D. radiodurans is a remarkable bacterium that can survive extreme levels of radiation, cold, dehydration, vacuum, and acid. It is known as a polyextremophile and has been listed as the world’s toughest bacterium in The Guinness Book of World Records . But which of these is most closely related to D. radiodurans? Is it another extremophile, a common soil bacterium, or something else entirely?
The Genus Deinococcus
To answer this question, we need to look at the genus Deinococcus, which is the taxonomic group that D. radiodurans belongs to. Deinococcus is one of three genera in the order Deinococcales, which is part of the phylum Deinococcus-Thermus . Deinococcus-Thermus is a distinct lineage of bacteria that diverged from other groups early in the evolution of life .
Deinococcus contains 12 species, all of which are radioresistant, meaning they can withstand high doses of ionizing radiation . D. radiodurans is the type species and the best studied member of this genus. It was discovered in 1956 in a can of ground meat that had been irradiated to sterilize it . Since then, it has been isolated from various habitats, mostly soil and feces based .
The Closest Relative of D. Radiodurans
The closest relative of D. radiodurans is another species of Deinococcus, called D. geothermalis. This species was isolated from hot springs in Italy and Iceland, and is also thermophilic, meaning it can grow at high temperatures . D. geothermalis shares 91% of its 16S rRNA gene sequence with D. radiodurans, which is a common marker for bacterial phylogeny . It also has a similar genome structure, consisting of two circular chromosomes and two plasmids .
D. geothermalis and D. radiodurans have many similarities in their cell structure and metabolism. They both form spherical cells that stick together in pairs or tetrads. They both have multiple copies of their genes, which help them repair their DNA after radiation damage. They both are gram-positive, aerobic, and heterotrophic, meaning they have a thick cell wall, use oxygen, and obtain carbon from organic sources .
The Differences Between D. Radiodurans and D. Geothermalis
Despite their close relationship, D. radiodurans and D. geothermalis have some notable differences. The most obvious one is their temperature preference. D. radiodurans is a mesophile, meaning it grows best at moderate temperatures, around 30-37°C. D. geothermalis is a thermophile, meaning it grows best at high temperatures, around 50-65°C .
Another difference is their resistance to other stress factors. D. radiodurans is more resistant to desiccation, UV light, and hydrogen peroxide than D. geothermalis. D. geothermalis is more resistant to acid and alkali than D. radiodurans . These differences reflect their adaptation to different environments, where they face different challenges.
The Implications of Their Relationship
The relationship between D. radiodurans and D. geothermalis has implications for the origin and evolution of radioresistance in bacteria. It suggests that radioresistance is an ancient trait that was present in the common ancestor of Deinococcus, and that it was maintained and diversified in different species . It also suggests that radioresistance is not a specific adaptation to radiation, but a by-product of a general stress response that protects the cells from various environmental insults .
The relationship between D. radiodurans and D. geothermalis also has implications for biotechnology and astrobiology. Both species have potential applications in bioremediation, the use of microorganisms to degrade or detoxify pollutants. D. radiodurans can degrade organic compounds such as toluene and phenol, and can also reduce toxic metals such as chromium and uranium . D. geothermalis can degrade aromatic compounds such as naphthalene and phenanthrene, and can also produce hydrogen gas from glucose .
Both species also have potential applications in astrobiology, the study of life in the universe. D. radiodurans and D. geothermalis are models for studying the limits of life and the possibility of life on other planets. D. radiodurans has been shown to survive in simulated Martian conditions, and has also been sent to space and exposed to the vacuum and radiation of outer space . D. geothermalis has been shown to survive in simulated hydrothermal vent conditions, and has also been proposed as a candidate for life on Europa, a moon of Jupiter that has a subsurface ocean .
D. radiodurans is a fascinating bacterium that can survive extreme levels of radiation and other stress factors. It belongs to the genus Deinococcus, which contains 12 radioresistant species. The closest relative of D. radiodurans is D. geothermalis, a thermophilic bacterium that lives in hot springs. They share many similarities in their genome, cell structure, and metabolism, but also have some differences in their temperature preference and resistance to other stress factors. Their relationship has implications for the origin and evolution of radioresistance, as well as for biotechnology and astrobiology.