Shift of algal symbiont communities in the coral Acropora digitifera: Shuffling and switching during thermal stress and recovery

Abstract

The algal symbionts (Symbiodiniaceae) of reef-building corals are known for their host-specific associations, yet they can undergo restructuring through shuffling and switching under environmental disturbances, particularly thermal stress. Following the 2019 thermal stress event (degree heating weeks, DHW = 2.43 °C-weeks), Acropora digitifera colonies were tagged and sampled over a seven-month period to examine the dynamics of algal symbiont communities using high-throughput ITS2 rDNA amplicon sequencing. Several A. digitifera colonies exhibited signs of bleaching and reduced symbiont density but subsequently recovered once temperatures returned to normal. Molecular profiling revealed both symbiont shuffling and transient switching. Community restructuring included shifts among dominant Cladocopium ITS2 type profiles, the temporary appearance of Symbiodinium lineages, and the persistence of low-abundance Durusdinium during the recovery phase. Ordination analyses further indicated that symbiont community composition was significantly structured by sampling time, reflecting both the 2019 thermal anomaly and seasonal variability. Together, these findings indicate that A. digitifera can withstand moderate thermal disturbances through dynamic yet reversible restructuring of its symbiont community. By capturing these temporal shifts, this study highlights the ecological flexibility of coral–symbiont partnerships and provides baseline insights into coral resilience under increasing climate-driven thermal stress.