Characterization of sinking and suspended microeukaryotic communities in spring Oyashio waters

Microeukaryotes are important components of sinking particles contributing to carbon export from the surface to deep oceans. The knowledge of the sinking microeukaryotic communities and their dynamics is limited. We applied 18S rDNA metabarcoding method to investigate the microeukaryotic communities in sinking and suspended particles distinguished by marine snow catchers (MSC) during spring in the Oyashio region. Sinking particles displayed distinct communities and lower diversity than did suspended particles. The community compositions of the sinking particles varied with depth, suggesting that microeukaryotes were selectively removed through disaggregation or decomposition during settling. Prymnesiophyceae and diatoms were effectively removed, as indicated by their decreased abundance in the sinking particles at increasing depths. Conversely, phototrophic dinoflagellates maintained a higher abundance in the sinking particles across depths, indicating resistance to disaggregation and decomposition. Heterotrophic dinoflagellates and Spirotrichea were enriched in sinking particles and marine stramenopiles (MAST) groups were enriched in suspended particles. Sinking heterotrophic protist communities in the deep layers were similar to those in the surface layers, whereas they differed from the suspended ones in the same layer. Therefore, heterotrophic protists in surface layers were transported to deeper layers. Overall, our results demonstrate the functional differences among microeukaryotes in the biological carbon pump. ### Competing Interest Statement The authors have declared no competing interest.