Asymmetric Osmoadaptive Responses in Intermediate-Salinity Microbial Communities Revealed by Metatranscriptomics.

Salinity is a dominant ecological driver shaping microbial community structure and function in hypersaline environments. Here, we investigated transcriptional responses to rapid salinity fluctuations using metatranscriptomic analyses of an intermediate-salinity brine sample from the Santa Pola solar salterns (Alicante, Spain). To this end, two experimental conditions were applied: salinity increase (12.4% to 17%) and salinity dilution (12.4% to 7%). Differential gene expression, functional enrichment, and protein isoelectric point (pI) distributions were analyzed to characterize osmoadaptive mechanisms. Salinity increase triggered a stress-dominated response characterized by upregulation of compatible solute biosynthesis (e.g., glycine betaine and ectoine), protein turnover, and chaperone activity, alongside repression of translation, energy metabolism, and transport systems. In contrast, salinity dilution induced metabolic reactivation, including enhanced translation, energy production, and osmolyte degradation pathways, indicating recovery from osmotic stress. Functional shifts were accompanied by changes in proteome physicochemical properties, with increased salinity promoting a shift toward higher pI proteins, consistent with salt-out strategies. These findings reveal a highly dynamic and asymmetric transcriptional plasticity, where osmotic upshift imposes stronger constraints than downshift, driving coordinated metabolic reprogramming and proteome restructuring in intermediate-salinity microbial communities.