Molecular Activity Mediates the Composition and Assembly of Dissolved Organic Matter in Lake Sediments

Lake sediments are hotspots for carbon transformation and burial, where dissolved organic matter (DOM) interacts with microorganisms to regulate global carbon cycling. The potential for individual DOM molecules to undergo biochemical transformations, i.e., their activity, is a critical molecular trait affecting DOM turnover in environment. However, the composition of sediment DOM and how its assembly mechanisms are related to molecular activity remains poorly understood. Here, 63 freshwater sediments were collected from tropical to cold temperate climatic zones in China. We explored the molecular composition and assembly of sediment DOM and the underlying mechanisms driven by climate, physicochemical factors, and microbes along the molecular activity gradient. Sediment DOM was dominated by lipid- (34.8%) and lignin-like compounds (33.01%), and the latter were enriched as molecular activity of DOM increased. Besides, DOM composed of more active molecules had greater compositional similarity across different climatic zones, and was inclined to assemble deterministically. This was supported by the fact that as potential transformations of molecular assemblages increased from 0.4 to 14, the assembly of these molecules was structured by a shift from stochastic to deterministic processes, with the latter accounting for ≥ 75% thereafter. Overall, the molecular assemblage was primarily structured by physicochemical factors, including sediment total organic carbon and electrical conductivity. As molecular activity increased, however, assemblage was increasingly affected by climate and bacterial communities, consistent with the enhanced complexity of bacterial-molecular networks. Collectively, our study highlights that the intrinsic activity of DOM molecules determines their fate through distinct biotic and abiotic mechanisms. ### Competing Interest Statement The authors have declared no competing interest.