Effects of light on microbial degradation of dissolved organic matter: source-dependent regulation by lignin-like molecules.

The increasing input of endogenous and exogenous dissolved organic matter (DOM) into lakes, driven by intensified human activities and climate warming, is profoundly affecting the functioning of lacustrine ecosystems. However, the fate of DOM under the combined influence of light and microorganisms-especially in northern steppe lakes-remains poorly understood. Here, we integrated ultrahigh-resolution mass spectrometry (FT-ICR MS) and high-throughput sequencing in a microcosm experiment using water from Hulun Lake, the largest arid steppe lake in northern China, to compare the molecular transformation of endogenous algal-derived DOM (ADOM) and exogenous livestock manure-derived DOM (MDOM). Treatments included dark controls, light-only, microbial-only, and light-microbial coupling. Using high-resolution mass spectrometry and multi-group structural equation modeling, our results demonstrate that light regulates microbial processing of DOM through source-dependent mechanisms. Photodegradation suppressed the bioavailability of ADOM by 4.9%, largely due to combined substrate competition and reactive oxygen species (ROS) stress on key bacterial degraders. In contrast, light synergistically enhanced the bioavailability of MDOM by 5.4% through substrate facilitation. Molecular-level analysis revealed that lignin-like molecules from different sources exhibit distinct responsiveness to photochemical and microbial processing, which emerged as the key mechanistic driver for the contrasting degradation patterns. Microbial co-occurrence networks further linked specific bacterial taxa to DOM component turnover and ROS sensitivity. These findings highlight the central role of light in modulating DOM balance and source-sink dynamics in arid boreal lakes, providing novel, mechanism-based insights into the biogeochemical cycling of DOM in lake ecosystems.