The seascape of VOC production and cycling in a tropical coral reef

Volatile organic compounds (VOCs) have been proposed to indicate coral reef health, but little is known about their cycling processes in coral reefs and the roles of reef components. We studied the distribution and cycling of ocean-leaving VOCs (dimethylsulfide (DMS), carbonyl sulfide (COS), CS2, dimethyl disulfide, isoprene, CH3I, CH2ClI, and bromomethanes CH2Br2 and CHBr3) across a coral reef in Mo’orea (French Polynesia) that has a fast and unidirectional water flow. Repeated sampling of transects between the open ocean and the reef outflow channel, across the shelf, the reef crest, and the back-reef lagoon, showed that reef waters were depleted in dissolved organic carbon, chlorophyll, phytoplankton, and bacteria, and enriched in nutrients. All studied VOCs increased in concentration after oceanic waters crossed the reef crest, with bromomethanes showing the largest increase. Incubation experiments of back-reef waters around midday suggested that: (a) photochemical reactions were a major source for COS and major sink for DMS; (b) microbial plankton were the main daytime source for DMS, isoprene, and CH3I, and an important sink for COS; (c) seaweeds were the main source of CH2ClI, CHBr3, and CH2Br2; and (d) carbonate sediments were a major source for CS2 and CH2ClI, an important source for DMS and isoprene, and the main sink for COS. The dominant coral Pocillopora sp. was a source only for DMS and COS. Decomposing seaweed rafts were an important but unquantified source for all VOCs except CH3I. In April 2018, the reef was a net producer of VOCs compared to the ocean, with the anticipation that production would increase if corals were lost and replaced by seaweeds. Using VOCs and other chemical tracers of reef waters, we estimated that one third of the water entering the reef is recirculated water from the same reef, with implications for ecosystem self-recruitment and genetic maintenance.