Phenotypic plasticity and evolution of thermal tolerance in bacteria from temperate and hot spring environments

Phenotypic plasticity allows individuals to respond to the selective forces of a new environment, followed by adaptive evolution. We do not know to what extent phenotypic plasticity allows thermal tolerance evolution in bacteria at the border of their physiological limits. We analyzed growth and reaction norms to temperature of strains of two bacterial lineages, Bacillus cereus sensu lato and Bacillus subtilis sensu lato, that evolved in two contrasting environments, a temperate lagoon (T) and a hot spring (H). Our results showed that despite the co-occurrence of members of both lineages in the two contrasting environments, norms of reactions to temperature exhibited a similar pattern only in strains within the lineages, suggesting fixed phenotypic plasticity. Additionally, strains from the H environment showed only two to three degrees centigrade more heat tolerance than strains from the T environment. Their viability decreased at temperatures above their optimal for growth, particularly for the B. cereus lineage. However, sporulation occurred at all temperatures, consistent with the known cell population heterogeneity that allows the Bacillus to anticipate adversity. We suggest that these mesophilic strains survive in the hot-spring as spores and complete their life cycle of germination and growth during intermittent opportunities of moderate temperatures. The limited evolutionary changes towards an increase in heat tolerance in bacteria should alert us of the negative impact of climate change on all biological cycles in the planet, which at its most basic level depends on microorganisms.