A summer study of the vertical structure of bi-layered microbial mats was carried out on Byers Peninsula (Livingston Island, South Shetland Islands). These benthic communities had a common basic structure that consisted of two distinct layers differing in composition, morphology and colour. Our sampling focused on mats showing more layering, which thrived over moist soils and at the bottom of ponds. The photosynthetic pigments analysis performed by high-performance liquid chromatography demonstrated a major occurrence of cyanobacteria and diatoms on these mats, the former being more abundant in relative terms on the surface and composed by morphospecies grouping into orders Oscillatoriales, Nostocales and Chroococcales. The areal chlorophyll a concentrations were slightly higher in the deeper layer although not significantly. Our microscopic and chemical analyses showed that non-active biomass accumulates at the surface. Hence, the upper layers showed the sheath pigment scytonemin and higher amounts of exopolysaccharides, as a strategy to cope with environmental stress. On the other hand, the basal layer was composed of more active photosynthetic microbiota, which also revealed a more balanced stoichiometry. Here we exemplify how environmental stresses are potentially overcome by physiological mechanisms developed by microbial mats which also shape their vertical structure.

The work reported here represents a study into the total fluorescence exhibited by a broad selection of model, extremophilic and photosynthetic bacterial strains, over a great range of excitation and emission wavelengths from ultraviolet (UV) through visible to near infrared. The aim is to identify distinctive fluorescent features that may serve as detectable biosignatures of remnant microbial life on the Martian surface. A lab-bench fluorescence spectrometer was used to generate an excitation–emission matrix (EEM) for the unpigmented Escherichia coli, radiation-resistant Deinococcus radiodurans, Antarctic Dry Valley isolates Brevundimonas sp. MV.7 and Rhodococcus sp. MV.10, and the cyanobacterium Synechocystis sp. PCC 6803. Detailed EEMs, representing the fluorescence signature of each organism, are presented, and the most significant features suitable for biosignature surveys are identified, including small-molecule cellular metabolites, light-harvesting photosynthetic pigments and extracellular UV-screening compounds. E. coli exhibits the most intense emission from tryptophan, presumably due to the absence of UV-screening pigments that would shield the organism from short-wavelength light-exciting intracellular fluorescence. The efficacy of commonly available laser diodes for exciting cellular fluorescence is treated, along with the most appropriate filter wavelengths for imaging systems. The best combination of available laser diodes and PanCam filters aboard the ExoMars probe is proposed. The possibility of detecting fluorescence excited by solar UV radiation in freshly exposed surface samples by imaging when both sunlit and shadowed, perhaps by the body of the rover itself, is discussed. We also study how these biological fluorophore molecules may be degraded, and thus the potential biosignatures erased, by the high flux of far-ultraviolet light on Mars.

Ab initio calculations for scytonemin, an important ultraviolet (UV)-radiation protective biomolecule synthesized by extremophilic cyanobacteria in stressed terrestrial environments, are reported for the first time. Vibrational spectroscopic assignments for the previously studied Raman spectra assist in the identification of the major features in the observed data. Calculations of the electronic absorption spectra confirm the capability of this molecule to absorb in all three regions of the UV, UVA, UVB and UVC, and also illustrate the need for a dimeric species in this respect. The presence of significant steric hindrance between the two halves of the dimeric molecule about the C—C bridging bond in scytonemin forces the molecule significantly out of planarity, contrary to assumptions made in the literature; however, it appears that the monomer is capable of absorbing to only a limited extent in the UVB and UVC regions only, so conferring a special emphasis upon the need for the dimerization to remove the lower-energy UV radiation whilst still affording protection for the chlorophyll with transmission of the visible radiation required for photosynthesis. The observation of vibrational band wavenumber coincidences for the first time between the infrared and Raman spectra confirm the non-planar structural prediction from the calculations. The results of this study provide information about the protective chemical strategies of terrestrial extremophilic cyanobacteria and provide a basis for the search for molecules of this type in the astrobiological exploration of Mars.

