Drastic coupling of dissolved organic matter molecules with bacterial and fungal communities in a major global river...

Dissolved organic matter (DOM) constitutes one of the largest carbon pools in aquatic ecosystems and plays a pivotal role in the global carbon cycle. Nevertheless, how riverine DOM and microbial communities associate across extreme elevational and environmental gradients remains poorly understood. We investigated the spatial variation in DOM molecular profiles along the Yarlung Tsangpo River, one of the world's major rivers with rapid elevation gradient, and examined its coupling with bacterial and fungal communities using high-resolution mass spectrometry, high-throughput sequencing and multivariate statistical analyses. Along the continuum from glacial headwater to low-altitude rainforest, bioavailable components including amino sugar-like, carbohydrate-like, protein-like and lipid-like decreased consistently, while recalcitrant lignin-like and condensed aromatics fractions increased significantly. DOM exhibited a distinct transition from microbially derived, high-molecular-weight compounds to more aromatic, oxidized, and low-molecular-weight substances. Additionally, the molecular specificity of DOM declined gradually, with the proportion of unique molecular formulas reaching 30.6% in the glacial headwater and dropping sharply to 3.7%-7.1% in fluvial reaches, and the mean Bray-Curtis dissimilarity was highest in the glacial headwater (0.49) and remained lower in river sections (0.23-0.27). The glacial headwater was enriched in carbohydrate-like, protein-like, and lipid-like compounds, while river were dominated by lignin-like, demonstrating the progressive homogenization of DOM along environmental gradients. In contrast, species richness, community dissimilarity and functional diversity of bacteria and fungi were significantly elevated in low-altitude rainforest regions, accompanied by a substantial increase in the scale and complexity of microbial-DOM co-occurrence networks across river segments. These findings indicate that bioavailable DOM fractions are progressively depleted, leading to the increasing dominance of ubiquitous, structurally stable recalcitrant compounds in the DOM pool. This study reveals the co-occurring pattern of progressive DOM molecular composition homogenization and increasing complexity of microbial interaction networks. It provides molecular and microbial insights for predicting the shifts in DOM composition along environmental gradients under climate warming and evaluating the stability of cross-ecosystem carbon pools.