Rhizaria are unexpectedly abundant and exhibit taxonomic and trophic diversity in the eastern subarctic Pacific

Rhizaria are a diverse supergroup of large marine protists that are often overlooked due to their fragility, lower abundances, and wide size range relative to other plankton. Despite their global distribution, Rhizaria ecology and biogeography is poorly understood due to a paucity of datasets and use of differing methodologies. Here we present the first characterization of Rhizaria ecology in the northern Gulf of Alaska (NGA), a variable yet productive subarctic ecosystem with important fisheries that is experiencing long-term warming. Seawater samples were collected from CTD-secured Niskin bottles at stations within the NGA Long-Term Ecological Research study area during summer 2023. We report some of the highest Rhizaria abundances (25 cells L-1) from any ocean environment to date and thus suggest a restructuring of the current biogeographical paradigm that posits highest abundances at the equator and decreases at higher latitudes. Acantharia was the most ubiquitous subgroup. Distinct depth niches were also revealed: Foraminifera dominated surface waters, Radiolaria exhibited a cosmopolitan distribution, and Phaeodaria were the deepest living. Prey captures and algal interactions primarily occurred offshore in the upper water column. A wide range of taxa had captured prey while the hosts to presumptively symbiotic algae were mainly Foraminifera and Acantharia. We highlight Rhizaria as key players in NGA food web dynamics as evidenced by their wide depth distributions, taxonomic diversity, and variable nutrition strategies. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=97 SRC="FIGDIR/small/652060v1_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@1cb948corg.highwire.dtl.DTLVardef@857c74org.highwire.dtl.DTLVardef@1adcaeborg.highwire.dtl.DTLVardef@e5332b_HPS_FORMAT_FIGEXP M_FIG Graphical Abstract.Distribution of Rhizaria subgroups in the northern Gulf of Alaska (left). Proposed revision of the biogeographical distribution of Rhizaria in the Pacific and Southern Oceans (right). C_FIG HighlightsO_LIThe N. Gulf of Alaska contains some of the highest Rhizaria abundances yet reported C_LIO_LIAcantharia was the most abundant taxon C_LIO_LIRhizaria subgroups inhabited distinct depth niches C_LIO_LIA wide range of taxa had captured prey C_LIO_LIForaminifera and Acantharia were the most common hosts to algal cells C_LI