Impacts of repeated photochemical and microbial processes: Selectively shaping of the dissolved organic matter pool

Abiotic photochemistry and microbial degradation are the two main removal processes of marine dissolved organic matter (DOM). However, the combined and repeated effects of irradiation and biodegradation on DOM remains poorly resolved due to their complex interactions. To disentangle the effects of abiotic photochemistry from photobiology, we alternately exposed coastal DOM to repeated exposures to simulated solar radiation and then to microbial communities in darkness. Our results demonstrated selective impacts on the DOM pool by photochemical and microbial degradation. Photodegradation resulted in the loss of fluorescent (both protein- and humic-like), the enrichment of aliphatic and nitrogen-containing compounds, and an increase in microbial diversity. However, biodegradation drove changes in key molecules (significantly altered) and enhanced the contribution of alicyclic compounds and aromatic compounds containing carboxyl/ester functional groups. Network analysis implicated the irradiation adapted (i.e., Methylophagaceae) microbes in DOM transformations involving the gain and loss of methyl groups, while the non-irradiation adapted (i.e., Alteromonadaceae) microbes appeared to alter DOM composition by the gain and loss of oxygen atoms. Our findings distinguish the selective contributions of irradiation and biodegradation processes and point to the complex interactions between photochemical and biological processes that jointly shape the DOM pool.