Molecular Size Mediates Sunlight Effects on the Biodegradation of Aquatic Dissolved Organic Matter: Insights from the Reservoir Wetland and Incubation Experiment

Dissolved organic matter (DOM) in aquatic ecosystems is crucial to climate, water quality, and water sanitation. However, little is known about how photochemical and biological processes interact to regulate the cycling of DOM in water environments. Here, we investigate the photobiochemical processing of different molecular weight (MW) fractions of aquatic DOM by incubating them with microbes in the dark and light/dark cycling environments via microcosm experiments. Light exposure has distinct effects on the medium (MMW; 1–20 kDa) and low (LMW; <1 kDa) MW fractions and can enrich photoautotrophic bacteria and taxa that could benefit the utilization of aromatic compounds. The decay of the MMW fraction displays the highest light sensitivity, with light increasing the net depletion of tannin- and condensed aromatic-like molecules and production of labile substrates (e.g., protein-like) from photoautotrophic bacteria. In contrast, the net decomposition of the LMW fraction is the least promoted by light exposure, as light induces the net production of complex aromatic molecules that are more persistent. Our findings provide new insights into the photobiochemical and molecular mechanisms underlying the ecological impacts of aquatic DOM and their relevance for wider processes.