Fourier Transform Infrared Spectral Detection of Life ... - OSA Publishing

Fourier Transform Infrared Spectral Detection of Life in Polar Subsurface Environments and Its Application to Mars Exploration Louisa J. Preston,a,* Diane Johnson,a Charles S. Cockell,b Monica M. Gradya,c a

The Open University, Department of Physical Sciences, Milton Keynes MK7 6AA, United Kingdom University of Edinburgh, School of Physics and Astronomy, James Clerk Maxwell Building, Peter Guthrie Tait Road, Edinburgh EH9 3FD, United Kingdom c Natural History Museum, Department of Earth Sciences, Cromwell Road, London SW7 5BD, United Kingdom b

Cryptoendolithic lichen communities of the Dry Valleys, Antarctica, survive in an extremely inhospitable environment, finding refuge in microscopic niches where conditions suitable for life exist. Such ‘‘within-rock’’ communities may have evolved on Mars when conditions for life on the surface deteriorated to such an extent that they could no longer survive. Fourier transform infrared spectroscopy of unprepared whole-rock Antarctic Beacon sandstones was used to vertically profile molecular vibrations of fatty acids, proteins, and carboxylic acids created by endolithic communities. Spectral biosignatures were found localized to lichen-rich areas and were absent in crustal regions and the bulk rock substrate. These cryptoendolithic profiles will aid similar spectroscopic investigations of organic biosignatures during future Martian subsurface studies and will help in the identification of similar communities in other localities across the Earth. Index Headings: Fourier transform infrared spectroscopy; FT-IR; Biosignature; Organics; Cryptoendoliths; Antarctica; Mars.

INTRODUCTION Astrobiology and the search for life in the solar system require the study of environments and organisms on the Earth that might be able to act as analogs. In particular, to determine whether life exists or once existed in the past on Mars, the identification of biosignatures within terrestrial environmentally extreme locales is of paramount importance. A biosignature is any substance, such as an element, isotope, or molecule, which provides scientific evidence of past or present life. The search for such signatures of life within Earth-based analog environments informs researchers of the potential habitability of an extraterrestrial locality. It also indicates the potential for the long-term preservation of signs of past life and thus guides the selection of which biosignatures to target on other planetary bodies, such as Mars, and the most appropriate analytical methods to detect them. Extreme, Mars-like environments are found across the Earth and can mimic past or present aspects of the planet through mineralogical, geochemical, or biological characteristics. The conditions documented in Antarctica most closely resemble those currently observed on the surface of Mars, including low temperatures, low Received 17 December 2014; accepted 10 March 2015. * Author to whom correspondence should be sent. E-mail: [email protected]. DOI: 10.1366/14-07843

Volume 69, Number 9, 2015

humidity, low annual precipitation, desiccating highspeed winds, and a high incidence of solar ultraviolet (UV) radiation.1 In such environments, where conditions are particularly damaging to biomolecules (e.g., DNA), protected niches are necessary for the survival of microbial life and the subsequent preservation of their biosignatures over geological time. The inside of rocks can provide such a protected place because they offer a thermally more favorable environment than that experienced outside on the rock surface due to absorption of solar radiation and buffering against rapid temperature changes caused by wind.2 The McMurdo Dry Valleys of Southern Victoria Land, Antarctica, are home to a variety of microorganisms termed endoliths that have used just this strategy for survival and colonized the inside of rock matrices.3,4 Endolithic organisms can be present as two types: chasmoendoliths, living in rock fissures and cracks and cryptoendoliths, inhabiting structural cavities of porous rocks. Cryptoendoliths are unable to penetrate or bore into a substrate through active dissolution of minerals, and their communities have a photosynthetic primary producer; therefore, preexisting cavities and the translucency of the rock substrate are important life-limiting factors. Rocks are ideal habitats in extreme locales as they can attenuate the damaging UV radiation present, such as that observed within the harsh Antarctic environment. They will, however, also reduce the penetration of light in the visible region between 400 and 700 nm that is required for photosynthesis,5 thus limiting the physical extent to which photosynthetic organisms can grow. As such, the nature of light and nutrient gradients within the Beacon sandstones of the Antarctic Dry Valleys has created an interesting sample set to observe complex communities within a vertical zonation of layers and provides an excellent analog to the conditions found on the surface of Mars. A symbiotic relationship has developed between pigmented fungi, non-pigmented fungi, and algae, creating a lichendominated ecosystem.4,6,7 In the McMurdo Dry Valleys, these cryptoendolithic lichen communities can tolerate a range of extreme environmental conditions and have a suite of survival mechanisms, suggesting they may be similar to early microbes that could have flourished on or near the surface of an early Mars.8 They are also suitable analogs for those microbes that may be preserved or even currently reside protected beneath the surface of Mars.

