Diatom community assemblages and microbial degradation resistance modulate local heterogeneity in diatom contribution to carbon export in the western North Pacific Subtropical Gyre

Diatoms typically make a high contribution to carbon export (defined as more carbon exported relative to their production) in productive coastal and upwelling regions. However, oligotrophic subtropical gyres are vast but uniformly nutrient-poor, supporting only very low diatom abundances. This raises a critical question: can diatoms still achieve a high contribution to carbon export in such nutrient-limited regions, and if so, what factors enable this? Here, we integrated taxonomic, sediment trap, and metagenomic analyses at five stations in the western North Pacific Subtropical Gyre. Our observations reveal distinct niche partitioning: Navicula and Rhizosolenia tend to be enriched in the nutrient-depleted surface mixed layer, while Nitzschia, Chaetoceros, and Thalassiosira prevail in the deep chlorophyll maximum, reflecting hydrographic and nutrient influences on community assembly. Within our limited station coverage, measured intact cell fluxes ranged from 103 to 105 cells m−2 d−1, with estimated carbon fluxes of 0.11–313.03 µg C m−2 d−1. The total fluxes of diatom cells, biogenic silica, and carbon export were highest at Station K2b, which is influenced by the Kuroshio. At this station, the contribution of diatom export remained high, with the large, carbon-rich Rhizosolenia species dominating the sinking flux. Community assembly, characterized by functional traits such as cell size, carbon content, and fucose-containing sulfated polysaccharides (FCSPs) production, influences export composition and magnitude. Metagenomic analyses indicate a widespread capacity for degrading common diatom polysaccharides, but genes encoding the essential enzyme (GH107) for cleaving FCSPs are nearly absent across all stations. This deficit, together with the prevalence of FCSP-producing diatoms, suggests that biochemical resistance may assist carbon export. Our findings provide preliminary support that diatom community assemblages (shaped by hydrodynamics and nutrient supply) and microbial degradation resistance (via limited FCSP-targeting enzymes) modulate local heterogeneity in diatom export contribution. We conclude that predicting the biological pump's response to global change may require accounting not only for which diatoms are present, but also for which organic byproducts are protected from rapid remineralization. These results are based on a limited number of stations and a single season, and therefore warrant further testing.