Interactions between riverine sediment organic matter molecular structure and microbial community as regulated by heavy metals

Heavy metals (HMs) exert a profound influence on soil carbon storage potential. The microbially-mediated association between HM content and carbon structure in riverine sediments remains unclear in lotic ecosystems. We investigated the spatiotemporal variations of HMs content, carbon content and microbial communities in riverine surface sediments, and further explored the chemical structure of sediment organic carbon (OC_sed), the molecular composition of dissolved organic matter (DOM), and their interactions with microorganisms. The spatial-temporal variations in the chemical structure of OC_sed, excluding O-alkyl C, were minimal, whereas the molecular composition of DOM underwent substantial fluctuations with seasons and sites. Significantly positive correlations were observed between Cu, Zn, Pb, and OC_sed content. However, within a certain content range, HMs can promote the mineralization risk of OC_sed, as reflected in their ability to increase the proportion of unstable O-alkyl C and decrease the proportion of stable carbon fractions (aromatic C, alkyl C, and phenolic C). Additionally, appropriate contents of HMs also improved the abundance and diversity of bacteria and fungi. Bacteria consumed more stable OC under HMs enrichment, whereas fungi increased the consumption of DOM fractions (condensed aromatic hydrocarbons and amino sugars). Our findings contribute to the understanding of the molecular mechanisms of carbon storage in HM-rich riverine sediments.