Transformation of algal-dissolved organic matter via sunlight-induced photochemical and microbial processes: interactions between two processes

Algal-dissolved organic matter (ADOM) is an important fraction of dissolved organic carbon (DOC) in eutrophic water. Although ADOM is known to be readily transformed by microbes, the role of sunlight-induced photochemical process and the interactions between two processes on ADOM transformation remains unclear. In this study, three types of treatments for ADOM, including photochemical process under natural solar light (L treatment), microbial process (M treatment), and the simultaneous photochemical plus microbial process (L&M), were performed for 18 days. Our results showed that M treatment was more effective for the loss of DOC, chromophoric DOM (CDOM) at short wavelengths (a254 and a280), than L treatment, while L treatment was more effective for the transformation of a350 and the fluorescent components of the ubiquitous humic-like component and the tryptophan-like component. Comparison in the decay kinetics of DOC and CDOM in the three treatments showed that the simultaneous photochemical and biological processes exhibited an inhibitory effect on DOC decay rate but not the percentage of labile DOC fraction. Higher relative abundance of protein-like substances was found after L&M treatment, while the relative abundance of humic-like substance and aromaticity increased after M treatment, and the low molecular-weight compounds were produced after L treatment. Our results emphasized the importance of photochemistry in processing ADOM to mediate the chemodiversity in natural water.