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hidden diversity

Algal symbionts give corals their colour and provide nutrition but are lost when the corals bleach due to increased sea temperature.

Reef-building stony corals are a partnership between an animal host and symbiotic microalgae that live in the animal tissues. As with other plants, the algal symbionts use energy from sunlight to make sugars through photosynthesis, and this contribution is critical to the survival of the host because the coral receives most of its nutrition from the algae. Many external stress events (freshwater, unusually cold or hot temperatures) can cause coral bleaching (i.e., the paling of the coral tissues due to the loss of its algae and/or their photosynthetic pigments), but none is so devastating over large spatial scales as the abnormally warm sea temperatures linked to global climate change.

The symbiosis between the coral host and its algal symbiont is sensitive to even small temperature increases; an increase of just 1.0–1.5 °C is typically enough to reach the point where the symbiosis breaks down and the coral host loses the majority of the algal symbionts.

Past research by AIMS scientists has shown that different populations of the same coral species can be dominated by different genetic types of microalgae and that some of these symbioses are more heat stable than others. Further work has shown that some coral colonies contain more than one type of symbiont and that the symbiont community of a coral colony can change over time. The change is brought about by changing the order of dominance between coexisting types ("symbiont shuffling") rather than uptake of new symbiont types from the environment ("symbiont switching").

Initial reactions to these research findings have been varied but one of the most compelling arguments against the importance of symbiont shuffling as a mechanism to acclimatize to climate change is that only few corals have been shown to possess more than one type of symbiont.

The latest results by AIMS scientists in collaboration with the Netherlands' University of Groningen show that the perception of low symbiont diversity within individual corals colonies is a significant underestimate that has been driven by the insensitivity of the current generation of genetic screens. The development of new techniques–100 times more powerful than conventional methods–has provided the first evidence that many coral colonies store an unrecognised diversity of microalgae, which make the potential of symbiont shuffling far greater than is currently thought.