Barriers to Coral Recovery: Dissolved Organic Exudates of Successional Algal Communities Hinder the Recovery of Bleached Coral Reefs in French Polynesia

Marine heat waves are becoming more frequent and severe, leading to the mass mortality of reef-building coral. In 2019, a severe coral bleaching event resulted in the loss of more than 50% coral cover around the island of Moorea in French Polynesia. This bleaching event left behind expansive areas of bare coral skeleton, leading to uncertainty regarding the recovery trajectory of this reef. This study utilizes metabarcoding to characterize the successional communities that colonize the coral skeleton, beginning with a recent mortality event through the initial turf-algal overgrowth stage and finally to the late-stage macroalgal dominated community. The concentration, bioavailability and structural complexity of dissolved organic matter (DOM) excreted into the water column by each of the three successional community states was also analyzed. Recent studies have identified an altered reef microbiome as a source of disease, deoxygenation and disrupted nutrient cycling on disturbed reefs. The alteration of the microbiome is largely due to changes in their food source, the dissolved organic matter, released by coral, algae, and other primary producers that comprise a coral reef. This study detected distinct differences in the nutrient profiles, chemical composition, and concentration of DOM across the three successional community stages. The lability and nutrient content of DOM can directly impact microbial community function, leading to modifications in the natural community assemblage and overall abundance of reef microbes. On perturbed coral reefs, this often leads to key biogeochemical feedbacks that contribute to phase-shifts from calcifying reef-building corals to their competitors, fleshy macroalgae. As of 2024, very little coral recruitment has occurred and macroalgal communities dominate the forereef of Moorea. We hypothesize that a change in the quality of the DOM and hence the microbial dynamics of the reef are leading to a “ghost-reefscape” in which the persisting coral structure is inhabited by algal communities that hinder the settlement and recovery of foundational reef-building coral.