0003-7028/15/6909-1059/0 Q 2015 Society for Applied Spectroscopy

APPLIED SPECTROSCOPY

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FIG. 1. (a) Sample BP6 with black and white cryptoendolith zones indicated by arrows. (b) SEM BSE image highlighting the distinct boundary between the uncolonized surficial crustal layer and the dark colonized layer beneath. (c) High-magnification SEM BSE image of a pore space colonized by fungal hyphae.

EXPERIMENTAL SETUP Sample Description. The samples of this study are centimeter-sized fragments (Fig. 1) from Devonian (395– 345 Ma) quartzite sandstones of the Beacon Supergroup collected by a British Antarctic Survey expedition to Terra Nova Bay and McMurdo Base during the Antarctic summer of 1995–1996.9 The unprepared rock fragments were naturally air-dried, although this was not a requirement for their analysis or for this study. These sandstones contain a widespread millimeter-thick cryptoendolith habitat that in situ extends beneath the surface from Victoria Land through the Transantarctic Mountains, East Antarctica.10 A distinct zonation is observed in the samples: a surficial orange crustal layer, black and white millimeter-sized layers, a leached white horizon, and a lower pale orange rock substrate. An additional green zonation can be commonly observed but is absent in the samples of this study. Cryptoendolithic lichens form this upper black differentiated zone11 and are composed of dark brownish, grayish, or greenish masses of fungal hyphae enclosing groups of algal cells. The white zone commonly seems to be formed by lichens, but it may contain colorless soredia, i.e., fungal hyphae wrapped around cyanobacteria or green algae.11 The samples in this study, named BP6, only contain the black and white zones (Fig. 1a). They are mineralogically very mature, with well-sorted and well-rounded quartz sandstones with minor clays, and range in grain size from 0.1 to 0.8 mm (Fig. 1b). There is abundant pore space observed in the main body of the

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sample, but lacking within the cryptoendolith zones due to colonization (Figs. 1b and 1c). For a detailed description of the nature and characteristics of these sandstones, see Blackhurst et al.12 Fourier Transform Infrared (FT-IR) Spectroscopy. Investigations of different species of bacterial cells by using Fourier transform infrared spectroscopy (FT-IR) have been undertaken previously by Naumann et al.13 and Helm et al.14 These studies indicated that absorption spectra could bear specific fingerprints of microbial cell constituents. In addition, reflectance techniques have been demonstrated to be reliable for the identification of organic functional groups within fossil materials and ancient rocks.15–22 The sensitivity of FT-IR analyses to organic molecular vibrations within geological materials depends on the contributing mineral phases and the granular state of the sample. This sensitivity can be increased to detect organic molecules in parts per billion if the organic material can be extracted, but this extraction will cause permanent damage to the sample. Spectral identification of the mineralogy and organic biosignatures within unprepared, air-dried, whole-rock cryptoendolith samples was conducted using a Thermo Nicolet Nexus FT-IR spectrometer coupled with a Continulm infrared (IR) microscope. This system allowed visual assessment of the cryptoendolith layers within the samples and condensed the IR beam for spectral acquisition. Reflectance measurements were carried out within a 100 lm aperture square over a spectral range of 600 to 4000 cm 1 at a resolution of 4 cm 1. A gold mirror standard was used and atmo-

FIG. 2. (a) SEM BSE image of the cryptoendolith zone showing colonization of pore spaces and filament draping over neighboring quartz grains. Colonies appear to be spreading outward of the pore spaces, growing onto the quartz grains. (b) Magnified view of fungal hyphae, with some in an almost 3D state and others flattened against the crystal faces. (c) SEM BSE image of the uncolonized lower sandstone, showing the absence of fungal hyphae. (d) Magnified view of (c).

