Hypersaline lakes harbor more active bacterial communities

Abstract

Extremophiles employ a diverse array of resistance strategies to thrive under harsh environmental conditions but maintaining these adaptations comes at an energetic cost. If energy reserves to drop too low, extremophiles may enter a dormant state of reduced metabolic activity to survive. Dormancy is frequently offered as a plausible explanation for the persistence of bacteria under suboptimal environmental conditions with the prevalence of this mechanism only expected to rise as stressful conditions intensify. We estimated dormancy in ten hypersaline and freshwater lakes across the Western United States. To our surprise, we found that extreme environmental conditions did not induce higher levels of bacterial dormancy. Based on our approach using rRNA:rDNA gene ratios to estimate activity, halophilic and halotolerant bacteria were classified as inactive at a similar percentage as freshwater bacteria, and the proportion of the community exhibiting dormancy was considerably lower (16%) in hypersaline than freshwater lakes across a range of cutoffs estimating activity. Of the multiple chemical characteristics we evaluated, salinity and, to a lesser extent, total phosphorus concentrations influenced activity. But instead of dormancy being more common as stressful conditions intensified, the percentage of the community residing in an inactive state decreased with increasing salinity in freshwater and hypersaline lakes, suggesting that salinity acts as a strong environmental filter selecting for bacteria that persist and thrive under saltier conditions. Within the compositionally distinct and less diverse hypersaline communities, abundant taxa were disproportionately active and localized in families Microbacteriaceae (Actinobacteria), Nitriliruptoraceae (Actinobacteria), and Rhodobacteraceae (Alphaproteobacteria). Our results demonstrate that extreme environments may not necessarily be stressful or suboptimal for highly adapted extremophiles causing them to need dormancy less often to survive.