Thermal stress responses and adaptive mechanisms in the coral holobiont of the Genus Orbicella

Increasing temperatures and increasingly frequent bleaching events have severe impacts on coral populations and their ability to sustain themselves and the ecosystem depending on them. This review analyzes the state of research around thermal stress adaptations in Orbicella corals, an important reef builder species of the Western Atlantic, their symbiotic algae and their microbial communities. After discussing the anatomy of the coral holobiont, its constituent parts and their role in the overall metabolism, I report studies on the relationship between biotic and abiotic factors and the composition of the holobiont. Repeated exposure to high temperatures seems to create partial tolerance to bleaching, while certain populations living in extreme or marginal conditions have developed their own genotype and symbiont populations, hosting more tolerant invasive species that have an effect on the overall coral metabolism. In the end, differences due to environmental factors in Orbicella microbial communities are discussed, and the conclusion brings forth possible future studies and methods, while introducing two frameworks around the study of corals and their holobiont: the concept of the “core microbiome” and possible changes to the way corals are studied on the human and organisational level.