spheric water (H2O) and carbon dioxide (CO2) subtractions made. No further processing of data (e.g., smoothing or Fourier self-deconvolution) was used, to avoid the introduction of artifacts. Band positions are reported simply as the observed maxima, rather than maxima obtained from second derivations or from curvefits. Ten analyses were taken at several spots, creating transects across the sample, to obtain a spectral profile of the cryptoendolith layer and uncolonized sample beneath. Scanning Electron Microscopy. High-magnification imaging of the samples was performed with an FEI Quanta 200 three-dimensional (3D) scanning electron microscope with an accelerating voltage of 20 kV and a beam current of 1.2 nA. To avoid exposing the sample to coating contamination, especially carbon, we recorded backscatter electron images in environmental mode at a pressure of 0.1 mbar, thus removing the need for application of a conductive coating that would otherwise be necessary for examination under a high vacuum.

RESULTS Cryptoendolith Distribution. There is a distinct boundary between the orange crustal zone, the cryptoendolith layers, and the pale orange bulk rock substrate beneath; this boundary is visible at both the millimeter and micrometer scales (Fig. 1b). The blackpigmented cryptoendolith horizon and to a variable extent the lower white zone are visible to the naked eye and form a ,1 mm thick layer (arrows in Fig. 1a) 1 mm beneath the surface. Examination of this region

by using scanning electron microscopy backscattered electron (SEM-BSE) imaging identified eukaryotic cryptoendolithic lichen communities draped around and across the quartz crystals and also concentrated in the pore spaces (Figs. 2a and 2b). The morphologies observed correspond to those described by Friedmann4 of loose filaments 3 lm in width. The upper black zone is composed of masses of fungal (mycobiont) hyphae that can be commonly observed to enclose groups of algal (phycobiont) cells. The hyphae form small spherical bodies (or presquamule-like forms) that are matted together and, as seen in Fig. 1c, those deepest within the pore spaces appear to be crushed or deflated. The hyphae that extend across the quartz grains are observed to maintain an almost 3D structure, whereas others are flattened against the crystal faces. Colonies appear to be spreading outward from the pore spaces, growing onto the quartz grains (Figs. 2a and 2b), with no apparent algal cell connection. The bulk rock substrate beneath is visibly devoid of lichen communities (Figs. 2c and 2d). Surficial Crustal Zone. The FT-IR spectra obtained from within the 1 mm orange crustal layer (Fig. 3a) are characterized by reflectance bands at 1200–1000 and 850–750 cm 1, indicative of the homogenous quartz composition of the samples. Reflectance maxima at 1187, 1135, and 1104 cm 1 are assigned to the Si–O–Si asymmetric stretching fundamental vibration, whereas reflectance bands at 801 and 783 cm 1 are created by the Si–O–Si symmetric stretching fundamentals. Cryptoendolithic Lichen Layer. Quartz crystals with filaments draped over their surface (Fig. 3b) and


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FIG. 3. FT-IR average spectral profiles from the four main layers of sample BP6, from the surficial crustal material (a), through the cryptoendolith layers (b and c), to the main quartz substrate beneath (d).

interstitial filament–filled pore spaces (Fig. 3c) show similar spectral organic functional group distributions. These cryptoendolith colonies contain reflectance decreases or increased absorbance of aliphatic structures as indicated by the aliphatic alkyl group (CHx) bands in the region 2930–2850 cm 1. Absorptions centered at 2930 and 2860 cm 1 are due to asymmetric stretching vibrations and symmetric stretching vibrations from CH2 methylene groups, respectively. A high-intensity absorption between 1880 and 1860 cm 1 and lower intensity absorptions between 1710 and 1700 cm 1 are assigned to oxygenated functions derived from C=O stretching of carbonyl and carboxyl groups. A broad minor absorption at about 1629 cm 1 might be due to the aromatic C=C ring stretching or an amide I vibration. A broad absorption centered at 1456 cm 1 is assigned to the symmetric bending vibrations of CH2 and asymmetric bending vibrations of CH3. An additional weak broad absorption of CH3 symmetric bending is observed at 1372 cm 1 . Absorption bands between 900 and 750 cm 1 (centered at 859, 837, and 813 cm 1) might be attributable to aromatic C–H out-of-plane deformations. The FT-IR spectra from this layer have a low overall percentage reflectance due to the strong absorptions assigned to the v1 and v2 fundamental modes of H2O located at 3679 and 1598 cm 1, and a broad O–H asymmetric stretching vibration at around 3350– 3370 cm 1. Reflectance maxima of variable intensity are located between 1200 and 900 cm 1 and 790 cm 1 due to the Si–O–Si asymmetric and symmetric stretching fundamentals from the surrounding quartz grains, respectively. Rock Substrate. The FT-IR spectra obtained from the rock substrate below the cryptoendolith layer (Fig. 3d) are characterized by intense reflectance bands at 1200– 1000 and 850–750 cm 1, indicative of the samples’ homogenous quartz composition. Reflectance maxima at 1178 and 1108 cm 1 are assigned to the Si–O–Si asymmetric stretching fundamental vibration. Bands at

