Despite the crucial role of microorganisms to sustain life on Earth, there is little research on the evolution of thermal tolerance of bacteria in the face of the challenge that global warming poses. Phenotypic adaptation to a new environment requires plasticity to allow individuals to respond to selective forces, 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 thermal 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 co-occurrence of members of both lineages in the two contrasting environments, norms of reactions to temperature exhibited a similar pattern only within the lineages, suggesting fixed phenotypic plasticity. Additionally, within the B. cereus lineage, strains from the H environment showed only two to three oC more heat tolerance than strains from the T environment. The limited evolutionary changes towards an increase in heat tolerance in bacteria should alert us of the negative impact that climate change can have on all biological cycles in the planet.