Microbe threat to Mars


Experiments suggest an ancient survival strategy on Earth could benefit bacterial hitch-hikers. Andrew Masterson reports.


Stromatolites in northern Western Australia -- made by a biofilm-forming organism.
Stromatolites in northern Western Australia -- made by a biofilm-forming organism.
Ted Mead / Getty

The presence of water on Mars increases the possibility that human missions to the planet will inadvertently contaminate it with Earth species. And new research has uncovered the most likely potential threat.

Some planetary scientists have suggested the danger of microbial contamination on Mars is reduced because any water on the planet – such as that identified at the equatorial Gale Crater in 2015 – is hyper-briny and likely shot through with bio-unfriendly ingredients such as high levels of sulfate.

Research led by astrobiologist Adam Stevens of the University of Edinburgh in Scotland, however, indicates that some types of terrestrial life might be tough enough to survive in such harsh conditions.

Stevens and his colleagues looked at biofilm-forming microbes. These are species that clump together on surfaces and embed themselves in a matrix of polysaccharides, proteins, DNA and fats (known collectively as “extracellular polymeric substances”).

Biofilm-making microbes are found among bacteria and archaea, and the process is also conducted by some more complex species, including certain fungi and algae.

Embedding in a biofilm offers increased protection from the environment, and typically affects gene regulation. Biofilm microbe colonies tend to grow very slowly, be highly resilient, and extremely long-lived.

These are qualities that might make them well suited to gaining a foothold in the wet areas of Mars, Stevens’ team discovered.

In a series of experiments, the astrobiologists used a species of bacteria called Sphingomonas desiccabilis, found in the soil on the US Colorado Plateau. The microbe can survive in extremely dry conditions, and accumulates in self-made crusts.

The scientists created a number of brines thought to be identical to, or very similar to, those currently on Mars, and exposed the bacteria to them. Plankton cells were also exposed, as controls.

The results showed that while some brines were so concentrated that they killed everything put in them, others allowed S. desiccabilis to survive considerable lengths of time before succumbing.

The longevity of the bacteria was extended if they were fully dried out before immersion.

In a paper currently on the pre-print server biorix, the researchers conclude that biofilms provide protection in conditions “potentially analogous” to those on Mars.

“Our results show that contaminant biofilm-forming microorganisms may have a greater chance of surviving,” in briny areas on the planet, they write, “with implications for planetary protection in missions that aim to explore these regions.”

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Andrew Masterson is news editor of Cosmos.
  1. https://www.nature.com/articles/ngeo2412
  2. https://www.biorxiv.org/content/early/2017/01/10/099374
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