The limits of microbial life.


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The limits of microbial life.

Microbial life has adapted to all ecosystems and can be found even in – what we consider – extreme environments. Microbes are found in acidic lakes, hot springs, under glaciers, but also 10 km under the surface, at the bottom of the ocean. These microorganisms are called extremophiles or polyextremophiles when they can survive one or more ‘extreme’ environmental conditions. Whether microbes can simultaneously adapt to very low pH, high salt and high temperature was still unclear… until now!

In their study, Jodie Belilla and collaborators went to the Dallol geothermal area in Ethiopia. As the centre of a volcanic area, it combines polyextreme gradients such as low pH (pH = 0), high salinity (at least 30% salinity) and high temperature (up to 110°C), making it the ideal place to look for polyextremophiles. Compared to ocean surface water that has an average surface temperature of 17°C, salinity between 3 and 5 %, and a pH around 8 – this is definitely extreme.     

The research team collected samples in different areas of Dallol, extracted and sequenced DNA in an attempt to identify specimens from the three domains of life (bacteria, archaea and eukaryotes). They observed extremophiles, primarily archaea, in hypersaline (>30%) but otherwise neutral areas (pH = 6 and temperature =20 – 30°C). Although archaea can be found in most ecosystems (as can bacteria and eukaryotes), they are often considered extremophiles and their abundance in these Dallol systems further support the claim. 

Image: By sampling in different areas, the authors found the limits of life.

However, the authors were surprised to see that the most extreme areas of Dallol (with pH < 3, temperatures > 35°C and salt > 30%) did not support microbial life. After rigorous testing using culturing and microscopy, they came to the same conclusion: they’ve found the limits of life in Dallol. 

It appears that the combination of hyperacid-hypersaline conditions serves as a determinant factor in contributing to an inhospitable environment. They suggest that adaptations to survive simultaneously a very low pH and high salinity are not compatible beyond those limits, regardless of temperature. Authors think that commonly found membrane protection mechanisms in acidic environments, such as K+ accumulation or ‘salt-in’ strategy, isn’t possible in these conditions. Future studies could help identify these limitations in more detail and the molecular mechanisms limiting the adaptation of life in combination of extreme environments.

Link to the original post: Belilla, J., Moreira, D., Jardillier, L. et al. Hyperdiverse archaea near life limits at the polyextreme geothermal Dallol area. Nat Ecol Evol 3, 1552–1561 (2019)

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Written by: Lucie Malard