| 21 Jan 2026
Trait-based mechanisms underpin regional hotspot of diatom-driven carbon export in an oligotrophic gyre
Abstract. The oligotrophic subtropical gyres, vast yet nutrient-poor, pose challenges to our understanding of efficient carbon sequestration. Here, we integrate taxonomic, sediment trap, and metagenomic analyses to investigate the mechanisms underlying regionally heterogeneous and efficient diatom-mediated carbon export in the western North Pacific Subtropical Gyre. We discovered that within a vertically stratified nutrient regime, diatom communities displayed clear niche partitioning: Navicula and Rhizosolenia were enriched in the nutrient-depleted surface mixed layer, while Nitzschia, Chaetoceros, and Thalassiosira tended to dominate the deep chlorophyll maximum – reflecting hydrographic control over community assembly. This trait-based community structuring directly influenced the composition and magnitude of diatom carbon export, with fluxes ranging from 10³ to 10⁵ cells m⁻² d⁻¹ and an estimated 0.13–194.85 μg C m⁻² d⁻¹. Total carbon export and export efficiency (carbon exported relative to production) was markedly enhanced at station affected by the Kuroshio (K2b), which was mainly driven by the large, carbon-rich Rhizosolenia, delineating a distinct regional hotspot. Critically, metagenomic analysis revealed a limited presence of bacteria genes encoding key carbohydrate-active enzymes capable of degrading diatom-derived fucose-containing sulfated polysaccharides (FCSP), indicating a key biochemical mechanism that may reduce organic matter remineralization and enhance flux preservation. Our findings establish a multi-process framework wherein hydrodynamic regimes select for export-prone diatom communities with specific functional traits (e.g., size, carbon content), and the biochemical resistance of their organic byproducts may synergistically promote efficient carbon export. This study deciphers the interacting controls on carbon sequestration heterogeneity in the oligotrophic ocean, with crucial implications for predicting the biological pump's response to global change.
General Comments
This manuscript addresses an important question regarding diatom-driven carbon export in the western North Pacific Subtropical Gyre. However, the strength of the evidence does not match the ambition of the conclusions. The study relies on very limited spatial and temporal coverage, substantial methodological assumptions, and indirect inference to support claims of a “regional hotspot” and a trait-based mechanistic framework linking community composition, biochemical resistance, and export efficiency. Given the extremely small effective sample size—particularly the reliance on a single station (K2b) to define a “regional hotspot”—the conclusions appear overstated and insufficiently supported. In its current form, the study reads more as a preliminary case observation than as robust evidence for a regional-scale ecological pattern.
Specific Comments
1. Only five stations were sampled in summer, and sediment trap data were successfully obtained at just three stations (K2b, M22, K8a). Winter observations were limited to two stations and lacked sediment trap measurements entirely.
2. Carbon biomass was estimated using genus-level average carbon content, largely derived from literature compilations and biovolume-to-carbon conversions. Oligotrophic regions are typically dominated by small and fragile diatom taxa, whose cellular carbon content may deviate substantially from literature averages derived under different conditions.
3. Diatom counts were conducted after sedimentation concentration and 8 µm mesh filtration. The authors acknowledge that this procedure may underestimate large or fragile taxa such as Rhizosolenia. However, Rhizosolenia is precisely the dominant contributor to the proposed K2b “hotspot.”