Temporal and spatial dynamics of coral symbiont assemblages are affected by local and global impacts.

The influence of abiotic variables and anthropogenic pressure on symbiodiniaceans associated with the scleractinian corals Mussismilia hispida and Siderastrea stellata were assessed quarterly at Armação dos Búzios, Brazil, for over 18 months. Thirty-eight Symbiodiniaceae ITS2 rDNA phylotypes were found by metabarcoding, with eight comprising new phylotypes. Both hosts maintained their generalist pattern, with 1-3 dominant lineages. An environmental pressure index and changes in seawater temperature explained the variations in the structure and diversity of Symbiodiniaceae assemblages over time and space. A mild bleaching event affected the photosymbiotic assemblage structure, even in non-bleached colonies. The highly dynamic and diverse photosymbiont assemblages were constantly driven by the influence of environmental variables and human-induced impacts. Furthermore, new strains of Symbiodiniaceae might be associated with lower temperatures caused by upwelling, which is characteristic of this subtropical coral community, highlighting the region's idiosyncrasy and the need for further studies of this coral system.

Marine Heatwave Caused Differentiated Dysbiosis in Photosymbiont Assemblages of Corals and Hydrocorals During El Niño 2015/2016.

Reef corals have been threatened by climate change, with more frequent and intense bleaching events leading to extensive coral mortality and loss of coral cover worldwide. In the face of this, the corals' photosymbiont assemblages have received special attention as a key to better understand the bleaching process and its recovery. To assess the effects of thermal anomalies, the coral Mussismilia harttii and the hydrocoral Millepora alcicornis were monitored through the El Niño 2015/2016 at a Southwestern Atlantic (SWA) coral reef. A severe bleaching event (57% of colonies bleached) was documented, triggered by a < 3 °C-week heatwave, but no mortality was detected. The hydrocoral was more susceptible than the scleractinian, displaying bleaching symptoms earlier and experiencing a longer and more intense bleaching event. The composition of photosymbionts in the M. alcicornis population was affected only at the rare biosphere level (< 5% relative abundance), with the emergence of new symbionts after bleaching. Conversely, a temporary dysbiosis was observed in the M. harttii population, with one of the dominant symbiodiniaceans decreasing in relative abundance at the peak of the bleaching, which negatively affected the total ß-diversity. After colonies' complete recovery, symbiodiniaceans' dominances returned to normal levels in both hosts. These results highlight critical differences in how the two coral species cope with bleaching and contribute to the understanding of the role of photosymbionts throughout the bleaching-recovery process.