Temperature is a key variable in biological processes. However, a full understanding of biological temperature adaptation needs to be improved, in part because of the unique constraints between different evolutionary lineages and physiological groups.
This begs the question, how does life adapt to different temperatures? To unravel this question, a research team led by Paula Prondzinsky and Shawn Erin McGlynn of the Tokyo Institute of Technology’s Earth-Life Science Institute (ELSI) recently examined 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 lack nuclei and are believed to have been the precursors of eukaryotic cells). Methanogens are ideal organisms to study temperature adaptation because 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 types of methanogens. They divided the methanogens into three groups based on the temperatures in which they thrived: thermotolerant (high temperatures), psychrotolerant (low temperatures), and mesophilic (ambient temperatures).
The Genome Taxonomy Database was then used to create a database of 255 genomes and protein sequences. The Database of Growth TEMPeratures of Normal and Rare Prokaryotes was then used to obtain temperature information for 86 methanogens held in laboratory collections. The result was a database that linked genome content to growth temperature.
Scientists then used a software called OrthoFinder to pinpoint different ortho groups — sets of genes descended from a single gene that was present in the last common ancestor of the species in question.
They then divided these orthogroups into three categories: core (present in more than 95% of species), shared (present in at least two species but less than 95% of organisms), and unique (present in only one species) ( present in only one species). According to their research, all animals share about a third of the methanogenic genome. They also found 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. It was also discovered that the “age” of these small genomes was older than that of psychrotolerant strains. These results suggest that genome size is more dependent on temperature than on the course of evolution, because thermotolerant species were discovered in different groups.
They also argue that instead of contracting as methanogenic genomes evolved, they increased, contradicting the theory of “thermoreductive genome evolution,” according to which organisms lose genes as they adapt to higher temperatures.
Analyzes performed by the researchers also revealed that methanogens could thrive in this wide range of temperatures without the need for many unique proteins. In fact, their genomes encoded similar proteins for most of them.
They speculate that the underlying mechanism of temperature adjustment could be cellular control or small-scale compositional changes. They investigated this by analyzing the amino acid composition of the methanogens, the building blocks of proteins.
They found that certain temperature groups were enriched in certain amino acids. They also found variations in the composition of the amino acids related to their proteome charge, polarity and unfolding entropy, all of which affect protein structure and, consequently, its functionality. They found that thermotolerant methanogens generally had more charged amino acids and functional ion transporter genes than psychrotolerant ones.
Psychrotolerant organisms, on the other hand, have an abundance of proteins and uncharged amino acids essential for cellular structure and movement. The fact that the scientists couldn’t identify specific roles that each member of a temperature group shared suggests that temperature adaptation happens gradually and in small increments rather than requiring drastic changes.
Paula Prondzinsky said: “This indicates that the first methanogens, which evolved when Earth’s conditions were hostile to life, may have been similar to the organisms found on Earth today. Our findings could indicate properties and functions present in the earliest microbes and even contain clues about whether microbial life originated in warm or cold environments. We could extend this knowledge to understand how life can adapt to other types of extreme conditions, not just temperature, and even can unravel how life on other planets may evolve.”
- Paula Prondzinsky1,2,*, Sakae Toyoda2, Shawn Erin McGlynn1,3,4*, The methanogen core and the pangenome: conservation and variability among biology’s extreme growth temperatures, DNA research, DOI: 10.1093/dnares/dsac048