Marine Syndiniales Parasites Exhibit Strong Temporal Variability and Ecological Connectivity in Coastal Protist Communities

Syndiniales (marine alveolates) are a ubiquitous group of protist parasites that infect a wide range of hosts, including many dinoflagellates known to produce harmful blooms. Despite the importance of parasitism as an agent of plankton mortality, parasite-host dynamics and temporal variability in parasite communities remain poorly understood. For a full year, we sampled weekly in the Skidaway River Estuary (GA) to investigate temporal shifts in Syndiniales relative abundance, diversity, and composition using 18S amplicon sequencing data. Co-occurrence networks were constructed from weekly relative abundances, which allowed for visualization of significant interactions between Syndiniales and putative hosts. Over the year, Syndiniales exhibited strong temporal variation, with highest relative abundances from June-October (7-28%) compared to November-February (0.01-6%). Tight clustering of Syndiniales communities was observed from June-October and best explained by shifts in temperature. Syndiniales amplicon sequences represented 20% of all associations in co-occurrence networks and were significantly correlated to known dinoflagellate hosts (e.g. Akashiwo and Gymnodinium) and other protist groups including diatoms, ciliates, and cryptophytes. Positive associations were often observed between multiple Syndiniales and a single dinoflagellate species, implying less-selective parasite-host infection dynamics. These findings suggest that parasitism in coastal protist communities varies temporally, and accounting for such variability may help to constrain parasitic mortality within ecosystem and biogeochemical models.