Come rain or shine: Depth not season shapes the active protistan community at station ALOHA in the North Pacific Subtropical Gyre

Abstract

Protists are extremely diverse morphologically and physiologically, and they play important ecological roles at multiple trophic levels as primary producers and consumers in nearly all microbial communities. In spite of their fundamental importance in marine ecosystems, protistan diversity and distribution has yet to be comprehensively characterized in much of the world ocean, particularly in the vast expanses below the euphotic zone. We examined protistan community structure and species diversity in the oligotrophic North Pacific Subtropical Gyre. Our primary goal was to better characterize the breadth of metabolically active protistan species throughout the water column spanning 12 depths from 5 m to 770 m (~600 m below the euphotic zone) across three seasons using 18S rRNA gene V4 amplicon sequencing of RNA (cDNA).Protistan community structure changed markedly across relatively narrow ranges of increasing depths between 75 m-100 m, and again between 175 m-300 m. Changes were driven by depth-specific distributions among major protistan taxa associated with the upper mixed layer, deep chlorophyll maximum and aphotic zone, respectively, in this permanently stratified water column. Diatoms and some heterotrophic protists (MAST, choanoflagellates) were important contributors in the upper mixed layer, while haptophytes and pelagophytes increased in relative abundances in the lower euphotic zone. Radiolaria, ciliates and syndinians (putative parasitic protists within the order Syndiniales) increased in relative abundances below the euphotic zone. Overall, the highest taxonomic richness of amplicon sequence variants (ASVs) was observed in aphotic samples. Additionally, the dominant ASVs within some taxonomic groups that were observed in deep water were different than those observed in the upper water column, implying adaptations to specific depth strata rather than passive transport of surface-dwelling species. In contrast to depth-related changes, seasonal changes in protistan community structure were not significant.