Disturbance of microbial communities living on corals due to environmental stress factors contributes to coral mortality, describes a paper published in Nature Communications this week. The study shows that overfishing and nutrient pollution interact with elevated temperatures, resulting in larger populations of pathogenic bacteria living on corals and an increase in coral disease.

Worldwide, coral reefs are declining as a result of ocean warming, nutrient pollution, sedimentation and overfishing. Both corals and their associated microbes are known to be sensitive to these stress factors, but how exactly the interactions between these factors influence coral decline is not well understood.

In order to disentangle the effects of different environmental stress factors on microbial communities and coral health, Rebecca Vega Thurber and colleagues conducted a three-year-study in which they independently manipulated fish presence and nutrient pollution (nitrogen and phosphorus levels) in experimental plots on a reef in the Florida Keys, United States, located between the Atlantic Ocean and Gulf of Mexico. They found that each of these stress factors destabilized the microbial communities and increased coral mortality and that these effects were amplified when temperatures rose. Furthermore, the addition of nutrients increased algal cover and killed corals that were bitten by normally-benign parrotfish by facilitating predation by opportunistic bacteria.

These findings provide insight into which factors govern reef resilience and show that changes in species’ interactions within coral reefs coupled with climate change can fundamentally reorganize ecological communities down to the microbial level.