Hydrothermal activity and water mass composition shape microbial eukaryote diversity and biogeography in the Okinawa Trough

Abstract

Abstract Microbial eukaryotes (protists) contribute substantially to ecological functioning in marine ecosystems, but factors shaping protist diversity, such as dispersal barriers and environmental selection, remain difficult to parse. Deep-sea water masses, which form geographic barriers, and hydrothermal vents, which represent isolated productivity hotspots, are ideal opportunities for studying the effects of dispersal barriers and environmental selection on protist communities. The Okinawa Trough, a deep, back-arc spreading basin, contains distinct water masses in the northern and southern regions and at least twenty-five active hydrothermal vents. In this study we used metabarcoding to characterize protist communities from fourteen stations spanning the length of the Okinawa Trough, including three hydrothermal vent sites. Significant differences in community by region and water mass were present, and protist communities in hydrothermally-influenced bottom-water were significantly different from communities in other bottom waters. Amplicon sequence variants enriched at vent sites largely derived from cosmopolitan protists and many were identical to sequences recovered from other vents. However, a putatively symbiotic ciliate appeared endemic to Okinawa Trough vents. Results support the view that most protists are cosmopolitan opportunists, but moderate endemism exists. Furthermore, dispersal limitation in back-arc basins may contribute to allopatric differentiation, especially if protists rely on endemic hosts.