A new study published in the journal Nature by Mukan Ji from the Australian Centre for Astrobiology and co-authors may have just extended the known limit for how dry and cold an environment life can tolerate—not only on Earth, but also on Mars and places outside our Solar System.
The research team studied microbes found in regions of Antarctica that aren’t permanently covered by ice and snow. Based on genetic analysis, they found evidence that bacteria in these locations meet their energy needs by oxidizing hydrogen and carbon monoxide—trace gases found in the cold, Antarctic air. Their genetic tool set gives them the ability to fix carbon dioxide directly from the air, but not in a way that photosynthetic organisms do.
This is astounding for several reasons. Generating energy from the metabolic oxidation of hydrogen and carbon monoxide is not unknown, but it occurs mostly under oxygen-free (anaerobic) conditions. These Antarctic microbes do it under conditions where oxygen is plentiful, and where hydrogen and carbon monoxide are found only in tiny concentrations—far less than a thousandth of a percent. We would expect photosynthesis to be the main energy source even in the extreme environmental conditions found in Antarctica, but that’s not the case for the soil bacteria studied by Ji et al.
The question is whether this unusual “lifestyle” is found only at the two sites they sampled, or whether it’s representative of the whole Antarctic continent. If the latter, why would it be preferable to photosynthesis? In the extreme cold of Antarctica, the bacteria might be active for only a few hundred hours during the year. Energy efficiency is important under those conditions, and assuming the study results are confirmed, we have to conclude that this type of metabolism is simply more energy-efficient in that particular environment.
Taking that line of thought a bit further, we might wonder what happens in warmer deserts such as the Atacama in South America. Even the driest areas in that region have salt crusts where microbes can make a living using the hygroscopic properties of salts to extract water directly from the atmosphere. Among the microbes living in these crusts are cyanobacteria that use sunlight (which is much stronger in the Atacama than in Antarctica) for photosynthesis. But maybe we’ve missed finding organisms that draw energy from trace gases in the atmosphere.
Even if it turns out this type of metabolism is only found in cold, dry environments, there are plenty of places in the Solar System where those conditions apply. The first that comes to mind is Mars, but the icy moons of the outer Solar System might also qualify. Organisms on those worlds may require very little from their soil substrate, fulfilling most of their needs—particularly their energy needs and their need for organic carbon—from the air. And that may broaden our conception of what makes a suitable habitat. What’s the old saying? One can live from love and air.