Abstract The deep chlorophyll maximum (DCM), a critical layer in the ocean characterized by the highest biomass and productivity, determines marine productivity, biogeochemical cycling and carbon sequestration. However, a comprehensive understanding of microbial community structure and metabolic activities in this layer is lacking in several parts of the oceans. Here, we characterized the whole spectrum of proteins covering three size fractions (0.7-200 µm, 0.2-0.7 µm, 10 kDa-0.2 µm) in the DCM of the South China Sea using a metaproteomic approach. A total of 17 724 non-redundant proteins were confidently identified. Proteins from Cyanobacteria, SAR11, nitrite-oxidizing bacteria, Archaea, eukaryotic phytoplankton and phototroph-associated viruses were abundant. These organisms were actively involved in diverse biogeochemical processes including light-dependent energy transduction, carbon fixation, nitrification, sulfur metabolism, dissolved organic matter (DOM) uptake, and C1 and methylated compounds oxidation. Furthermore, chemolithoautotrophic activity of Nitrospinae and Thaumarchaea complemented carbon fixation pathways in this habitat. Notably, photoheterotrophic activity of SAR11 and PVC (Planctomycetes, Verrucomicrobia and Chlamydiae) bacteria and mixotrophic activity of photoautotrophs suggested diverse regulation channels of light on microbe-mediated DOM recycling. This in-depth metaproteomic study provides a holistic view of microbial community and metabolic activities in the DCM, and uncovers novel biogeochemical processes, especially those previously ignored but potentially active in the smallest fraction.

中文翻译：

---

通过尺寸分级的宏蛋白质组学剖析海洋深层叶绿素最大层的微生物群落结构和代谢活动

摘要 深叶绿素最大值 (DCM) 是海洋中具有最高生物量和生产力的关键层，它决定了海洋生产力、生物地球化学循环和碳固存。然而，在海洋的几个部分缺乏对该层微生物群落结构和代谢活动的全面了解。在这里，我们使用元蛋白质组学方法表征了南海 DCM 中涵盖三个尺寸分数（0.7-200 µm、0.2-0.7 µm、10 kDa-0.2 µm）的整个蛋白质谱。总共有 17 724 种非冗余蛋白质被可靠地鉴定。来自蓝藻、SAR11、亚硝酸盐氧化细菌、古细菌、真核浮游植物和光养相关病毒的蛋白质丰富。这些生物积极参与各种生物地球化学过程，包括光依赖能量转导、碳固定、硝化、硫代谢、溶解有机物 (DOM) 吸收以及 C1 和甲基化化合物氧化。此外，Nitrospinae 和 Thaumarchaea 的化学自养活动补充了该栖息地的碳固定途径。值得注意的是，SAR11 和 PVC（Planttomycetes、Verrucomicrobia 和 Chlamydiae）细菌的光异养活性以及光合自养生物的混合营养活性表明，微生物介导的 DOM 循环具有多种光调节通道。这项深入的元蛋白质组学研究提供了 DCM 中微生物群落和代谢活动的整体视图，并揭示了新的生物地球化学过程，