Novel bacterial strains of bacteria first isolated on the International Space Station, did the space environment lead to these genetic changes?

Question

Question

Methylobacterium ajmalii sp. nov., Isolated From the International Space Station (Bijlani et al. Frontiers in Microbiology, 12, p. 534, 2021) is a thorough analysis of "novel strains" of bacteria isolated from the International Space Station (Characterization of the total and viable bacterial and fungal communities associated with the International Space Station surfaces)

Question: Bijlani et al. 2021 is written at a very dense and technical level that is challenging for me to understand. I'd like to know:

1. Does the article suggests these may be newly discovered species or only that they are novel strains of known species of Earth bacteria?
2. Does the article suggest that the environment on the ISS selected for these species or strains, and they resulted from some genetic changes due to the ISS environment, or if we looked hard enough we'd expect to find the same ones on Earth and they just happened to be identified first on the ISS because of the rigorous studies of those samples?

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Background

I was lead to the paper after seeing the ink the BBC's Could humans have contaminated Mars with life? which is primarily about a different topic.

The paper is one of 10 papers the 2021 topic Extremophiles: Microbial Genomics and Taxogenomics.

The ISS is a unique closed environment with plenty of surfaces with sometimes problematic mold growth, and a population of usually 3-6 humans with regular rotations every 5 to 6 months who maintain an array of biological experiments including a sustained effort to try to grow vegetables and flower hydroponically and in soil.

The atmosphere is maintained similar to Earth's surface (oxygen/nitrogen ~1 bar) with 60% relative humidity and often has a CO2 concentration slightly higher than the astronauts would prefer.

Answer 1

1. The authors propose that this is a distinct species based on a number of physiological and genetic tests. To quote the summary of your linked paper

In summary, the phylogenetic and genetic distinctiveness and differential phenotypic properties were sufficient to categorize these three ISS strains as members of a species distinct from other recognized Methylobacterium species. Therefore, on the basis of the data presented, strains IF7SW-B2T, IIF1SW-B5, and IIF4SW-B5 represent a novel species of the genus Methylobacterium, for which the name Methylobacterium ajmalii sp. nov. is proposed.

This indicates (emphasis mine) that they think that these isolates are distinct enough from species that we have found on Earth to make the isolates a species. This is a species that we haven't found on Earth, but unsurprisingly it is closely related to species from the same genus that have been found on Earth and on the ISS

2. The authors make no statements about how the species came to be there that I could see, only that it is a novel species from the ISS, which has not been found on Earth. No mechanisms for arisal are postulated, and the authors wisely did not speculate on this at all (speculation on results is frowned upon in science). One of the reasons they did not speculate on this will be that testing mechanisms whereby speciation has occurred is very difficult, especially as they would need to mimic in the lab the conditions in the ISS and show which conditions were important for the speciation. These experiments would be technically challenging and very time consuming.

It is worth noting that we don't really know the rate of species differentiation on Earth, let alone in space, partly because Earth is so covered in bacteria that proving that one evolved in the environment is difficult as it is hard to prove that you didn't just miss it last time you looked. To quote the paper I linked above:

For example, a single gram of soil can harbour up to 10¹⁰ bacterial cells and an estimated species diversity of between 4·10³ to 5·10⁴ species .

However, the ISS is one environment where it might be easier to see these sorts of events because it is relatively micro-organism free, and we have prior samples to go back and compare to. For some comparison of the numbers, I turn to the second paper you linked, which found:

Of the total bacterial and fungal isolates that grew, 133 bacterial isolates and 81 fungal isolates were identified by Sanger sequencing... ...When the ASVs were summarized to the genus level, 121 taxa were detected, 77 of which could be assigned to known genera

and

Overall, the number of bacteria (combination of R2A and BA growth) isolated from the ISS from all 24 samples ranged from 6.7 × 10³ to 7.8 × 10¹⁰ CFU/m²

So quite high numbers of cultivatable bacteria (CFU = colony forming units, a measure of how many bacteria will grow per sampling), indeed as high as those found in a gram of soil, but very low species diversity compared to the soil.

It is still difficult to prove that this is a new species evolved strictly on the ISS, but it does seem logically likely that evolution will happen on the ISS, as it does anywhere that life is present (as far as we know).