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Extreme Habitats

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.

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