Chemical stress drives opposing bacterial biomass responses at the single-cell level.

Microbial biomass is a central functional trait governing nutrient turnover and trophic transfer in aquatic ecosystems, yet how chemical stress reshapes biomass at the level of individual cells remains poorly resolved. Conventional toxicity assays primarily detect population-level growth inhibition and overlook sublethal physiological responses and heterogeneity within microbial populations. Here, we quantify single-cell dry-mass, a label-free proxy for cellular biomass and biosynthetic state, in the fast-growing marine bacterium Vibrio natriegens. Across thousands of cells, we assessed how exposure to copper, zinc, diclofenac, bisphenol A, bisphenol E, and bisphenol Z alters cellular biomass. Chemical stress elicited contrasting responses: copper, zinc, and diclofenac reduced median dry-mass by up to 36%, whereas bisphenols increased median dry-mass by up to 24%. These changes were not simply proportional to population-level growth inhibition. Beyond shifts in central tendency, stress altered the shape and variability of biomass distributions, revealing additional distribution-level responses that are not detectable in bulk measurements. These results identify single-cell dry-mass as an integrative trait of microbial stress physiology and show that chemical perturbations can drive opposing cellular biomass responses that are not fully captured by population growth measurements. By linking chemical stress to microbial biomass traits, this work provides a controlled framework for testing cellular-scale responses in more complex ecological settings.