Molecular insights into the coupled transformation of dissolved organic matter and arsenic in lake sediment driven by algal input under redox fluctuations.

Algal organic matter (AOM) released during algal degradation promotes the mobilization of arsenic (As) from lake sediments into the overlying water. This process has been well studied under static anaerobic or aerobic conditions. However, owing to the complexity of AOM, the molecular mechanisms driving its dynamic changes and their impact on the coupled transformation of dissolved organic matter (DOM) and As in sediments remain poorly understood under redox fluctuations. To address this gap, we investigated the effects of AOM input on As mobility and speciation by characterizing sediment DOM via Fourier-transform ion cyclotron resonance mass spectrometry and three-dimensional excitation-emission matrix fluorescence spectroscopy. In addition, we quantified As metabolism genes under sequential anaerobic, aerobic, and anaerobic conditions over 42 days. The exogenous AOM input contributed more to As release than endogenous sediment DOM, which ultimately provided more labile carbon (i.e., CHONS compounds) that was preferentially used by arsenate-, iron-, and sulfate-reducing bacteria (with 3.28-, 1.27-, and 4.88-fold increases in Geobacter, DsrB, and arsC gene copies, respectively). This process directly or indirectly enhanced As mobility during the initial anaerobic-aerobic stage. Subsequently, condensed aromatics and highly polymerized organic substances formed to further promote As migration during the final anaerobic stage. Integrating the As flux revealed a cumulative As release of 3371 µg∙m-2 driven by AOM input during the incubation period, which was 4.79 times that without AOM input. Overall, our findings indicated that redox fluctuations contributed more to As mobility when AOM was present compared with static conditions. These findings provide a theoretical basis for managing As risks in eutrophic lakes.