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801 and 782 cm 1 are the Si–O–Si symmetric stretching fundamentals. No H2O, hydroxide (OH), or CO2 reflectance bands are observed, other than those from background values. Interpretations and Implications. High-precision, nonintrusive FT-IR spectroscopy has the ability to identify biomolecules in their natural state and also to locate them spatially within samples relative to the biological communities. There is a need to analyze the distribution of key molecules of life in undisturbed, unprocessed material, not only to detect and quantify them but also to allow for their distribution to be mapped across the Earth and ultimately on other planetary bodies. The samples within this study required no preparation, with analyses conducted along natural fracture surfaces that exposed the profile of the cryptoendolithic communities. For the determination of potential Antarctic-like cryptoendolith horizons on Mars, however, tools capable of removing weathered surficial crusts and coring to meter depths will be needed to enable better spectral (and to variable extent visual) identification. Several tools with these capabilities have, and will be, sent to Mars, such as the rock abrasion tool on Mars Exploration Rover,23 the rotary percussive drill on Curiosity,24,25 and the future ExoMars drill.26,27 Newly exposed surfaces created during these activities will have unpolished variable topographies. Fourier transform infrared spectral analyses of these rough surfaces will absorb energy and scatter light as a function of particle size, homogeneity, and packing density, thereby influencing the quality of any spectrum obtained. As such, a sample with a smaller particle size and increased homogeneity is preferred, but not always available, especially when such samples are collected and processed remotely on another planet. The rough natures of the samples in this study are intended to mimic those on Mars and have therefore produced spectra of variable noise levels and intensity. Spectra obtained from smooth quartz crystal faces produced the

cleanest, highest percentage reflectance spectra, whereas those from the cryptoendolith-filled pore spaces produced the lowest intensity, noisiest spectra. Both spectral resolutions could and should be expected from analyses on Mars, but they will still provide valuable information on mineralogy, grain size, and surface roughness, as well as any biomolecules present. Biomolecules within inhabited horizons of rock substrates on Mars would be identifiable using spectroscopic techniques, including spectral microscopes such as MicrOmega,28 and those combined with drilling tools, such as the ExoMars Ma-MISS spectrometer.26,27 The former microscope is composed of two microscopes, with one of the microscopes operating in the visible range and the other microscope operating in the nearinfrared (MicrOmega/IR). This spectrometer will cover the spectral range of 0.9–4 lm (11 111–2500 cm 1), with a spatial sampling of 20 lm per pixel. This range encroaches into the mid-IR range covered in this study, to include the OH vibrations of H2O molecules and the 3000–2800 cm 1 range of CH2 and CH3 aliphatic hydrocarbons. MicrOmega will be able to identify carbon-rich phases at the microscopic scale and the mineralogical phases with which they are associated,29 as observed within FT-IR spectra of this study. Analogous studies of terrestrial cryptoendolith horizons and our ability to spectrally detect organics within them are therefore important for interpreting future mineralogical, and hopefully organic, spectral signatures on Mars. Antarctic biological micro-niches are a type of rockdwelling analog community that has been designated as the most relevant putative martian analog for biomolecular analysis, together with niches that include fossil biomolecules. Previous studies have focused on the use of laser Raman spectroscopy for biomolecular analyses of terrestrial analog materials, including Antarctic cryptoendoliths, due to its proposed inclusion as part of a lander or rover instrumentation suite,30–39 with several groups developing and testing miniature Raman spectrometers for this purpose32,38,40–44. The ExoMars Raman laser spectrometer (RLS) will perform Raman spectroscopy on crushed powered samples, collected by the rover’s drill system, and will represent the first time that a Raman spectrometer will be launched on a planetary mission. Biosignatures detectable by Raman spectroscopy include photosynthetic pigments such as chlorophyll; accessory pigments such as phycocyanin,8,45 bacteriorhodopsin,46 and UV-screening compounds such as scytonemin;47 and photoprotection and antioxidant molecules such as b-carotene and other carotenoids.8,40 These biomolecules have all been identified within cryptoendolithic horizons from the Antarctic Dry Valleys9,36,39 and even within the same sample as this study, but as of yet they have not been studied using FT-IR. Considering that two IR-capable instruments are included in the ExoMars payload in addition to the RLS, it is important to study biomolecules within the Mars-like Antarctic environments by using both forms of vibrational spectroscopy. Infrared has long been used to study extraterrestrial geological materials and organic matter, such as occur within meteorites, micrometeorites and interplanetary dust particles, but IR also has been used for astronomical observations. As a

