Symbiont shuffling dynamics associated with photodamage during temperature stress in coral symbiosis

Reef-building corals usually form a symbiotic relationship with various photosynthetic dinoflagellates, which may determine the physiology and stress tolerance of their hosts. The mechanisms governing symbiont dynamics are still poorly understood, but may be driven by temperature. Therefore, we performed indoor experiments to examine the photochemical efficiency and symbiont dynamics in the coral Pocillopora damicornis with thermo-sensitive Cladocopium (PdC) or thermo-tolerant Durusdinium (PdD) under 6-day temperature stress and after 90-day post-stress recovery. Regardless of the symbiont type, photochemical damage was induced by both heat and cold stress. We observed PdC with greater photodamage had a faster increase in the proportion of Durusdinium when exposed to heat or cold stress, whereas an inverse shuffling to Cladocopium in both PdC and PdD after the recovery period. This is the first record of symbiont shuffling from the originally dominant Durusdinium to Cladocopium in adult corals under laboratory conditions and the degree of symbiont shuffling was highly correlated with the photodamage. Overall, our findings suggest that symbiont shuffling can act as a critical mechanism allowing corals to rapidly adjust to environmental changes, which has implications for the capacity of corals to prevail in oceans with greater temperature fluctuations in the future.