Diversity and putative interactions of parasitic alveolates belonging to Syndiniales at a coastal Pacific site

The dinoflagellate lineage Syndiniales currently consists entirely of parasitic species that fall into five well‐supported clades. Environmental sequencing studies worldwide have found an abundance of Syndiniales in a variety of marine ecosystems, but very little is known about the majority of Syndiniales species including two entire clades which have only been observed in sequence data. Syndiniales are known to have a wide range of hosts, but only a few dozen interactions have been confirmed through observation of actual infections. Here, we describe the diversity of Syndiniales found at the Scripps Institution of Oceanography pier over the course of a year based on 18S sequencing. We find Syndiniales to be the most species (amplicon sequence variant)‐rich taxonomic group and for its members to be present and abundant throughout the year. We used several analytical techniques to identify potential parasite–host interactions which we were then able to visualize over time. Using mock communities and size fractionation of seawater, we suggest that the majority of Syndiniales sequences that are found in environmental studies belong to the free‐living dinospore stage rather than representing active infections.