We have found tantalising hints that not only was there once life on Mars, but that it might still exist on the Red Planet. Now, a study here on Earth has made that idea more plausible by showing that salt-loving, or halophilic, microbes can grow in conditions close to those that might be found in little pockets of highly salty water just below the surface of Mars.
鈥淲e show that these halophilic microorganisms can actively grow under a combination of anoxic [oxygen-free], low-pressure atmosphere conditions and a high-salt liquid media that contains harmful Martian chemicals that are similar to bleach, which are the perchlorates,鈥� says at the University of Florida. 鈥淎nd that鈥檚 something that nobody had been able to show before.鈥�
Robinson and his colleagues designed their experiments based on the idea that there might be pockets of liquid, salty water just below the surface of Mars. We haven鈥檛 found any, says Robinson, but work by other researchers suggests they might exist.
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The microbe used in the experiments is Haloferax volcanii, an archaeon 鈥� the other kind of simple cell besides bacteria 鈥� that thrives in extremely salty waters such as the Dead Sea. It鈥檚 relatively easy to grow compared with many other microbes that live in extreme environments, known as extremophiles.
The team put these microbes in water containing 225 grams of salt per litre, and kept it at a temperature of 21掳C (about 70掳F) and a pressure of 24 millibar for 160 days. Varying levels of nitrates and chlorates were added to different batches. For controls, the microbes were grown in the same media but at Earth atmospheric pressure: 1013 millibar.
While growth was slower in the low-pressure condition, it did still occur, as shown by the growth medium becoming cloudier 鈥� a standard measure of growth 鈥� and the biological reduction of nitrate and perchlorate. A scanning electron microscope also revealed extensive biofilm formation.
鈥淪o there鈥檚 pretty robust evidence that points towards active growth and not just survival,鈥� says Robinson. Many previous studies have demonstrated that microbes can survive in Mars-like conditions, he says, but very few have shown growth, and none have shown growth of salt-loving extremophiles.
But do pockets of salty water really exist under the Martian surface? We don鈥檛 know, in part because since the Viking missions in 1976 found possible signs of life, other missions have avoided the most promising areas, says Robinson. 鈥淭hey specifically avoided places that were deemed potentially habitable, because we don鈥檛 want to contaminate regions where there might be life with our own spacecraft and landers.鈥�
It is surprising that any Earth microbe can grow in these conditions at all, says team member at the Los Angeles Natural History Museum, given that nowhere on Earth鈥檚 surface has such low pressure. But if life did evolve on Mars billions of years ago, when conditions were more favourable, it would have had ample time to adapt to such conditions, he says.
However, the pressure and temperature combination used might be higher than those found on Mars, and the team did add yeast extract to the growth medium as a carbon source. 鈥淪o the growth medium is Mars-like in the sense that it has high salinity and the chlorates and nitrates in it. But it鈥檚 not Mars-like in the sense that we鈥檙e giving a complex carbon source that would not be found on Mars today,鈥� says Robinson. 鈥淚t鈥檚 a starting point for future experiments.鈥�
In these experiments, the team plans to replace the yeast abstract with acetate, a potential carbon source thought to be found on Mars today.
at the University of Edinburgh, UK, says that temperatures on the surface of Mars can exceed 21掳C during the day. 鈥淏ut here, the pressure is only around 6 to 12 millibar. The difference is significant because, in the realistic range, water cannot stay liquid. No liquid water, no life.鈥�
Below the surface where pressures could reach 24 millibar, the temperatures are well below freezing, says McMahon.
Robinson thinks the conditions used in the study are plausible on Mars. But the team is already investigating whether cold-loving halophiles can grow at temperatures of between 0 and 4掳C (32 and 39掳F) and pressures of between 7 and 12 millibar, he says.
There is also the issue of what lasting food source there could be for microbes on Mars. The most likely source is the carbon dioxide in the atmosphere 鈥� microbes called methanogens can feed on CO2 and spew out methane, which has been detected on Mars. There are a few salt-loving methanogens on Earth, says Robinson, which the team hopes to try to grow in Mars-like conditions.
bioRxiv
