Limits of bacterial osmoadaptation during planktonic and biofilm growth: a step toward effective biofouling control

Salinity is a key parameter for bacterial survival and growth. Halophilic and halotolerant bacteria can adapt to elevated salinity, but the energetic demands of osmoadaptation increase under fluctuating salt concentrations, potentially constraining growth and persistence. A new concept in reverse osmosis (RO) filtration is batch operation with oscillating rather than constantly high brine salinity. We hypothesize that fluctuating salinity can diminish biofouling in such RO systems. To test this hypothesis, we examined the survival and activity of Aliivibrio fischeri and Pseudomonas fluorescens under fluctuating salinities in planktonic and biofilm cultures as representative of halophilic and halotolerant species, respectively, and common members of biofouling communities. At 28°C, P. fluorescens grew at 0%-6% salinity with fastest growth rate at 0%-1%. At 7%-10% salinity, P. fluorescens remained viable but did not grow. At 22°C, A. fischeri grew at 0.5%-7% salinity, with fastest growth rate at 2%-3%, but unlike P. fluorescens, it lost viability outside this growth range. Cultures did not respire at salinities that did not support growth, suggesting that survival under such salt stress does not depend on high metabolic activity. Furthermore, cell-specific aerobic respiration rates in A. fischeri correlated with growth rate but not osmotic stress. Biofilm formation did not enhance the osmotic stress tolerance of the two bacteria. Our results indicate that high constant salinity favors the halophilic A. fischeri over the halotolerant P. fluorescens, but oscillating salinity (e.g., 0%-7%) favors neither. Oscillating salinity may, therefore, offer a new mechanism for controlling microbial growth that circumvents community adaptation to environmental conditions.IMPORTANCEReverse osmosis filtration is a widely used technology to address the scarcity of clean freshwater. However, the efficiency of reverse osmosis systems is challenged by microbial biofouling, as microbial communities adapt to the environmental conditions within the system and form biofilms on the membranes. This study investigated the impact of fluctuating salinity on the growth and survival of halophilic and halotolerant bacteria. The findings suggest that oscillating salinity disrupts the growth and viability of both types of bacteria, in both planktonic cultures and biofilms. The study, thus, supports the hypothesis that fluctuating salinity in reverse osmosis systems could reduce biofouling by impeding microbial adaptation to salinity. This represents a promising new strategy for microbial control in reverse osmosis systems, potentially enhancing performance by minimizing biofouling through an environmentally friendly approach.