Date Published: May 7, 2019
Publisher: Public Library of Science
Author(s): Tomoyuki Kosaka, Yasuyuki Nakajima, Ayana Ishii, Maiko Yamashita, Saki Yoshida, Masayuki Murata, Kunpei Kato, Yuki Shiromaru, Shun Kato, Yu Kanasaki, Hirofumi Yoshikawa, Minenosuke Matsutani, Pornthap Thanonkeo, Mamoru Yamada, Shihui Yang.
The Intergovernmental Panel on Climate Change recommends keeping the increase in temperature to less than a two-degree increase by the end of the century, but the direct impact of global warming on ecosystems including microbes has not been investigated. Here we performed thermal adaptation of two species and three strains of mesophilic microbes for improvement of the survival upper limit of temperature, and the improvement was evaluated by a newly developed method. To understand the limitation and variation of thermal adaptation, experiments with mutators and by multiple cultures were performed. The results of experiments including genome sequencing and analysis of the characteristics of mutants suggest that these microbes bear a genomic potential to endure a 2–3°C rise in temperature but possess a limited variation of strategies for thermal adaptation.
According to the fifth Assessment Report of the Intergovernmental Panel on Climate Change regarding global warming over the next 100 years, in a scenario with the lowest average temperature rise, the temperature is predicted to rise by about 2 degrees, and in a scenario with the highest average temperature rise, the temperature is expected to rise by about 4 degrees. The fifth Assessment Report points out the possibility of various effects on society and the environment if the temperature continues to rise as it has been. Global warming may affect not only environments but also the diversity and populations of organisms. Each microorganism possesses a critical high temperature (CHT) that is an upper limit of survival as well as an optimum temperature for growth, probably due to the integration of various factors including general and unique metabolisms. Mesophilic microbes that have CHTs near the range of global warming would be crucially damaged by such a temperature rise. Their growth may be prevented or they may die, resulting in alterations in local ecosystems in which they are included. Alternatively, they might survive by adaptation to a temperature rise if effective mutations are accumulated. However, there has been no report on their capacity for adaptation to a rise in temperature and the variation in their strategies for adaptation, though there are many reports on E. coli regarding improvement of thermal stability and regarding the perspective of evolution to adapt to fixed or stepwise increasing temperatures [1–7]. In order to obtain clues for understanding the survival of microbes under the condition of progressive global warming and for preventing the extinction of species, we carried out in vitro thermal adaptation experiments using three strains of microbes with different CHTs: two strains of Zymomonas mobilis and one strain of Escherichia coli. The upper limit of thermal adaptation was further examined by using mutators that are defective in mismatch repair and thus have a high mutation frequency . The strategic variation for thermal adaptation was investigated by repetitive cultivations of 4 lines of the same strain that were performed in parallel. The thermal adaptation of isolated mutants was evaluated by determination of their CHTs, by physiological characterization and by genomic sequencing analysis. The results suggested the genomic capacity of these microbes for survival in global warming. Here we also discuss functional overlapping of thermal adaptive mutations with thermotolerant genes, which are essential for survival around the CHT, in mesophilic microbes including Z. mobilis and E. coli [9–12].
Previous adaptation experiments were carried out by repetitive cultivations in minimum or rich media at fixed temperatures from 20°C to 42°C [3, 5–7] or by a stepwise increase in temperature to 45.5°C  or 48.5°C [1, 4], and thermoadapted mutants isolated by a stepwise temperature upshift in a rich medium were shown to survive at equivalent high temperatures in a minimum medium . In this study using thermal adaptation experiments in a rich medium with two different species and three strains with distinct CHTs, there was an improvement in the CHTs up to 2 degrees (Table 1) and application of mutators to further improve the CHT resulted in an increase of one more degree in the CHT of Z. mobilis TISTR 548, thus giving a total increase of 3 degrees in the CHT. The increase in CHT of 3 degrees may be close to the limit of thermal adaptation considering the application of a mutator. Therefore, it is assumed that the CHT improvement potential in the genome is around 3 degrees in these mesophiles. Although there is no information on generation speed or mutation rate in nature, an increase of 2–3 degrees in global temperature is considered to have a strong impact on mesophiles and the ecosystems including these microbes.