Strom, S. L. | bioRxiv Science

bioRxiv2025

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

ecology↗

bioRxiv2023

Strong and efficient biological carbon pump in the Northern Gulf of Alaska during summer

Sinking marine particles, one pathway of the biological carbon pump, transport carbon to the deep ocean from the oceans surface, thereby contributing to atmospheric carbon dioxide modulation and benthic food supply. Few in situ measurements exist of sinking particles in the Northern Gulf of Alaska (NGA); therefore, regional carbon flux prediction is poorly constrained. In this study, we aim to (1) characterize the magnitude and efficiency of the biological carbon pump and (2) identify drivers of carbon flux in the NGA. We deployed drifting sediment traps to simultaneously collect bulk carbon and intact sinking particles in polyacrylamide gels and measured net primary productivity from deck-board incubations. Through deployments during the summer of 2019, we found high carbon flux magnitude, low attenuation with depth, and high export efficiency. We quantitatively attributed carbon flux between ten particle types, including various fecal pellet categories, dense detritus, and aggregates using polyacrylamide gels. The contribution of aggregates to total carbon flux (41 - 93%) and total carbon flux variability (95%) suggests that aggregation processes, not zooplankton repackaging, played a dominant role in carbon export during the summer of 2019 in the NGA. Furthermore, efficient export correlated significantly with the proportion of chlA > 20 {micro}m, total aggregate flux, and proportion aggregate flux. These results suggest that this stratified, small-cell-dominated ecosystem can have sufficient aggregation to allow for a strong and efficient biological carbon pump. These are the first measurements of carbon flux and the first integrative description of the BCP in this region. Significance StatementO_ST_ABSNovelty and significanceC_ST_ABSWe use a comprehensive approach that brings together sediment trap sampling and imaging, optically measured distribution of sinking and suspended particles, and incubations to make the first description of the biological carbon pump in the Northern Gulf of Alaska. We found high carbon flux magnitude, low attenuation with depth, and high export efficiency with a phytoplankton community consisting of mostly pico-and nanoplankton. Notably, just 25% of carbon flux out of the euphotic zone was as recognizable fecal pellets; instead, we demonstrate that aggregation processes were the main driver of carbon flux. Additionally, size-fractionated chlorophyll-a (> 20 {micro}m) strongly correlated with export efficiency across our region. These results lead us to question our expectations about what conditions and processes can create strong and efficient flux events in the Gulf of Alaska. Breadth of InterestThis study is the first description of the biological carbon pump in the Northern Gulf of Alaska and greatly improves biogeochemical constraints on this system. We report observed primary production, carbon flux, export ratio, carbon flux attenuation, and carbon flux by 10 particle types, which can be used to test regional climate models. This study builds on previous studies published in L&O: Strom et al. 2007; Ebersbach & Trull 2008; McDonnell & Buesseler 2010, 2012; and Durkin et al. 2016. Author contribution statementSO, SS, and AM: conceptualization, methodology, and investigation. SS, AM, GH: funding acquisition and project administration. SO, TK, AM: formal analysis. GH, TK, and AM: supervision. SO: visualization, writing-original draft preparation. SO, GH, TK, SS, and AM: writing-reviewing and editing.

biochemistry↗