Rapid Adaptation to Road Salts in a Freshwater Microbial Eukaryote

Humans are changing habitat for wildlife in myriad ways and for populations to persist, they must adapt to this change. In parts of the world that experience snow and ice, road salts are often used to make driving safer in the winter. Runoff from these roads increases the salinity in nearby bodies of water, which has been shown to have detrimental physiological and ecological effects in freshwater ecosystems; however, the evolutionary consequences of salinization remain unclear. Tetrahymena are microbial eukaryotes that live in freshwater habitats and serve as an important link in the microbial food loop. In this study, we tested how T. thermophila can evolve in response to increasing concentrations of road salts in their environment. Using experimental evolution, we found that T. thermophila populations adapted to survive and grow in concentrations of up to 18 g/L of NaCl and 17 g/L MgCl₂, approximately twice the salinity tolerance of ancestral populations. However, populations adapted to the highest salt concentrations experience fitness trade-offs of reduced survival and growth rate and longer lag times when grown in salt-free environments. These results demonstrate the rapidity with which microbial populations can respond to anthropogenic changes to their environment, yet highlight the potential costs associated with this adaptation.