Lichens, cyanobacteria and molds growing in humidity of simulated Martian atmosphere

Martian conditions in miniature - In the Mars simulation chamber, DLR researchers recreated the atmospheric composition and pressure, the planet's surface, the temperature cycles and the solar radiation incident on the surface. The activity of polar and alpine lichen was investigated under these conditions.

A series of experiments by DLR (German aerospace company) in Mars simulation chambers and on the ISS show that some Earth life (Lichens and strains of chrooccocidiopsis, a green algae) can survive Mars surface conditions and photosynthesize and metabolize, slowly, in absence of any water at all. They could make use of the humidity of the Mars atmosphere. Though the absolute humidity is low, the relative humidity at night reaches 100% because of the large day / night swings in atmospheric pressure and temperature. This is relevant to the search for native life on Mars and also to planetary protection, the need to protect Mars from Earth life if we wish to study native life in the habitats in its original state.

Humidity observations of the Mars atmosphere by Curiosity and Viking

The humidity variations on Mars are mainly due to variations of temperature of the air, with colder air having a higher relative humidity for the same water content. The volume mixing ration of water vapour varies up to around 70 ppm, and is anti-correlated with the humidity, with the highest humidity at times of least VMR.

Curiosity has measured relative humidity readings of up to 70% in winter, measured at a height of 1.6 meters above ground level (see Sect 14. This is in winter with a temperature range of around 50 C and mean temperature around -63 C, so lowest temperature around -88 C (see their figure 5), and the highest humidity is normally reached between 04:00 and 06:00 LMST. The lowest humidity readings are between 10:00 and 18:000 LMST, when they are typically less than 5%

Curiosity hasn't directly observed the frosts that Viking observed at somewhat higher latitudes. But there is indirect evidence that frosts may form at times.

Lichens relying on 75% night time humidity

The lichens studied in these experiments have protection from UV light due to special pigments only found in lichens, such as parietin and antioxidants such as b-carotene in epilithic lichens. This gives them enough protection to tolerate the light levels in conditions of partial shade in the simulation chambers and make use of the light to photosynthesize. Indeed, UV protection pigments have been suggested as potential biomarkers to search for on Mars.

An experiment on the ISS as part of Expose-E in 2008-2009 showed that one lichen, Xanthoria elegans, retained a viability of 71% for the algae (photobiont) and 84% for the fungus (mycobiont) after 18 months in the ISS, in Mars surface simulation conditions, and the surviving cells returned to 99% photosynthetic capabilities on return to Earth. This was an experiment without the day night temperature cycles of Mars and the lichens were kept in a desiccated state so it didn't test their ability to survive in niche habitats on Mars. This greatly exceeded the post flight viability of any of the other organisms tested in the experiment.

Another study in 2014 by German aerospace DLR in a Mars simulation chamber used the lichen Pleopsidium chlorophanum. This lives in the most Mars like environmental conditions on Earth, at up to 2000 meters in Antarctica. It is able to cope with high UV, low temperatures and dryness. It is mainly found in cracks, where just a small amount of scattered light reaches it. This is probably adaptive behaviour to protect it from UV light and desiccation. It remains metabolically active in temperatures down to -20 C, and can absorb small amounts of liquid water in an environment with ice and snow.

As measured in Antarctica, the relative humidity in the lichen's niche microhabitat varies from 57 to 79% as the temperature varies from -6 to -8% and externally it varies from 23% to 46% as the external temperature varies from 8 to - 8 C.

In this experiment the temperature varied between +21 °C and -50 °C. Relative humidity is higher in cold air, for the same concentrations of water vapour, and as the temperature varied, the relative humidity varied between 0.1% and 75%. The atmosphere consisted of 5% CO₂,4%N₂, and 1% O₂ at 800 Pa or about 0.79% of Earth's sea level atmospheric pressure. This approximates conditions that are encountered in the equatorial and lower lattitude regions of Mars.

When exposed to full UV levels in a 34-day experiment in a Mars simulation chamber at DLR, the fungus component of the lichen Pleopsidium chlorophanum died, and it wasn't clear if the algae component was still photosynthesizing.

However, when partially shaded from the UV light, as it is in its natural habitats in Antarctica, both fungus and algae survived, and the algae remained photosynthetically active throughout. Also new growth of the lichen was observed. Photosynthetic activity continued to increase for the duration of the experiment, showing that the lichen adapted to the Mars conditions.