nondestructive tool, this technique is a tried and tested method of organic material analysis both on Earth and in space. Raman spectroscopy, although complementary to IR spectroscopy and also commonly cited as a nondestructive technique, requires that the laser remain at a low power as it can locally heat the sample, damaging and potentially destroying the organic biomolecules present. In addition, on the ExoMars rover, Raman spectroscopic analyses can only be conducted on powdered samples, and powdering itself is a destructive process. Infrared analyses on Mars, however, will not destroy or damage samples through the laser nor require the samples to be powdered as analyses can be conducted on completely undisturbed or mechanically exposed surfaces. It is therefore recommended to analyze samples on Mars by using both IR and Raman spectroscopy, preferably IR first. There is a need to analyze the distribution of key biomolecules in undisturbed field-fresh material, so as to detect and quantify them on Earth and Mars and also to spatially locate them. Fourier transform infrared spectroscopy used in this study identified biomolecules in their natural state; it also identified their spatial distribution within the cryptoendolithic lichen horizons of the Beacon sandstones, and their absence from the surficial crustal zone and rock substrate below. Several organic functional groups were identified that were not documented in previous studies that used Raman spectroscopic techniques.9 Many important stretching and bending vibrational modes associated with lipids (documented by Tamm and Tatulian48) were identified in the cryptoendolithic horizons. In particular, CH2 aliphatic hydrocarbons are observed, most likely the result of the lipid bilayers in the hyphae cell membranes.14 Only the symmetric and asymmetric CH2 stretching vibrational modes are observed in the 3000–2800 cm 1 region, indicating the highly aliphatic nature of the inhabited areas and potentially the lichen themselves. The predominance of methylene-chain CH2 over end-methyl CH3, as observed in this study, has been previously used to indicate a dominant presence of long and straight aliphatic chains (i.e., fatty acids16). The IR spectra of polypeptides exhibit several amide absorption bands that represent different vibrational modes of the peptide bond. An absorption band at 1629 cm 1 may be derived from the amide I C=O stretching vibrational mode49 and indicate the presence of proteins within the samples. Finally, absorptions related to carbonyl and carboxyl groups of carboxylic acids are detected at various wavenumber positions. The preservation potential of biomolecules can be listed in the order lipids . carbohydrates . proteins . nucleic acids.50 Consideration of the preservation potential of all types of biomolecules suggests that the various classes of lipids, especially glycolipids and lipopolysaccharides, are the most resistant to degradation in all types of depositional environments.51 Previous studies of biomolecule preservation20 have indicated that, over geological time, the observation of amide absorption bands within rocks decreases, whereas aliphatic hydrocarbons become more persistent. These cryptoendolith horizons contain functional group vibrations of lipids, carbohydrates, and potentially proteins.


