Scientists posit microbes on Enceladus and Mars

Separate studies suggest plausible scenarios for archaean and bacterial communities on the red planet and the frozen moon. Andrew Masterson reports.

An artists's impression of a geyser erupting on the surface of Saturn's moon Enceladus. The outflow contains methane, a possible signature of microbial activity.
An artists's impression of a geyser erupting on the surface of Saturn's moon Enceladus. The outflow contains methane, a possible signature of microbial activity.
Stocktrek Image/Getty Images

At least some of the methane detected by the Cassini space probe around Saturn’s moon Enceladus could have been produced by microbes, according to researchers.

The finding is contained in one of two recent papers that posit the existence of microbial life in space. The second concerns the potential for dormant bacteria on Mars.

In the Enceladus paper, published in the journal Nature Communications, a team led by systems biologist Simon Rittmann of Universität Wien in Austria focuses on a type of single-celled microorganism called archaea.

These microbes are not bacteria, but constitute an entirely different kingdom of life. They have a unique biochemistry that allows them (depending on the species) to exploit substances such as ammonia, metal ions and hydrogen gas as energy sources. Widespread in variety and distribution, archaea live everywhere from deep ocean hydrothermal vents to the human gut.

Rittmann and his colleagues suggest that such microbes (or analogues thereof) might also live on Enceladus – and that Cassini might have detected evidence of them.

The scientists look at a particular subclass of archaea known as methanogens, which use molecular hydrogen and carbon as energy sources, and release methane as a by-product of their metabolism. To test their hypothesis, the researchers grew three species of methanogenic microbes under conditions that mimicked the gas composition and atmospheric pressures thought to be present on Enceladus.

They found that one species, Methanothermococcus okinawensis, thrived in the tough conditions, continuing to produce methane even in the presence of additional chemicals that stymied the other two contenders.

The scientists report that M. okinawensis achieved a very respectable carbon-dioxide-to-methane conversion rate of 72%. Other calculations found that a predicted type of crystal-forming mineralisation on Enceladus, called serpentinisation, might also result in enough molecular hydrogen to provide a substrate for methane production.

Therefore, Rittman and colleagues conclude, “some of the [methane] detected in the plume of Enceladus might, in principle, be produced by methanogens”.

Meanwhile, over in South America, another group of researchers has been investigating microbial health and activity in the Atacama Desert in Chile. The desert in one of the driest places on Earth – known as a hyperarid zone – where rain falls as infrequently as once a decade.

A team led by Dirk Schulze-Makuch of the Centre of Astronomy and Astrophysics at Germany’s Technical University Berlin set out to measure what, if any, bacterial life could survive in such an inhospitable environment.

The results, compiled over a couple of years, surprised them. In the desert sands they found DNA belonging to a wide range of bacterial species, particularly Actinobacteria and hyperarid specialists Geodermatophilaceae. They also found genetic material indicating the presence of diverse, if smaller, populations of archaea and fungi.

The next task was to run a separate analysis to discover whether the DNA collected was simply the remnants of long dead organisms or viable material indicating the presence of active or alive-but-dormant ones.

This analysis was conducted over three years, starting with samples taken in the wake of a rare rain event in 2015. The results showed high levels of intracellular DNA – considered a proxy for living microbes – immediately after the rainfall. The level then decreased sharply, in inverse proportion to the density of extracellular – dead – DNA over the subsequent two rain-free years.

The results, write Schulze-Makuch and colleagues in the journal PNAS, show that even in hyperarid environments microbes can survive for long periods – lying dormant and then becoming active following an increase in moisture.

Could the dry surface of Mars thus harbour stubbornly dormant microbes, awaiting only water in order to spring back to life? The scientists consider the scenario at least plausible.

“The insights gained from the hyperarid core of the Atacama Desert can serve as a working model for Mars, where environmental stresses are even harsher,” they conclude.

“If life ever evolved on Mars, the results presented here suggest that it could have endured the transition from the early aquatic stage, through increasing aridity cycles, and perhaps even found a subsurface niche beneath today’s severely hyperarid surface.”

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Andrew Masterson is news editor of Cosmos.
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