This is remarkable as the fungus is an aerobe, growing in an atmosphere with no appreciable amount of oxygen and 95% CO₂. It seems that the algae provides it with enough oxygen to survive. The lichen was grown in Sulfatic Mars Regolith Simulant - igneous rock with composition similar to Mars meteorites, consisting of gabbro and olivine, to which quartz and anhydrous iron oxide hematite (the only thermodynamically stable iron oxide under present day Mars conditions) were added. It also contains gypsum and geothite, and was crushed to simulate the martian regolith. This was an ice free environment. They found that photosynthetic activity was strongly correlated with the beginning and the end of the simulated Martian day. Those are times when atmospheric water vapour could condense on the soil and be absorbed by it, and could probably also form cold brines with the salts in the simulated martian regolith. The pressure used for the experiment was 700 - 800 Pa, above the triple point of pure water at 600 Pa and consistent with the conditions measured by Curiosity in Gale crater.

The experimenters concluded that it is likely that some lichens and cyanobacteria can adapt to Mars conditions, taking advantage of the night time humidity, and that it is possible that life from early Mars could have adapted to these conditions and still survive today in microniches on the surface.

Black fungi and black yeast relying on 70% night time humidity

In another experiment, by Kristina Zakharova et al., two species of microcolonial fungi – Cryomyces antarcticus and Knufia perforans - and a species of black yeasts–Exophiala jeanselmei were found to adapt and recover metabolic activity during exposure to a simulated Mars environment for 7 days. They depended on the temporary saturation of the atmosphere with water vapour like the lichens. The fungi didn't show any signs of stress reactions (such as creating unusual new proteins).

There Cryomyces antarcticus is an extremophile fungi, one of several from Antarctic dry deserts. Knufia perforans is a fungi from hot arid environments, and Exophiala jeanselmei is a black yeast endolith closely related to human pathogens.

In this experiment, the temperature cycled between 21 °C and -50 and the relative humidity varied up to 70% at the lowest temperatures, with pressure 700 Pascals or about 0.69% of Earth sea level.

The experimenters concluded that these black fungi can survive in a Mars environment.

Effects of micropores in salt pillars

In experimental studies of salt pillars in the Atacama desert, microbes are able to access liquid at extremely low relative humidities due to micropores in the salt structures. They do this through spontaneous capillary condensation, at relative humidities far lower than the deliquescence point of NaCl of 75%.

'The Atacama desert hosts the closest analogue of what a real, live Martian might be like', in its salt rock formations.

Micro-environmental data measured simultaneously outside and inside halite pinnacles in the Yungay region (table 2 from )

Variable	Halite exterior	Halite interior
Mean annual RH, %	34.75	54.74
Maximum annual RH, %	74.20	86.10
Minimum annual RH, %	2.90	2.20

The researchers, Wierzchos et al., did detailed studies with scanning electron microscopes. At 75% relative humidity then brine was abundant inside the salt pillars. As the humidity was reduced, even at 30% RH, the cyanobacteria aggregates shrunk due to water loss, but still there were small pockets of brine in the salt pillars.

"Endolithic communities inside halite pinnacles in the Atacama Desert take advantage of the moist conditions that are created by the halite substrate in the absence of rain, fog or dew. The tendency of the halite to condense and retain liquid water is enhanced by the presence of a nano-porous phase with a smooth surface skin, which covers large crystals and fills the larger pore spaces inside the pinnacles... Endolithic microbial communities were observed as intimately associated with this hypothetical nano-porous phase. While halite endoliths must still be adapted to stress conditions inside the pinnacles (i.e. low water activity due to high salinity), these observations show that hygroscopic salts such as halite become oasis for life in extremely dry environments, when all other survival strategies fail.

Our findings have implications for the habitability of extremely dry environments, as they suggest that salts with properties similar to halite could be the preferred habitat for life close to the dry limit on Earth and elsewhere. It is particularly tempting to speculate that the chloride-bearing evaporites recently identified on Mars may have been the last, and therefore most recently inhabited, substrate as this planet transitioned from relatively wet to extremely dry conditions"

Microbes also inhabit Gypsum deposits (CaSO₄.2H₂O), however Gypsum doesn't deliquesce. Researchers found that the regions of the desert that had microbial colonies within the gypsum correlated with regions with over 60% relative humidity for a significant part of the year. They also found that the microbes imbibed water whenever the humidity increased above 60% and gradually became desiccated when it was below that figure.

See also

• Possible present day habitats for life on Mars