Corals at the extreme: partitioning the response of coral holobionts to marginal habitats

While coral reefs worldwide are threatened by unprecedented environmental change, some reef-building corals can already be found living under extreme conditions within marginal habitats. Learning how corals can survive high temperature fluctuations and multiple other stressors experienced in mangroves, relative to typical reefs, is a key step in understanding the adaptive capacity of reef-building corals to future environmental change. The role of the coral host, symbiotic algae, and diverse microbiota, and how these components of the holobiont interact to define the adaptive capacity of reef-building corals requires further exploration. In this thesis, the thermal tolerance limits of conspecific corals from a mangrove versus a reef habitat were tested in a 20-day heat-ramping experiment. Heating corals beyond their regional thermal maxima caused severe decreases in productivity, irrespective of which habitat the coral came from, but corals from the mangrove habitat suffered less thermally induced bleaching. Amplicon sequencing coral holobionts from reef and mangrove habitats in Indonesia and the Seychelles revealed significant habitat-dependent differences in coral microbiome compositions. A potentially novel coral-bacteria symbiosis between a mangrove-dwelling merulinid coral and an unclassified spirochaete, which accounted for 47% of the coral’s bacterial community, was also uncovered, though its role in the holobiont remains unknown. Reciprocal translocations of corals between reef and mangrove habitats resulted in rapid reorganisation of coral-associated bacterial communities. Within four days of translocation, coral-associated bacterial communities had changed. Corals demonstrated local adaptation and exhibited increased survivability when back-transplanted in their native habitat than when cross-transplanted to a new habitat. Experimental manipulation of the coral microbiome by antibiotic treatment demonstrated its sensitivity to disturbance, with rapid shifts in bacterial abundance, diversity, and composition taking place within 36 hours. These findings demonstrate the conservation value of mangrove coral habitats and highlight the rapid habitat-dependent flexibility of the coral microbiome.