Time-series dynamics in symbiont communities of acroporid corals undergoing cold stress in winter

The bleaching of corals due to cold stress has been seldom studied compared with bleaching events resulting from heat stress. In this study, using a camera connected to an observatory system, we discovered an Acropora cytherea -like coral colony that underwent cold bleaching in winter at a depth of approximately 20 m in Okinawa, Japan. This main colony and 8 nearby colonies with similar morphological traits were selected for study, and their genetic lineages were identified. The dynamics in the symbiont communities associated with the coral colonies during 2015 and 2016 were determined by metabarcoding. Bleaching in the winter of 2015-2016 was less severe than that in the winters of 2013-2014 and 2014-2015, and water temperatures were higher in the winter of 2015-2016. Analyses of genetic markers revealed that the 9 studied colonies were A . cytherea and A. hyacinthus species complexes with multiple genetic lineages. The symbionts in the 9 colonies included 2 major defining intragenomic variants, C50a and C50c, which are related to Cladocopium C3. The ratio of symbiont genotypes in the 8 nearby colonies remained largely unaltered throughout the sampling period. Although the ratio changed continually in the bleached colony, the alterations were not related to the seasonal fluctuations in temperature. This study revealed that certain C3 strains can endure low temperatures in winter and suggests that different genetic strains within the same genus can exhibit physiological differences.