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

Photochemical 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. In this study, we investigated the repeated effects of photodegradation and biodegradation on the molecular composition of DOM and the microbial community in seawater collected off Xiamen Island, China. Our study overall demonstrated the selective impacts on the DOM pool and microbes 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 and enhanced the contribution of alicyclic compounds and aromatic compounds containing carboxyl/ester functional groups. Network analysis demonstrated the importance of irradiation adapted (i.e., <i>Methylophagaceae</i>) and<i> </i>non-irradiation adapted (i.e., <i>Alteromonadaceae</i>) microbes in the transformation process among the key molecules through potentially (de)methylation and (de)oxidation, respectively. Our findings distinguish the selective contributions of irradiation and biodegradation processes and clearly emphasize the importance of studying how photochemical and biological processes jointly shape the DOM pool.