Life Supporting Environments Against Solar Radiation
Photosynthetically active radiation (PAR, 400-700 nm) would have been vital for the development of photosynthetic microbial communities on early Earth and early Mars 3.5 billion years ago. However, the UV component of solar radiation (UVC at <280 nm, UVB at 280-320 nm and UVA at 320-380 nm) will have a concurrent deleterious effect on any cells or biomolecules. Various factors could protect them from UV damage during their probably brief exposure before re-burial in shallow silt and evaporite crystals.
The reddish colour of the Martian surface is due to the presence of ferric iron. Iron in the profile may be reduced, but either way, when normally UV transparent quartz is doped with iron, it becomes a good environmental UV filter. It was found that 1 mm-thick silica gels containing only 0,1% FeCl3 could attenuate UVC by 37:1 whilst still transmitting 85% of visible light (PAR).
Using the primitive green photosynthetic bacterium Chloroflexus aurantiacus as a tool, and natural materials as filters, showed that quartz sand, basaltic sand and calcium carbonate all attenuated UVC much more than PAR. Chloroflexus aurantiacus forms anoxic stromatolitic mats which grow well under doses of UVC that severely depress growth of unprotected cells of Escherichia coli. Low concentrations of Fe3+ in the stromatolitic environment provided a very effective UV absorbing screen.
Certain evaporitic salts fortuitously absorb UV radiation in a manner analogous to iron in the quartzite endolithic niche. Cellular pigmentation and the effectiveness of iron and other salts as a UV-absorbing screen in sediments and microbial mats are likely survival strategies for early phototrophs in the Precambrian in the absence of an ozone shield on Earth and equivalent Hesperian period on Mars.
If any cells or biomolecules emerge in streams onto the surface of Mars, turbulence will prevent them being exposed to UVB or UVC for any prolonged time. The system would act as an integrated dynamic optical filter, with time of exposure as much a protective factor as the UV screening capability of Fe-doped crystals and evaporitic salts.
For microbes or their products to emerge onto the surface of Mars in seepage channels, they must have originated in subsurface strata, either as former light-dependent photosynthetic communities which have since become either buried as dormant or fossilized cells and biomolecules, or as subsurface communities dependent on chemical energy derived from redox gradients. These life-forms are not only constrained by the water and light limits that are vital for photosynthetic communities, but also a variety of other factors, including temperature and its range.
Fire and Ice: Life Under Extreme Temperature Conditions
The temperature range for microbes in cold surface habitats can be narrow, such as for psychrophiles that function only below ~20°C. Examples of these from Antarctic hypersaline lakes include the micro-algae Dunaliella sp. and Chlamydomonas sp. that are able to remain motile at temperatures as low as 14°C. The psychrophilic bacterial population of hypersaline Deep Lake (salinity 32% and mean temperature 15°C) is almost entirely composed of halophilic microorganism Archaea, the oldest root of the bacterial evolutionary tree. Alternatively, psychrotolerant organisms can metabolise at 0°C whilst continuing to function when the temperature exceeds ~20°C, as in maritime Antarctic soils.
Conversely, certain bacteria can withstand the exceptionally high temperatures that have been demonstrated in the habitable zone of hydrothermal oceanic vents on Earth where the current maximal growth temperature recorded is 113°C for Pyrolobus fumarolii. It is suggested that the maximum temperature for bacterial survival may even be as high as 150°C, and strains of the hyperthermophiles Pyrolobus and Pyrodictium have been shown to survive autoclaving at 121°C for one hour. There may therefore be a similar substantial subsurface geothermally-heated microbial ecosystem in groundwater on Mars.
The Inerterrestrials: Subsurface Microbial Communities
Metabolism at low redox potential is a feature of anaerobic deep subsurface microbial communities on Earth and potentially on early and even present-day Mars. Anaerobic communities in both surface and subsurface communities show a diversity of redox couples which can yield energy and drive the carbon cycle. Methane fermentation by the Antarctic psychrotolerant archaebacterium Methanococcoides burtonii from hypersaline Ace Lake and hydrogen oxidation by the extreme halophilic thermophilic archebacterium Pyrolobus fumari in geothermal deep-ocean vents are contrasting examples.
In deep subsurface geothermally-warmed aquatic environments on Mars, there may be chemolithotrophic microbes which depend on minerals for energy and nutrients. These use the transfer of electrons down redox gradients as an alternative energy source to solar radiation. Likely electron donors would include inorganic sulfides, and acceptors might include hydrogen.
Analogous deep subsurface microbial communities, occurring at least 750 m beneath the sea floor, are now well documented on Earth. With a viable population of up to 107cells cm-3 at 500 meter beneath the sea bed, they may comprise as much as 50% of the Earth's biomass.
Sulfate-reducing and Fe(III)-reducing bacterial communities are reported at 2,800 meter depth in a natural gas-bearing formation in Taylorsville Basin, in Virginia comprising up to 104 cells g-1 at 76°C at a salinity of 0.8 wt.% NaCl equivalent and under 32 Mpa pressure.
Microorganisms Who Love Acid
Acid-tolerance is another survival factor shown by various groups of primitive microbes of relevance to early Earth and potential relevance to Hesperian Mars. Some are photosynthetic, such as the primitive unicellular red alga Cyanidium caldarium, which can tolerate pH values of 0.05. Others are chemosynthetic, such as the extremely acidiphilic archaebacterium Sulpholobus acidocaldarius. They are often also thermophilic as befits their primitive origins.
Salt Tolerance: The Penalty of Liquid Water Below Normal Freezing Point
Hypersalinity per se as shown by the Antarctic archaebacter Halobacterium lacusprofundi from Deep Lake may be crucial for survival in hypothetical supercooled brine aquifers emerging onto the surface of Mars.
Biofilms and the Role of Environmental Gradients
Photosynthetic microbial biofilms such as cyanobacterial stromatolitic mats, result in highly concentrated biomolecular layers, which are tightly constrained by the penetration of PAR These accumulate 2-3 metres thick in Antarctic Dry Valley lakes. Using readily available solar energy, photosynthetic biofilms could have evolved during the wet Hesperian period on Mars. The mats would have subsequently become buried in paleolakes and similar fluvial habitats which have analogues in Antarctic Dry Valleys. If present on Mars, their concentrated stratification would make them more easily detectable by vertical sampling via a drilling technology or natural horizontal flushing through concentrated layers of dormant or fossilized layers of photosynthetic biomass into surface seepage channels.
Former Life on Mars and Other Terrestrial Bodies
Microbial communities themselves may not survive, but their fossil biochemicals may be preserved in frozen regolith. The survival of marker biomolecules for periods of geological time requires conditions which minimize their degradation by the attrition of internal molecular motion and damage by external radiation. Even in a highly desiccated state, gradual deterioration of macromolecules such as DNA and proteins at temperatures of ~20°C requires occasional periods of anabolic activity to repair the accrued damage.
Subzero temperatures reduce the rate of deterioration, but even in soils at temperatures below 20°C dormant microbes may require a pulse of growth-supporting conditions to effect biochemical repairs. This situation is alleviated in highly desiccated extremely cold conditions by the adsorption of biomolecules and cells onto mineral crystals which have been found to preserve the activity of immobilized enzymes such as nitrogenase after 1.8 Ga in Siberian permafrost soil.
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