- The modern world has presented many threats to the health and stability of ecosystems worldwide. One of the most biodiverse ecosystems, coral reefs, faces particularly strong pressures, and is already declining rapidly in complexity and area. Although the stressors that affect reefs are diverse, ranging from nutrient pollution to overfishing, invasive species to climate change, the impact of many of these stressors is ultimately mediated through interactions between the coral animal and its microbial associates, or microbiome. Some such interactions are readily apparent and have been studied for decades. For instance, coral bleaching, which is caused in part by increases in water temperature due to climate change, has devastated large swaths of reefs in recent years. The visual ‘bleaching’ that characterizes this phenomenon is the result of a breakdown in the symbiosis of the coral with photosynthetic algae of the family Symbiodiniaceae that normally live within its tissue. These algae provide the coral with essential energy, nutrients, and other services, but under temperature stress, they are expelled from the transparent tissue, leaving the white underlying coral skeleton visible and the animal without its food source.
However, other interactions between the coral and its microbiome are less well-defined. Many coral diseases, for instance, may be caused by the opportunistic overgrowth of fungi and bacteria in nutrient-rich water or under conditions of general stress. Even less clear is how bacteria may act as mutualists in the coral ‘holobiont’. Other cnidarians have been shown to require developmental stimulation from particular bacterial species, and non-Symbiodiniaceae microbes have also been hypothesized to act as nutritional symbionts or as defense against other, pathogenic microbes.
Scleractinian corals are diverse; having been evolving for more than 450 million years and including over 1,500 species. Because of this, they are likely to have many different modes of interaction with their microbiomes. To begin to better understand the similarities and differences among the microbiomes of corals, I conducted during my PhD the Global Coral Microbiome Project (GCMP), which sampled thousands of coral colonies from dozens of phylogenetically diverse species. Through the course of my work, I identified the similarities and differences between the microbiomes of these many species, showed the importance of considering shared evolutionary history in the analysis of such datasets, and developed new, rigorous methods of microbiome analysis that separate the effects of distinct evolutionary and ecological processes.