Many corals rely on a symbiotic algae called zooxanthellae for nutrient uptake through photosynthesis. Corals obtain about 60-85% of their total nutrition from symbiotic zooxanthellae. Slight increases in sea surface temperature can cause zooxanthellae to die. Coral hosts become bleached when they lose their zooxanthellae. Difference in symbionts, determined by genetic groupings, may explain thermal tolerance in corals. Research has shown that some corals contain thermally-resistant clades of zooxanthellae. Corals housing primarily clade D symbionts, and certain types of thermally-resistant clade C symbionts, allow corals to avoid bleaching as severely as others experiencing the same stressor. Scientists remain in debate if thermal resistance in corals is due to a mixing or shifting of symbionts, or thermally resistant vs. thermally-sensitive types of zooxanthellae. Species of coral housing multiple types of zooxanthellae can withstand a 1-1.5 °C change in temperature. However, few species of coral are known to house multiple types of zooxanthellae. Corals are more likely to contain clade D symbionts after multiple coral bleaching events.
Reef recovery
Research studies of the Mediterranean species of coral Oculina patagonica reveal that the presence of endolithic algae in coral skeletons may provide additional energy which can result in post-bleaching recovery. During bleaching, the loss of zooxanthellae decreases the amount of light absorbed by coral tissue, which allows increased amounts of photosynthetically active radiation to penetrate the coral skeleton. Greater amounts of photosynthetically active radiation in coral skeletons causes an increase in endolithic algae biomass and production of photoassimilates. During bleaching, the energy input to the coral tissue of phototrophic endoliths expands as the energy input of the zooxanthellae dwindles. This additional energy could explain the survival and rapid recovery of O. patagonica after bleaching events.
Managing coral reefs
In an attempt to prevent coral bleaching, scientists are experimenting by "seeding" corals that can host multiple types of zooxanthellae with thermally-resistant zooxanthellae. MPAs have begun to apply reef resilience management techniques in order to improve the 'immune system' of coral reefs and promote reef recovery after bleaching. The Nature Conservancy has developed, and is continually refining, a model to help manage and promote reef resilience. Although this model does not guarantee reef resilience, it is a comprehensible management model to follow. The principles outlined in their model are:
Representation and replication: Coral survivorship is ensured by representing and replicating resilient species and habitats in an MPA network. The presence of resilient species in management in MPAs will help protect corals from bleaching events and other natural disturbances.
Connectivity: Preserving the connectivity between coral reefs and surrounding habitats provides healthy coral communities and fish habitat.
Effective management: Resilience based strategies are based on reducing threats to maintain healthy reefs. Measurements of effective management of MPAs allows for adaptive management.
