Coral Reefs: A Reef Resilience Toolkit Module

Zooxanthellae Mechanisms

Several studies have revealed that some corals can vary the ratio of zooxanthellae clades, resulting in an improvement of their thermal tolerance. The selective exchange of zooxanthellae (from suboptimal to more heat tolerant clades) represents a mechanism for rapid acclimatization by corals to thermal stress. Changes in the dominant zooxanthellae types of a coral colony may occur through two processes1:

1. “shuffling” — changes in the relative abundance of zooxanthellae clades that are already present in the coral tissue
2. “switching” — uptake of new zooxanthellae clades from the environment

Thermal acclimatization/adaptation, often referred to as the adaptive bleaching hypothesis (ABH), is catalyzed by zooxanthellae changes. It is a natural operation of coral reefs, and shifts toward more heat tolerant populations of corals and zooxanthellae may arise naturally, but may be accelerated due to rising temperatures2,3.

Five fundamental assumptions underpin the ABH:

1. Multiple types of both coral host and Symbiodinium species commonly co-exist.
2. Both zooxanthellae and coral host have a degree of flexibility in their associations.
3. Coral types are physiologically different and influence important aspects of the coral’s physiology (especially stress-responses).
4. Coral bleaching provides an opportunity for re-population of a host with a different dominant zooxanthellae.
5. Stress-sensitive coral + zooxanthellae pair have competitive advantages in the absence of stress, which implies a reversion to stress-prone combinations under non-stressful conditions.

In the short term, corals with flexible symbioses may shuffle or switch zooxanthellae; and an increase in the abundance of thermally tolerant zooxanthellae strains (such as those of clade D) is expected with an increasing frequency of bleaching conditions. The potential to adapt to increasing sea surface temperatures depends on the extent of genetic variation for heat tolerance, the generation time of the coral host and zooxanthellae, and the strength of selection4.