Come Rain or Shine: Depth, Not Season, Shapes Protistan Community Structure in the NPSG

Protists are extremely diverse in form and trophic function, and form dynamic hubs in nearly all microbial communities as both primary producers and consumers. In spite of their importance, protist diversity and distribution has yet to be comprehensively characterized in the NPSG.

To better understand protist diversity and spatiotemporal distribution in the NPSG, we characterized the breadth of metabolically active protistan species along a high-resolution depth profile spanning 12 depths from 5- to 770-meters (~600 meters below the euphotic zone) across three seasons using 18S sequencing. Our results illustrated that seasonal changes in community structure were minor, however protist communities sharply changed across two narrow ranges of increasing depths between 75m-125m and 175m-300m. Stramenopiles, Alveolates, and Rhizaria (SAR) groups and Haptophytes were abundant in each season, and differentially distributed throughout the water column. Alveolate and Radiolarian sequences were dominant beneath the euphotic zone, whereas photosynthetic Stramenopiles and Haptophytes were abundant throughout the entire euphotic zone. Our results illustrate that depth, not season, shapes the protistan community structure in the NPSG.