Seasonal Cycles in a Seaweed Holobiont: A Multiyear Time Series Reveals Repetitive Microbial Shifts and Core Taxa.

Seasonality is an important natural feature that drives cyclic environmental changes. Seaweed holobionts, inhabiting shallow waters such as rocky shores and mud flats, are subject to seasonal changes in particular, but little is known about the influence of seasonality on their microbial communities. In this study, we conducted a three-year time series, sampling at two-month intervals, to assess the seasonality of microbial epibiota in the seaweed holobiont Gracilaria vermiculophylla. Our results reveal pronounced seasonal shifts that are both taxonomic and functional, oscillating between late winter and early summer across consecutive years. While epibiota varied taxonomically between populations, they were functionally similar, indicating that seasonal variability drives functional changes, while spatial variability is more redundant. We also identified seasonal core microbiota that consistently (re)associated with the host at specific times, alongside a permanent core that is present year-round, independent of season or geography. These findings highlight the dynamic yet resilient nature of seaweed holobionts and demonstrate that their epibiota undergo predictable changes. Therewith, this research offers important insights into the temporal dynamics of seaweed-associated microbiota and demonstrates that the relationship between seaweed host and its epibiota is not static but naturally subject to an ongoing seasonal succession process.