Coral-Algal Phase Shifts Impact Microbially-Mediated Organic Carbon Processing in Coral Reefs of Mo’orea, French Polynesia

Coral reefs are highly productive ecosystems where bacterioplankton play a critical role in recycling dissolved compounds, enabling corals to thrive in oligotrophic waters. Reefs worldwide exhibit trends of declining coral cover with concomitant increases in macroalgae. This project seeks to understand how these trends may alter microbial carbon and nutrient recycling supporting the resilience of reefs to climate change. Decadal data from the Moorea Coral Reef Long Term Ecological Research program (MCR LTER) show that reefs have suffered significant structural damages from past cyclonic events, ongoing anthropogenic impacts and predator outbreaks (Taramea or Crown-of-thorn starfish Acanthaster planci), resulting in phase shifts from coral to macroalgal dominance. Previous research suggests that phase shifts reduce microbial diversity, increase heterotrophic bacteria and disrupt bacterioplankton recycling, leading to inefficient productivity and further coral decline. Mesocosm experiments simulated coral-dominated, algae-dominated, and post-disturbance reefs (A. planci outbreak) to assess the effects of benthic cover on compositional dynamics of microbial communities and dissolved organic matter. Organic matter (fDOM and DOC) quantification was done via fluorescence spectroscopy and catalytic oxidation, while microbial community analysis was done using flow cytometry with amplicon next generation DNA sequencing. The results of this project will enhance our understanding of the interactions between bacterioplankton communities and benthic species, with implications for carbon fixation and remineralization processes in the oligotrophic ecosystems of the South Pacific. Additionally, findings from this study will provide a valuable baseline for assessing the ecological status of locally managed lagoons, supporting parallel research efforts in remote regions of Tahiti.