Properties of sediment dissolved organic matter respond to eutrophication and interact with bacterial communities in a plateau lake

Sediment dissolved organic matter (DOM) in inland waters is commonly affected by environmental changes. However, knowledge about how sediment DOM responds to eutrophication and the associations between sediment DOM and bacterial communities requires further investigation. We selected a sediment core from Dianchi Lake (China) that was dated from 1864 to 2019 by the activity of radionuclides (²¹⁰Pb and ¹³⁷Cs). δ¹³C_DOC changes fit well with the historical record that heavy eutrophic status in Dianchi Lake were observed since 1980s. Large amounts of dissolved organic carbon (DOC), chromophoric (CDOM) and fluorescent (FDOM) DOM accumulated at the top of the sediments during the eutrophication period (1982-present). The additional algae sources with a higher degradation rate altered the composition, aromaticity and humification of DOM. After long-term mineralization, the remaining DOM became more and more recalcitrant and kept a relatively stable level at older sediments. A co-occurrence network analysis revealed that Proteobacteria, Chloroflexi, Acidobacteriota, Bacteroidota and Desulfobacterota were the most abundant species at the phylum level and clustered into three primary modules. Different microbes shared unique preferences for niches, causing a heterogeneous bacterial distribution at different depths. We conducted Spearman's correlation and redundancy analysis (RDA) to explore potential interactions between bacterial community and sediment DOM. The richness and diversity of bacterial communities were positively related to DOM content, suggesting abundant DOM can produce more available resources for bacteria. RDA results showed some specific species might modify DOM composition and structure. This study suggests that sediment DOM properties were regulated by source transformation during eutrophication, and emphasizes the importance of microbial role on sediment biogeochemical process.