An experimental evolutionary study on adaptation to temporally fluctuating pH in Escherichia coli

In this study, we use the bacterium Escherichia coli to examine evolutionary responses to environmental acidity fluctuating temporally among pH 5.3, 6.3, 7.0, and 7.8 (5,000-15 nM [H(+)]). Two experimental protocols of temporal variation were used. One group (six replicate lines) of populations evolved for 2,000 generations during exposure to a cycled regime fluctuating daily between pH 5.3 and 7.8. The other group (also in six replicate lines) evolved during exposure for 2,000 generations to a randomly shifting regime fluctuating stochastically each day among pH 5.3, 6.3, 7.0, and 7.8. Adaptation to these fluctuating acidity regimes was measured as a change in fitness relative to the common ancestor by direct competition experiments in both constant and fluctuating pH regimes. For comparisons with constant pH evolution, a group evolved at a constant pH of 5.3 and another group evolved at pH 7.8 were also tested. This study initiated the first long-term laboratory natural selection experiment on adaptation to variable acidity and addressed key questions concerning patterns of adaptation (trade-offs, specialists, generalists, plasticity, transitions, and acclimation) in temporally fluctuating environments.