Coral-Symbiodiniaceae relationships can display a significant level of host-specificity at both the genus and species level, while simultaneously experiencing the occasional shuffling and switching of symbionts in response to environmental disturbances, particularly when exposed to marine heatwaves. During 2019-2020 marine heatwaves, Acropora digitifera colonies were tagged and sampled over a seven-month period for symbiont cell density and temporal fluctuation in symbiont diversity using ITS2 amplicon sequencing. In 2019, a thermal stress (degree heating weeks) of 2.43 °C-weeks was detected and resulting in some A. digitifera colonies showing signs of bleaching with reduced symbiont cell density, but all colonies were able to recover when the temperature returns to normal with no DHW detected in 2020. The Symbiodiniaceae ITS2 sequencing revealed the A. digitifera host undergoes symbiont switching (increased symbiont richness and diversity, but no complete takeover) and shuffling (shift of dominant symbiont community profile) during the thermal stress event and during the recovery. The symbiont switching was evidenced with temporary occupancy of Symbiodinium and its related ITS2 type but was later expelled when the host recovered. In the same period, Durusdinium type also increased in relative abundance as the host recovering. A shift in background community especially of Cladocopium was observed, with C3u type replacing C3gf.    Ordination analysis further supports the evidence of shuffling and switching as the community structure shifts during thermal stress year, recovery and post-recovery. Such study is important in understanding the recovery potential of coral species under marine heatwaves via symbiont switching and shuffling, especially with the inevitable mass coral bleaching event in 2024.

Coral,marine heatwave,Adaptive