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They do not show spectral evidence of nucleic acids; however, the positioning of the Si–O–Si vibrations of the quartz host rock could mask their signatures. These horizons therefore contain very well-preserved fungal hyphae and associated organic compounds; however, degradation of the biomolecules has occurred. These results indicate that biomolecules, specifically lipids, within the Dry Valley Beacon sandstones are excellent analogs for those that could be detected and preserved on Mars in similar endolithic habitats beneath its surface. A crucial ingredient for carbon-based life on the Earth is H2O. Cryptoendoliths favor the colonization of translucent rocks for the benefit of photosynthetic processes; however, the movement and availability of H2O within their habitat is also of paramount importance. Optimal activity for photometabolism occurs between 80 and 100% H2O content,52 so a porous and permeable substrate is a primary requirement for the habitat. OH and H2O absorption bands were observed within FT-IR spectra taken from the cryptoendolith horizon, but not within the surficial crustal zone or rock substrate. This observation indicates that the cryptoendolith layers are hydrated, providing a localized aqueous environment for the colonies. Fungal hyphae are observed in SEM-BSE images to be spreading out from inside the pore spaces to cover the neighboring quartz grains. This spreading is probably due to initial habitation of H2O-rich areas and then movement driven by a lack of space in the pores once the colonies reach a certain size. This spreading may also be a mechanism of stabilization for the colony. Many hyphae within the pore spaces appear to be crushed or deflated, perhaps due to a gradual dehydration of the sample while in storage. The use of FT-IR analysis has identified zonation within the samples of organic compounds and H2O content. Fourier transform infrared spectroscopy is therefore an invaluable tool to enable the identification of two key ingredients for a habitable environment on another planetary body, within a single analysis.

CONCLUSIONS The cryptoendolithic microorganisms of the Dry Valleys of Antarctica survive in an extremely inhospitable environment, finding refuge in microscopic niches where conditions suitable for life exist. Such endolithic communities may have evolved on Mars when conditions for life on the surface deteriorated to such an extent that they could no longer survive. One of the key ways to identify this life is through vibrational spectroscopy, preferably a combination of FT-IR and Raman spectroscopy. We have used FT-IR to profile molecular vibrations created by fatty acids, proteins, and carboxylic acids within cryptoendolith horizons from Antarctic Beacon sandstones. These spectral biosignatures are localized to lichen-rich areas and are absent in the crustal zone and rock substrates beneath. Water is also found to be concentrated within the cryptoendolithic lichen zones and absent from the rest of the sample. 1. I.B. Campbell, G.G.C. Claridge. ‘‘Antarctica: Soils, Weathering Processes and Environment’’. In: Developments in Soil Science, 16. Amsterdam: The Netherlands: Elsevier Science Publishers, 1987.

