Single-cell transcriptomics reveals a new marine parasite: Characterization and distribution of <i>Rozellomyces</i> from the Gulf of Alaska

Parasitic microorganisms play critical roles in shaping aquatic ecosystems, yet many lineages, such as those within the phylum Rozellomycota, remain poorly understood. Here, we describe a new rozellid isolate from the Gulf of Alaska. Phylogenomic analysis based on 238 genes confirms its placement within Rozellomycota and its distinctness from currently described taxa. We identified amplicon sequence variants (ASVs) from the metaPR² database to recover its geographic distribution. Our isolate is an endoparasite of the diatom <i>Thalassiosira</i>, a primary producer in marine ecosystems that plays a critical role in global carbon and nutrient cycling. Phylogenetic and ecological evidence supports the establishment of a new genus within Rozellomycota, <i>Rozellomyces</i>. Our findings provide new insights into the diversity, distribution, and ecological roles of Rozellomycota, highlighting the need for further research on these parasites in marine ecosystems.