In the search for possible life on exoplanets, i.e., planets outside our solar system, researchers look at whether the conditions on these planets could support lifeforms similar to those on Earth. They consider, for example, the temperature, the availability of water, and the chemical nature of the planet’s atmosphere. Exoplanets with a hydrogen-rich atmosphere are likely to exist and should be observable with current telescope technology. However, whether life can exist under such an atmosphere had not been thoroughly investigated so far.
Sara Seager, Massachusetts Institute of Technology (MIT), Cambridge, USA, and colleagues have found that microorganisms which do not usually live in H2-rich environments can survive and grow in a pure H2 atmosphere. The team grew cultures of Escherichia coli bacteria and Saccharomyces cerevisiae (yeast) and incubated them at 28 °C in culture bottles under an H2 atmosphere. The team also incubated the same microorganisms under air, 100 % He, or a gas mixture of 20 % CO2 and 80 % N2. The team used oxygen sensors for each culture bottle to ensure that the conditions for the H2, He, and CO2/N2 groups remained oxygen-free during the incubation period.
The researchers found that E. coli can live and reproduce normally under an H2 atmosphere. The maximal cell concentration is, however, lower than for E. coli grown in air. The team attributes this to the bacteria switching from aerobic respiration to less efficient metabolic processes based on anaerobic respiration or fermentation. Growth under He is similar to that under H2, while the CO2/N2 atmosphere led to slower growth. This could be due to increased acidity caused by CO2.
Yeast grew very similarly under all anerobic atmospheres (H2, He, and CO2/N2). It reached significantly lower growth rates and maximal cell concentrations under these conditions than in air. According to the team, these lower growth rates might be due to the fact that yeast needs O2 not just to produce energy, but also as a substrate for the biosynthesis of many crucial compounds.
Overall, both microorganisms were able to survive and reproduce under a hydrogen atmosphere, which leads the researchers to conclude that there might be a much broader range of planets that can support life than previously thought.
- Laboratory studies on the viability of life in H2-dominated exoplanet atmospheres,
S. Seager, J. Huang, J. J. Petkowski, M. Pajusalu,
Nat. Astron. 2020.