Phylogenetic studies suggest that LUCA, the last universal common ancestor of Bacteria and Archaea, was a thermophile.1 This implies that at some point in the history of Early Earth, mesophilic organisms may have evolved from thermophilic organisms, but laboratory experiments towards that are missing. The order Thermotogales contains mesophilic and thermophilic members with a broad temperature range2 , and it has been hypothesized that the former may have evolved from the latter within the order.3 T. maritima is one of the few known hyperthermophilic bacteria with an optimal growth temperature of 80 °C.4
In a first attempt, we characterized the phenotype of T. maritima at suboptimal temperatures. Interestingly, we observed growth at 45° C, 10 K below the published TMIN, at a specific growth rate of 0.01 h-1. Growth was accompanied by dramatic changes in morphology, with larger cells compared to their counterparts grown at 80 °C. We analysed fermentation products at different temperatures and observed higher concentrations of lactate during incubation at 45 and 50 °C.
To study adaptions towards lower temperatures, we then serially transfered T. maritima at 45 °C and 50 °C in an ongoing adaptive laboratory evolution experiment (ALE) (Fig: 1).
After 25 transfers at 45 °C and after 60 transfers at 50 °C two passaged populations (ATM45 and ATM50), show shifts in their growth rates compared to the type strain at different temperatures. While 80 °C seems to stay the optimum for one population (ATM50), growth rates at lower temperatures were increased in both adapted populations. In addition, ODMAX of ATM50 increased about 20-25 %. For ATM45 overall higher growth rates could be observed at all temperatures.
Currently, we are studying the molecular basis for these phenotypes. We anticipate that these adapted strains will allow us to gain insights into the evolution of thermophiles towards lower temperatures.
References
1. Weiss M., et al. NatMicrobiol (2016). 2. Mori K. et al. Microbes Environ. (2020)
3. Dipippo J. et al. Int J Syst Evol Microbiol.(2009) 4. Huber, R. et al. Arch. Microbiol. (1986)