Presentation by Matthew P. Hoch, Ph.D. and Mark A. McNab at the 2nd Belize National Research Conference, 2019.
Coral reefs are some of the most biological diverse and economically valuable ecosystems in the world. Nevertheless, reef building corals are in rapid decline globally due to multiple human induced stressors, including sea surface temperature rise due to climate change and excessive macroalgal cover due to cultural eutrophication and overfishing. Resiliency of corals to thermal stress may relate to the composition of their microbiome. However, organic exudates from reef macroalgae can drive deleterious changes in the microbiome of plankton and corals, favoring copiotrophic pathogens. A synergistic effect on microbiome structure and coral health is expected for corals stressed by high macroalgal exudate release and experiencing a thermal stress events. To test this, the microbiome and health of a cosmopolitan scleractinian coral, Porites astreoides, exposed to a short-term thermal stress event (4.9 ± 1.3 °C rise over 65 hours) in the presence or absence of organic exudates from either reef macroalgae, Dictyota sp. or Amphiroa sp. One pronounced response was the loss in relative abundance of the endosymbiotic bacterium Endozoicomonas sp. of the Gammaproteobacteria order Oceanospirillales in coral tissues, which was by 65% in thermal stress alone treatments, but by 96.5% when algal exudates was also present. However, the P. astreoides microbiome was not sensitive to organic exuades without thermal stress, as was seen for Pocillopora verrucosa microbiome. Plankton communities also had significantly different compositions and bacterial abundance with algal exudate treatment. Shifts in plankton microbiome composition due to increasing algal cover and organic exudate release rates may predispose corals to greater sensitivity to thermal stress events due to climate change.
Key words: barrier reef, corals, climate change, environmental impact