Search

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.

One way viruses can affect coral health is by infecting their symbiotic dinoflagellate partners (Symbiodiniaceae). Yet, viral dynamics in coral colonies exposed to environmental stress have not been studied at the reef scale, particularly within individual viral lineages. We sequenced the viral major capsid protein (mcp) gene of positive-sense single-stranded RNA viruses known to infect symbiotic dinoflagellates (‘dinoRNAVs’) to analyze their dynamics in the reef-building coral, Porites lobata. We repeatedly sampled 54 colonies harboring Cladocopium C15 dinoflagellates, across three environmentally distinct reef zones (fringing reef, back reef, and forereef) around the island of Moorea, French Polynesia over a three-year period, and spanning a reef-wide thermal stress event. By the end of the sampling period, 28% (5/18) of corals in the fringing reef experienced partial mortality versus 78% (14/18) in the forereef. Over 90% (50/54) of colonies had detectable dinoRNAV infections. Reef zone influenced the composition and richness of viral mcp amino acid types (‘aminotypes’), with the fringing reef containing the highest aminotype richness. The reef-wide thermal stress event significantly increased aminotype dispersion, and this pattern was strongest in the colonies that experienced partial mortality. These findings demonstrate that dinoRNAV infections respond to environmental fluctuations experienced in situ on reefs. Further, viral productivity will likely increase as ocean temperatures continue to rise, potentially impacting the foundational symbiosis underpinning coral reef ecosystems.