Microbial processes driving coral reef organic carbon flow

Coral reefs are one of the most productive ecosystems on the planet, with primary production rates compared to that of rain forests. Benthic organisms release 10-50% of their gross organic production as mucus that stimulates heterotrophic microbial metabolism in the water column. As a result, coral reef microbes grow up to 50 times faster than open ocean communities. Anthropogenic disturbances cause once coral-dominated reefs to become dominated by fleshy organisms, with several outcomes for trophic relationships. Here we review microbial processes implicated in organic carbon flux in coral reefs displaying species phase shifts. The first section presents microbial players and interactions within the coral holobiont that contribute to reef carbon flow. In the second section, we identify four ecosystem-level microbial features that directly respond to benthic species phase shifts: community composition, biomass, metabolism and viral predation. The third section discusses the significance of microbial consumption of benthic organic matter to reef trophic relationships. In the fourth section, we propose that the 'microbial phase shifts' discussed here are conducive to lower resilience, facilitating the transition to new degradation states in coral reefs.