Capacity for survival in global warming: Adaptation of mesophiles to the temperature upper limit

Abstract

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.

Improvement of Thermotolerance of Zymomonas mobilis by Genes for Reactive Oxygen Species-Scavenging Enzymes and Heat Shock Proteins

“…Bacterial growth was monitored by measuring the optical density of the culture on a spectrophotometer (HITACHI, U-200) at OD 550 . Determination of the CHT was carried out by the two-step cultivation assay as described previously (Kosaka et al, 2019). For measuring the cell length, 12-h cultivated cells in the first culture of the two-step cultivation were collected by low-speed centrifugation, washed, and resuspended in a saline solution.…”

“…The level of intracellular ROS was determined using a fluorescence probe, 2 ,7 -dichlorofluorescin diacetate (H2DCFDA) (Pérez-Gallardo et al, 2013), as described previously (Kosaka et al, 2019). Cells that were grown for 12 h at 38 • C in the first culture of the two-step cultivation assay were mixed with H2DCFDA at the final concentration of 5 µM, incubated for 30 min, and collected as a pellet by centrifugation (14,000 rpm) for 1 min.…”

“…Microorganisms intrinsically have an upper temperature limit for survival called a critical high temperature (CHT) (Matsushita et al, 2015;Kosaka et al, 2019). Genome-wide analysis of three mesophiles, Escherichia coli, Acetobacter tropicalis, and Zymomonas mobilis, by screening thermosensitive mutants either with a single-knockout mutant library or with a transposoninserted mutant library has revealed that about 1.5% of genomic genes, called thermotolerant genes, are responsible for cell survival at a CHT (Charoensuk et al, 2017;Murata et al, 2018), but there is no sufficient information to conclude that a mesophile with a larger number of genomic genes has a larger number of thermotolerant genes and thus tends to be more temperatureresistant.…”

“…The CHT differs to some extent from strain to strain even in the same species such that relatively thermotolerant strains can be isolated in tropical areas, assuming that thermotolerant strains have adapted to the environmental temperature. The thermotolerance of mesophiles, E. coli W3110, Z. mobilis CP4, and Z. mobilis TISTR 548, has been improved by in vivo thermal adaptation, suggesting that they have a genomic capacity for adaptation to higher temperature environments (Kosaka et al, 2019). The capacity, however, is limited to 2-3 • , and the variation of thermal adaptation is also restricted.…”

“…The capacity, however, is limited to 2-3 • , and the variation of thermal adaptation is also restricted. The change of only a few degrees is physiologically important, which was determined by an accurate method, called a two-step cultivation assay that eliminates the effects of start temperature and can distinguish CHT differences between two different strains of the same species (Kosaka et al, 2019).…”