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

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 chroococcidiopsis, 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 in winter reaches over 70% even in regions close to the equator, as measured directly in Gale Crater by Curiosity, because of the large day / night swings in atmospheric pressure and temperature. It may well reach much closer to 100% in regions where frosts are seen (including the Viking 2 lander site) and where ground hugging mists form, including equatorial regions in Valles Marineres and the Hellas basin.

This is relevant to the search for native life on Mars. It is also relevant 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.

Lichens surviving in Mars surface conditions
The Martian surface has high levels of UV, higher than found anywhere on Earth. Most lichens do not survive prolonged exposure to Mars surface conditions. However lichens at high altitudes and in polar regions need protection from high levels of UV too, if not quite as high as on Mars. The lichens studied in these experiments have special pigments to protect them, found only found in lichens, such as parietin. They also have protective antioxidants such as b-carotene. They are epilithic - grow on rock, and in our closet analogues to Mars conditions, for instance growing on rocks at high altitudes in Antarctica, they huddle in cracks with not much exposure to direct sunlight.

It turns out that all 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. UV protection pigments that help them to survive such conditions have been suggested as potential biomarkers to search for on Mars.

An experiment simulating a Mars environment in a small chamber external to 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 inn Mars surface simulation conditions. Moreover, those surviving cells returned to 99% photosynthetic capabilities on return to Earth.

This greatly exceeded the post flight viability of any of the other organisms tested in the experiment. 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. T

Lichens relying on 75% night time humidity to metabolize, photosynthesize, and develop new growth
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 from the atmosphere in an environment where it is only surrounded by 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 °C to -8 °C and externally it varies from 23% to 46% as the external temperature varies from 8 °C 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.

"This work strongly supports the interconnected notions (i) thatterrestrial life most likely can adapt physiologically to live on Mars (hence justifying stringent measures to prevent human activitiesf rom contaminating/infecting Mars with terrestrial organisms);(ii) that in searching for extant life on Mars we should focus on“protected putative habitats”; and (iii) that early-originating(Noachian Period) indigenous Martian life might still survive in such micro-niches despite Mars' cooling and drying during the last 4 billion years"

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).

The three organisms they used were
 * Cryomyces antarcticus, an extremophile fungi, one of several from Antarctic dry deserts.
 * Knufia perforans, a fungi from hot arid environments
 * 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 and that an unknown metabolic pathway may be responsible.

Locations of landers and rovers on Mars
Note the higher latitude of Viking 2 which observed the seasonal frosts. Mists in Valles Marineres suggest high humidity there. Curiosity observed humidites of up to 70% in winter.

Effects of micropores in gypsum and salt pillars in the Atacama desert
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%.

These micro-organisms can survive because of numerous micro-pores in the salt, less than 0.1 micrometers in diameter in the salt. Theoretically, this reduces the limit for trapping water from the atmosphere down to relative humidities as low as 50-55% instead of the usual limit of 75%. Also once the water is trapped, it is retained for a long time, as the air gets even drier, right down to an extremely low relative humidity of 20%.

These micro-organisms can survive because of numerous micro-pores in the salt, less than 0.1 micrometers in diameter in the salt. Theoretically, this reduces the limit for trapping water from the atmosphere down to relative humidities as low as 50-55% instead of the usual limit of 75%. Also once the water is trapped, it is retained for a long time, as the air gets even drier, right down to an extremely low relative humidity of 20%. Micro-environmental data measured simultaneously outside and inside halite pinnacles in the Yungay region (table 2 from )

Brines still present down to 30% external humidity in micropores in salt pillars
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."

Microbes imbibe water down to 60% external humidity in micropores in gypsum pillars
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.

Possible relevance to Mars
The Wierzchos et al conclude their paper by saying: :

"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"

In another paper by Vitek et al the authors mention the possibility that these micropre results could be relevant to present day Mars as well :

"The results presented here support the suggestion that evaporitic, chloride-bearing rocks revealed on Mars may represent a suitable habitat for life—in the past or even present—enabling photosynthetic activity and adaptation to a harsh UV and hyper-arid environment."

Wierzoch et al, in a section on "Implications for the search for life on Mars" write

"The work presented here and work conducted in  deserts  around  the  world  in  the  past 40 years,  indicate  that  any  putative  microbial  life  on  Mars would have likely colonized the insides of rocks and crusts, as they provide natural shelter against harmful radiation and also enhance the moisture conditions on a micro scale. Sulfate and chloride evaporitic deposits have been suggested as a possible oasis for an extant biota, or the last refuge for an extinct biota on the surface of Mars). In this context, terrestrial evaporite rocks, including the Ca-sulfate crusts of the dry core of the Atacama Desert as key terrestrial analogs of Martian aqueous deposits, might prove useful for assessing the habitability of the Martian surface, and develop or improve current strategies for the search for life on Mars"

Surface high relative humidity observations of the Mars atmosphere by Curiosity and indirectly by Viking


This section is based on the survey article by Martinez et al, "The modern near-surface martian climate: A review of in-situ meteorological data from Viking to Curiosity."

In short, Curiosity measured humidity directly 70% humidity at a height of 1.6 meters above ground level in winter, ground temperature

The humidity variations on Mars are mainly due to the daily and seasonal variations of temperature of the air. Colder air has a higher relative humidity for the same water content.

The amount of water vapour can be measured as the volume mixing ratio of water vapour (VMR). This varies between around 10 ppm in winter and 70 ppm in summer at the Curiosity site. You might expect the highest relative humidity then to be in summer, but no, it's in winter, because it is so much colder then. The summer relative humidity is about 10% and the winter relative humidity around 70%.

So, it's 10ppm (approx) for 70% RH in winter, and 70 ppm for 10% RH in summer (by a strange coincidence the numbers are reversed).

Curiosity made these measurements of 70% humidity 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 highest temperature around -38 C and lowest temperature around -88 C (see their figure 5 ), and the highest humidity is normally reached between 04:00 and 06:00 Local Mean Solar Time (LMST). The lowest humidity readings are between 10:00 and 18:000 LMST, when they are typically less than 5%.

The Viking landers didn't have humidity sensors. But the humidity can be estimated indirectly, with maximum volume mixing ratio of water vapour of 200 ppm for both spacecraft.

The Viking 2 observed frosts for 250 sols of the year (frost usually suggests a high relative humidity close to 100%).

Viking 1 and Curiosity didn't observe them directly. But there is indirect evidence that frosts may form at times at lower latitudes, including imaging of mists in the Valles Marineres, and possible observation of frost on the deck and calibration target of Opportunity.