Distinct, Depth-Stratified Communities of Heterotrophic, Mixotrophic, and Parasitic Dinoflagellates and Ciliates in the Subarctic Northeast Pacific Euphotic Zone.

Dinoflagellates and ciliates are important grazers of primary production in the Northeast Pacific but knowledge of their taxonomic composition and depth-distribution is limited. These organisms also display a variety of heterotrophic feeding modes including heterotrophy, mixotrophy, and parasitism. Here, we analyzed dinoflagellate and ciliate feeding modes and depth-distribution using high-throughput amplicon sequencing of the 18S rRNA V4 gene region on samples collected during the August-September 2018 EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) campaign near ocean station Papa. The parasitic dinoflagellate class Syndiniales comprised 52% of amplicon sequence variants (ASVs), only one of which could be identified to the genus level, highlighting the potential importance of unknown parasitic dinoflagellates to marine food webs. Frequent occurrences of carnivorous ciliates suggested grazer-on-grazer predation, driving carbon recycling through repeated trophic transfers. Over 90% of protist consumer ASVs were more likely to occur in either surface waters or below the mixed layer, highlighting significant vertical structure in the protistan consumer community. The activities of this diverse set of depth-stratified protist consumers likely modulate carbon flow in the upper ocean and may explain currently unattributed export production losses in the Northeast Pacific.