Salinity Stress Modulates the Dynamic Co-occurrence Interactions between DOM and Microbial Community Profiles in a Typical River–Estuary–Ocean Continuum: From the Pearl River to South China Sea

The pivotal role of salinity stress in regulating the microbial-driven dissolved organic matter (DOM) transformation in river–estuary–ocean continua has consistently been overlooked. The difference in the osmotic pressure caused by the salinity fluctuations between freshwater and seawater results in the formation of distinct microbial community profiles, subsequently triggering dynamic DOM transformation. However, the complexity of the dynamic interactions poses a vital challenge in unraveling the underlying mechanisms at the molecular level. To bridge this gap, the utilization of FT-ICR MS, integrated with co-occurrence network, emerges as a potent tool. In this study, we demonstrated that, despite the major influence of urbanization on the DOM input, as characterized by spectroscopic characteristics, its impact on the transformation processes of DOM is negligible when compared to the salinity stress. In contrast, salinity can trigger similar transformation patterns of DOM among diverse microbial populations, underscoring the pivotal role of salinity. Concurrently, salinity enhances microbial transformations of DOM (e.g., higher biological index and the ratio of product to precursor), and furthermore, the gradual increase in total nitrogen with increasing salinity may be correlated with the salinity-induced suppression of denitrifying bacteria. The co-occurrence network analysis offers mechanistic insights into delineating the intricate interplay of synergism and antagonism among microbial DOM transformations under salinity conditions.