Coral Bleaching Induced Mortality Transforms Local and Global Carbon Cycles: An Unrecognized Feedback Loop That May Accelerate Reef Decline

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Abstract. Increases in atmospheric carbon have led to widespread, frequent, and severe coral bleaching, resulting in global coral reef decline. Here, we show that bleaching corals severely impact the local carbon cycle by releasing significant amounts of dissolved organic carbon (DOC), which further stresses the local reef community and may trigger coral mortality. During a severe bleaching event in Mo’orea, French Polynesia, we measured DOC concentrations 37 % greater than Total Organic Carbon (DOC and Particulate Carbon combined), compared to non-bleaching conditions. In addition, this DOC was highest immediately adjacent to the reef, indicating that the corals were the source of this carbon. Further, when exposed to bleaching-derived DOC-rich exudate (~2 mM DOC), otherwise healthy corals experienced bleaching, tissue loss, and mortality within 48 hours. While this is an extreme amount of DOC to be found on a reef in situ, it identifies a potential mechanistic impacts of coral-derived DOC on healthy corals. By extending our findings to regional scales, we estimate that large scale bleaching events can significantly alter the marine carbon cycle. For example, a single bleaching event on the Great Barrier Reef could have released an estimated 150 Gmol C as DOC alone. Our research identifies a previously unrecognized mechanism of coral mortality during bleaching events, in which biogeochemical shifts across the reefscape that may result in a positive feedback loop that accelerates coral reef loss.

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