Limited influence of seasonality on coral microbiomes and endosymbionts in an equatorial reef

Coral-associated bacteria and Symbiodiniaceae are critical for maintaining the health of their coral hosts. Factors that are known to affect the coral microbiome include biogeography, seasonality, and environmental change, but longitudinal studies are sorely lacking. Many temporal studies rarely extend beyond a year, limiting the conclusions that can be drawn. In this study, we characterised both the bacterial and Symbiodiniaceae communities in three species of corals—Merulina ampliata, Pachyseris speciosa, and Porites lutea—found in an equatorial reef over a two-year period. Furthermore, we investigated the effect of monsoon seasons on the pattern of bacterial and Symbiodiniaceae communities using metabarcoding methods. We found significant differences in bacterial communities through time and space, but no consistent patterns of diversity between seasons and environmental conditions (temperature and rainfall) were observed. Conversely, Symbiodiniaceae communities did not differ over time and space. Taken together, our results suggest that bacterial communities, even within a stable climate regime, were highly dynamic through time, but were not affected by monsoonal variations. Finally, network analyses of bacteria and Symbiodiniaceae revealed multiple potential biogeochemical processes that worked in tandem between the members of the coral holobiont to support holobiont functioning. Longitudinal studies on host-associated microbial diversity are critical for drawing relevant conclusions about the long-term effects of environmental change on the coral holobiont, and for directing conservation efforts