Depth‐dependent metabolic stratification identifies the oxic deep chlorophyll maximum as a global biogeochemical hotspot

Abstract

The deep chlorophyll maximum (DCM) is an important feature of the marine photic zone where carbon fixation, nutrient transformations and particle‐associated export can be enhanced, collectively influencing marine productivity, biogeochemical cycling and carbon sequestration. However, how microbial metabolism in this layer differs from that in the overlying surface layer (SL) remains poorly characterized. Using depth‐resolved metagenomes from the Western Pacific Ocean, validated against global Tara Oceans datasets, we show that the DCM is strongly stratified metabolically even though its taxonomic composition differs only modestly from the SL, revealing a pronounced decoupling of function from taxonomy. Surface communities were enriched in light‐driven carbon fixation pathways that consume 3–3.5 adenosine triphosphate (ATP) per CO ₂ fixed, whereas DCM communities were enriched in pathways requiring only 1–2 ATP—the reductive tricarboxylic acid cycle and the dicarboxylate/4‐hydroxybutyrate cycle—consistent with an energetic niche axis between light‐rich surface and light‐limited subsurface waters. Fully oxygenated DCM waters (~ 190  μ mol O ₂ L ⁻¹ ) contained gene inventories associated with nitrification, denitrification, and sulfur oxidation, together with methane metabolism–associated genes, potentials conventionally restricted to oxygen‐minimum zones. These gene suites were consistently enriched at the DCM across all major ocean basins. Methane assimilation showed the same energetic pattern, with the ATP‐expensive serine cycle dominating the SL and the lower‐cost ribulose monophosphate route enriched in the DCM. Because warming intensifies stratification and is projected to deepen and sharpen DCM features, the metabolic organization of this layer is likely to become more, not less, important to upper‐ocean biogeochemistry.