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2. C.P. McKay, E.I. Friedmann. ‘‘The Cryptoendolithic Microbial Environment in the Antarctic Cold Desert: Temperature Variations in Nature’’. Polar Biol. 1985. 4: 19-25. 3. E.I. Friedmann, R. Ocampo. ‘‘Endolithic Blue-Green Algae in the Dry Valleys: Primary Producers in the Antarctic Desert Ecosystem’’. Science. 1976. 193(4259): 1247-1249. 4. E.I. Friedmann. ‘‘Endolithic Microorganisms in the Antarctic Cold Desert’’. Science. 1982. 215(4356): 1045-1053. 5. J.A. Nienow, C.P. McKay, E.I. Friedmann. ‘‘The Cryptoendolithic Microbial Environment in the Ross Desert of Antarctica: Light in the Photosynthetically Active Region’’. Microb. Ecol. 1988. 16: 271-289. 6. J.A. Nienow, E.I. Friedmann. ‘‘Terrestrial Lithophytic Communities’’. In: E.I. Friedmann, editor. Antarctic Microbiology. New York: Wiley-Liss, 1993. Pp. 343-412. 7. R. Ocampo-Friedmann, E.I. Friedmann. ‘‘Biologically Active Substances Produced by Antarctic Cryptoendolithic Fungi’’. Antarct. J. U.S. 1993. 28(5): 252-254. 8. D.D. Wynn-Williams, H.G.M. Edwards. ‘‘Antarctic Ecosystems as Models for Extraterrestrial Surface Habitats’’. Planet. Space Sci. 2000. 48(11): 1065-1075. 9. H.G.M. Edwards, N.C. Russell, D.D. Wynn-Williams. ‘‘Fourier Transform Raman Spectroscopic and Scanning Electron Microscopic Study of Cryptoendolithic Lichens from Antarctica’’. J. Raman Spectrosc. 1997. 28(9): 685-690. 10. N.C. Russell, H.G.M. Edwards, D.D. Wynn-Williams. ‘‘FT-Raman Spectroscopic Analysis of Endolithic Microbial Communities from Beacon Sandstone in Victoria Land, Antarctica’’. Antarct. Sci. 1998. 10(1): 63-74. 11. E.I. Friedmann, M. Hua, R. Ocampo-Friedmann. ‘‘Cryptoendolithic Lichen and Cyanobacterial Communities of the Ross Desert, Antarctica’’. Polarforschung. 1988. 58(2-3): 251-259. 12. R.L. Blackhurst, M.J. Genge, A.T. Kearsley, M.M. Grady. ‘‘Cryptoendolithic Alteration of Antarctic Sandstones: Pioneers or Opportunists?’’. J. Geophys. Res. 2005. 110(E12): E12S24. doi:10. 1029/2005JE002463. 13. D. Naumann, C.P. Schultz, D. Helm. ‘‘What Can Infrared Spectroscopy Tell Us About the Structure and Composition of Intact Bacterial Cells?’’. In: H.H. Mantsch, D. Chapman, editors. Infrared Spectroscopy of Biomolecules. New York: Wiley-Liss, 1996. Pp. 279310. 14. D. Helm, H. Labischinski, G. Schallehn, D. Naumann. ‘‘Classification and Identification of Bacteria by Fourier-Transform Infrared Spectroscopy’’. J. Gen. Microbiol. 1991. 137(1): 69-79. 15. M. Igisu, S. Nakashima, Y. Ueno, S.M. Awramik, S. Maruyama. ‘‘In Situ Infrared Microspectroscopy Approximately 850 Million-YearOld Prokaryotic Fossils’’. Appl. Spectrosc. 2006. 60(10): 1111-1120. 16. M. Igisu, Y. Ueno, M. Shimojima, S. Nakashima, S.M. Awramik, H. Ohta, S. Maruyama. ‘‘Micro-FTIR Spectroscopic Signatures of Bacterial Lipids in Proterozoic Microfossils’’. Precambrian Res. 2009. 173(1-4): 19-26. 17. M. Igisu, Y. Ueno, M. Shimojima, S. Nakashima, S.M. Awramik, S. Maruyama. ‘‘Micro-FTIR Spectroscopic Imaging of 1900 Ma Stromatolitic Chert’’. In: V. Tewari, J. Seckbach, editors. Stromatolites: Interactions of Microbes with Sediments. Dordrecht, The Netherlands: Kluwer Academic Publishers, 2011. Pp. 445-461. 18. L.J. Preston, G.K. Benedix, M.J. Genge, M.A. Sephton. ‘‘A Mulitdiscipinary Study of Silica Sinter Deposits with Applications to Silica Identification and Detection of Fossil Life on Mars’’. Icarus. 2008. 198(2): 331-350. 19. L.J. Preston, M.J. Genge. ‘‘The Rhynie Chert, Scotland and the Search for Life on Mars’’. Astrobiology. 2010. 10(5): 549-560. 20. L.J. Preston, J. Shuster, D. Fernańdez-Remolar, N.R. Banerjee, G.R. Osinski, G. Southam. ‘‘The Preservation and Degradation of Filamentous Bacteria and Biomolecules in Iron Oxide Deposits from Rio Tinto, Spain’’. Geobiology. 2011. 9(3): 233-249. 21. L.J. Preston, M.R.M. Izawa, N.R. Banerjee. ‘‘Infrared Spectroscopic Characterisation of Organic Matter Associated with Microbial Bioalteration Textures in Basaltic Glass’’. Astrobiology. 2011. 11(7): 585-599. 22. L.J. Preston, L.A. Melim, V.J. Polyak, Y. Asmerom, G. Southam. ‘‘Infrared Spectroscopic Biosignatures from Hidden Cave, New Mexico: Possible Applications for Remote Life Detection’’. Geomicrobiol. J. 2014. 31(10): 929-941. 23. S.P. Gorevan, T. Myrick, K. Davis, J.J. Chau, P. Bartlett, S. Mukherjee, R. Anderson, S.W. Squyres, R.E. Arvidson, M.B. Madsen, P. Bertelsen, W. Goetz, C.S. Binau, L. Richter. ‘‘Rock

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