Temperature is a key variable in biological processes. However, a complete understanding of biological temperature adaptation needs to be improved partly because of the unique constraints among different evolutionary lineages and physiological groups.
This begs the question, how does life adapt to different temperatures? To attempt to unravel this question, a research team led by Paula Prondzinsky and Shawn Erin McGlynn of the Earth-Life Science Institute (ELSI) at the Tokyo Institute of Technology recently investigated a group of organisms called methanogens.
Methanogens are single-celled microbes that produce methane and are part of the “Archaea” phylum (ancient, single-celled organisms that do not have cell nuclei and are thought to have been the predecessor to Eukaryotic cells). Methanogens are the ideal organisms to research temperature adaptation since they can survive in a wide range of temperature extremes, from -2.5 oC to 122 oC.
Scientists analyzed and compared the genomes of different species of methanogens. They divided the methanogens into three groups based on the temperatures they thrived in—thermotolerant (high temperatures), psychrotolerant (low temperatures), and mesophilic (ambient temperatures).
The Genome Taxonomy Database was then used to create a 255 genomes and protein sequences database. The Database of Growth TEMPeratures of Normal and Rare Prokaryotes was then used to get temperature information for 86 methanogens kept in laboratory collections. A database that connected genome content to growth temperature was the result.
Scientists then used a software called OrthoFinder to establish different orthogroups—sets of genes descended from a single gene present in the last common ancestor of the species under consideration.
Following that, they divided these orthogroups into three categories: core (present in over 95% of species), shared (present in at least two species but in less than 95% of organisms), and unique (present in just one species) (present only in a single species). According to their research, all animals share around one-third of the methanogenic genome. They also discovered that as evolutionary distance increases, the proportion of genes shared by different species decreases.
Interestingly, the scientists found that thermotolerant organisms had smaller genomes and a higher percentage of the core genome. Also, it was discovered that the “age” of these tiny genomes was older than that of psychrotolerant species. These results suggest that the size of the genome is more dependent on temperature than on the course of evolution because thermotolerant species were discovered in various groupings.
They also contend that rather than contracting as methanogen genomes evolved, they increased, which contradicts the theory of “thermoreductive genome evolution,” according to which organisms lose genes as they adapt to higher temperatures.
Analyses conducted by the researchers also revealed that methanogens could thrive in this wide temperature range without the need for many unique proteins. In actuality, their genomes encoded similar proteins for the majority of them.
They speculate that the underlying mechanism of temperature adaptation might be cellular control or smaller-scale compositional alterations. They investigated this by analyzing the amino acid makeup of the methanogens, which are the building blocks of proteins.
They discovered that particular temperature groups were enriched in certain amino acids. Also, they found compositional variations in the amino acids related to their proteome charge, polarity, and unfolding entropy, all of which impact protein structure and, consequently, its functionality. They discovered that, in general, thermotolerant methanogens had more charged amino acids and functional ion transport genes than psychrotolerant ones do.
In contrast, psychrotolerant organisms have an abundance of proteins and uncharged amino acids essential for cellular structure and movement. The fact that the scientists could not identify any particular roles that each member of a temperature group shared suggests that temperature adaptation happens gradually and in little steps rather than requiring drastic alterations.
Paula Prondzinsky said, “This indicates that the first methanogens, which evolved when the conditions on the Earth were hostile to life, may have been similar to the organisms we find on present-day Earth. Our findings could point toward traits and functions present in the earliest microbes and even hold clues as to whether microbial life originated in hot or cold environments. We could extend this knowledge to understand how life could adapt to other kinds of extreme conditions, not just temperature, and even unravel how life on other planets could evolve.”