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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.

海洋溶解态有机物（marine dissolved organic matter, DOM）的主要去除过程包括光化学降解与微生物降解。然而，由于光辐照与生物降解之间存在复杂的相互作用，二者联合及反复作用对DOM的影响机制仍未得到充分阐明。本研究以中国厦门岛周边海域采集的海水为研究对象，探究了光降解与生物降解的反复作用对DOM分子组成以及海水微生物群落的影响。研究整体揭示了光化学与微生物降解对DOM库及微生物群落的选择性影响：光降解可导致荧光组分（包括蛋白类与腐殖酸类）的流失，富集脂肪族含氮化合物，并提升微生物多样性；而生物降解则会驱动关键分子组分发生变化，增强脂环族化合物以及带有羧基/酯基官能团的芳香族化合物的占比。网络分析结果表明，适应光辐照的微生物（如噬甲基菌科（Methylophagaceae））与非适应光辐照的微生物（如交替单胞菌科（Alteromonadaceae））分别通过潜在的（脱）甲基化与（脱）氧化作用，在关键分子的转化过程中发挥重要作用。本研究明确区分了光辐照与生物降解过程的选择性贡献，并着重强调了探究光化学与生物过程如何共同塑造DOM库的重要性。