Increasing Freshwater Salinity Impacts Aerosolized Bacteria

Increases in the salt concentration of freshwater result in detrimental impacts on water quality and ecosystem biodiversity. Biodiversity effects include freshwater microbiota, as increasing salinity can induce shifts in the structure of native freshwater bacterial communities, which could disturb their role in mediating basal ecosystem services. Moreover, salinity affects the wave breaking and bubble-bursting mechanisms via which water-to-air dispersal of bacteria occurs. Given this dual effect of freshwater salinity on waterborne bacterial communities and their aerosolization mechanism, further effects on aerosolized bacterial diversity and abundance are anticipated. Cumulative salt additions in the freshwater-euhaline continuum (0-35 g/kg) were administered to a freshwater sample aerosolized inside a breaking wave analogue tank. Waterborne and corresponding airborne bacteria were sampled at each salinity treatment and later analyzed for diversity and abundance. Results demonstrated that the airborne bacterial community was significantly different (PERMANOVA; F_1,22 = 155.1, r² = 0.38, p < 0.001) from the waterborne community. The relative aerosolization factor (r-AF), defined as the air-to-water relative abundance ratio, revealed that different bacterial families exhibited either an enhanced (r-AF ≫ 1), neutral (r-AF ∼ 1), or diminished (r-AF ≪ 1) transfer to the aerosol phase throughout the salinization gradient. Going from freshwater to euhaline conditions, aerosolized bacterial abundance exhibited a nonmonotonic response with a maximum peak at lower oligohaline conditions (0.5-1 g/kg), a decline at higher oligohaline conditions (5 g/kg), and a moderate increase at polyhaline-euhaline conditions (15-35 g/kg). Our results demonstrate that increases in freshwater salinity are likely to influence the abundance and diversity of aerosolized bacteria. These shifts in aerosolized bacterial communities might have broader implications on public health by increasing exposure to airborne pathogens via inhalation. Impacts on regional climate, related to changes in biological ice-nucleating particles (INPs) emission from freshwater, are also expected.