Coupling between dissolved organic matter, algae, and microbes on coral reef platforms

This NSF funded project seeks to understand the microbiology of coral reefs in Moorea, with specific emphasis on understanding how increasing algal cover and declining coral cover may alter the microbial communities and their role in overall reef ecosystem function and coral health. The proposed research will investigate the coupling between algae and the utilization of dissolved organic matter (DOM) by marine heterotrophic microbes on coral reefs. Previous metagenomic studies of the microbial communities associated with near-pristine and degraded coral reefs demonstrated a shift from a microbial food web similar to the open ocean (Prochlorococcus spp. and SAR11-like bacteria) to a community dominated by "super-heterotrophs", most closely related to known pathogens like E. coli, Staphylococcus spp., Streptococcus spp., Enterobacter spp. and Vibrio spp. This shift is associated with a decline in coral cover and an increase in coral disease prevalence.


Our previous research has also shown that dissolved organic carbon (DOC) concentrations are lower on coral reef platforms compared to measurements of offshore waters (60-80 μM). On degraded reefs, we have observed DOC measurements as low as 30 - 40 μM, a value similar to concentrations observed in the deep Pacific Ocean. The observation of low DOC measurements on degraded reefs is decoupled from the high abundance of macroalgae, which one might expect would raise levels of DOC through the release of photosynthate into the water column. To explain this apparent paradox we propose that reef degradation, and the associated phase-shifts from coral to algal dominance, leads to elevated levels of algal exudates in the water column, which allows the microbial community to utilize the standing stock of semi-labile DOC. Our research seeks to understand the microbiology of coral reef degradation using metagenomics and biogeochemical assessment of DOM release associated with phase shifts from coral-dominated to algal-dominated reefs.

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