooxanthellae may be functioning as antioxidants, but further experiments are needed to substantiate this.