We used synchrotron light to determine VUV-UV absorption spectra (125–340 nm) of thin films of substances associated with UV resistance in specific groups of organisms or across limited phylogenetic boundaries: scytonemin, mycosporine-like amino acids, dipicolinic acid, β-carotene, melanin and flavonoids (quercitrin, isoquercitrin, robinin and catechin). The objective was to extend known UV absorption spectra into the vacuum UV, and to evaluate the likely effectiveness of these molecules in shielding DNA from the unfiltered solar UV found in space, using similarity with DNA absorption spectra as the primary criterion. The spectroscopy indicated that plant flavonoids would be ideal UV screens in space. We suggest that flavonoids represent primitive UV screens, and offer explanations (including horizontal gene transfer) for their presence in plants. We also discuss the possibility of improving UV resistance by increasing flavonoid accumulation through metabolic engineering, in the hope of better adapting life for space travel, i.e. for its dissemination away from the Earth (exospermia). Finally, we propose using plant seeds as exospermia vehicles for sending life (including artificial life) into space.

A study of Rivularia stromatolites demonstrated seasonal and non-seasonal banding patterns of calcification, ‘sun-screen’ scytonemin pigment and nitrogen-fixing heterocysts. Calcification was controlled by seasonal events with abiogenic ‘winter’ deposition and biogenic ‘summer’ deposition. Scytonemin was produced as a series of complex bands, probably as a response to summer Atlantic weather systems. Its production was also correlated in part with the appearance of heterocysts. The heterocysts were produced in bands, the pattern of which was probably controlled by an internal regulatory system. Raman spectra of modern and ancient (up to 4000 year old) Rivularia showed that scytonemin and carotenoid pigment can persist in dried material for >100 yr. The 4000 year old fossils did not reveal any useful biomarkers.

For activity and survival in extreme terrestrial Antarctic habitats, lithobiontic cyanobacteria depend on key biomolecules for protection against environmental stress and for optimization of growth conditions. Their ability to synthesize such molecules is central to their pioneering characteristics and major role as primary producers in Antarctic desert habitats. Pigmentation is especially important in protecting them against enhanced UVB damage during stratospheric ozone depletion (the Ozone Hole) during the Antarctic spring and subsequent photoinhibition in the intense insolation of the summer. To be effective, especially for the screening of highly shade-adapted photosystems of cyanobacteria, protective pigments need to be located strategically. Antarctic lithic cyanobacterial communities are therefore stratified, as in soil biofilms of Alexander Island, the benthic stromatolitic mats of ice-covered hypersaline lakes in the McMurdo Dry Valleys, and the endolithic communities within translucent Beacon sandstone outcrops of Victoria Land. The protective pigments include scytonemin, carotenoids, anthroquinones and mycosporine-like amino acids. To detect and locate photoprotective pigments in situ in free-living cyanobacteria and cyanolichens from hot and cold desert habitats, we have used Fourier-transform Raman micro-spectroscopy. With appropriate power inputs for labile molecules, this high-precision, non-intrusive laser-based technique can not only identify biomolecules in their natural state but also locate them spatially within the habitat relative to the components of the community, which require protection. In conjunction with direct and epifluorescence microscopy it provides a spatial and functional description of the protective strategy of a community. We present the unique Raman spectrum of scytonemin and use its primary and corroborative peaks to identify it within the plethora of other biochemical constituents of several natural cyanobacterial communities, including an Antarctic endolith. The remote-sensing aspect of this technique makes it suitable not only for spatial biochemical analysis of present and palaeontological Antarctic communities but also for analogous putative habitats on Mars.

If the depletion of the stratospheric ozone shield continues, the influx of solar UV-B radiation (280–320 nm) will increase in the near future. In photosynthetic organisms there are several targets for the deleterious UV-B radiation, such as proteins, DNA and membranes. Cyanobacteria use three different types of strategies to counteract UV damage: (i) stress avoidance by gliding mechanisms, (ii) stress defence by synthesis of UV-absorbing compounds, antioxidants and extracellular polysaccharides, and (iii) repair mechanisms including DNA repair and resynthesis of UV-sensitive proteins. In the past, most studies concentrated on the physiological aspects of UV tolerance whereas the molecular basis of UV tolerance in cyanobacteria is poorly understood. We will summarize the effects of and responses to UV-B at the physiological and molecular levels and present data on the influence of UV-B irradiation on the proteome of the terrestrial cyanobacterium Nostoc commune. SDS-PAGE and high-resolution two-dimensional gel electrophoresis were used to analyse the influence of UV-B on the proteome of N. commune. The resolution of SDS-PAGE turned out to be far too low to monitor the UV acclimation process. In contrast to the latter technique, two-dimensional electrophoresis showed that the UV-B response is extremely complex, involving the induction and the repression of a large number of proteins.