Protistan parasites along oxygen gradients in a seasonally anoxic fjord: A network approach to assessing potential host-parasite interactions

The seasonally stratified water column of Saanich Inlet, Vancouver, B.C., Canada provides a natural laboratory for examining how ocean deoxygenation shapes microbial community population dynamics and interactions. Microbial eukaryotes within these communities play significant roles as phototrophs, mixotrophs, symbionts, predators, parasites, and saprotrophs, with population abundances and interactions changing seasonally, and along water column redox gradients. Here, we use time series observations of ribosomal RNA and DNA diversity in combination with co-occurrence correlation and indicator species analyses to observe microbial eukaryote community dynamics over a 12-month period, uncovering significant correlations between parasitic dinoflagellates within Syndiniales and other eukaryotic taxa during months of peak water column stratification. Syndiniales operational taxonomic units (OTUs) were the most abundant indicator OTUs during stratification. Syndiniales OTUs exhibited significant correlations with a broad range of taxa, including members of Apicomplexa, Ciliophora, Dinoflagellata, Chlorophyta, Metazoa and a variety of Stramenopiles including MAST groups. Correlation analyses suggest Syndiniales may influence phytoplankton bloom dynamics in Saanich Inlet as well as abundances of protist and metazoan grazers that are indicator species for peak stratification periods. These observations shed light on the potential role of parasitic protists on organic matter release and bloom termination events during stratification and renewal periods in Saanich Inlet with potential implications for coastal marine foodwebs and fisheries productivity.