Salinity-driven adaptations and evolution of DNA viruses in estuarine-coastal ecosystems.

Salinity gradients drive microbial diversity and evolution in estuarine-coastal ecosystems, yet viral adaptation remains less well understood. We used metagenomics to study viral adaptation and functions in three representative estuarine-coastal regions in China. Our results reveal salinity-associated adaptations in DNA viruses, with viruses enriched in medium- to high-salinity environments exhibiting higher frequencies of acidic isoelectric points and charged amino acids compared to those enriched in low-salinity environments. Viral genomes encode diverse genes related to ion transporters and organic osmolyte metabolism, suggesting potential roles in osmotic stress responses. Viral microdiversity also varied systematically along the salinity gradient, indicating reduced genetic variation and stronger purifying selection under more saline conditions. Furthermore, we identified diverse AMGs linked to nutrient cycles, with salinity-driven enrichment revealing viral roles in host metabolism. Overall, our findings highlight salinity as a key driver of viral evolution and functional potential in estuarine-coastal ecosystems, providing new insights into how viruses adapt to environmental gradients. IMPORTANCE: Salinity is a defining environmental gradient in estuarine-coastal systems, yet its role in shaping viral molecular evolution remains poorly understood. By integrating metagenomes, viromes, and metatranscriptomes across three estuaries, this study demonstrates that salinity exerts a strong and consistent imprint on DNA viruses. Increasing salinity selects for viral genomes encoding ion-transport and osmolyte-related proteins and drives systematic shifts in viral proteome composition toward osmoadaptive physicochemical properties. At the population level, higher salinity is associated with reduced viral microdiversity and stronger purifying selection, indicating constrained evolutionary space under osmotic stress. Viral auxiliary metabolic gene repertoires are structured along salinity gradients, with functional differentiation in carbon, nutrient, and nucleotide metabolism. Together, these findings identify salinity as a key evolutionary filter linking viral physiological adaptation, evolutionary dynamics, and functional potential in estuarine and coastal ecosystems.