Cave Microbe-Mineral Suites: Best Model for Extraterrestrial Biosignatures! P.J
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Lunar and Planetary Science XXXII (2001) 2015.pdf CAVE MICROBE-MINERAL SUITES: BEST MODEL FOR EXTRATERRESTRIAL BIOSIGNATURES! P.J. Boston1, M.N. Spilde2, D.E. Northup1, and L.A. Melim3. 1 Biology Dept., Univ. New Mexico, Albuquerque, NM, 87131 ([email protected]), 2 Inst. Meteoritics, UNM, Albuquerque, NM, 3.Geology Dept., West. Illinois Univ., Macomb, IL 61455 Introduction: We now know that Earth’s subsur- traits to ultimately apply as templates to extraterrestrial face is a rich source of extremely diverse microorgan- environments on future missions. isms [1]. Caves, subsurface fissures, microcracks, and Lifestyle Biomarkers of the Small and Slimy: intergrain pore spaces all provide homes for microbial We are attempting to classify our many examples of life away from the ravages of the surface; ordinary biosignatures into “suites” that can be reliably applied weather, dessication, temperature fluctuations, ultra- to microbial cave communities across a number of violet radiation, and grazing by higher organisms chemically distinct environments. These include a [2,3,4,5,6]. In these protected subterrains, microbes very common tendency to form filaments that are carry out metabolism and often experience extensive in coated with minerals and a widespread tendency to situ lithification and subsequent preservation without form mats on surfaces as diverse as walls, ceilings, destruction common to surface materials or diagenesis floors, overhangs, and even deep within convoluted common to buried deposits. Thus, Earth’s subsurface rock piping of cave springs [6]. We are particularly offers one of the best of all possible site types to search interested in biomarkers that produce visible large for extant, recently alive, and long-dead microbial life- scale structures or material types easily seen with na- forms and characteristic lithologies that they leave be- ked-eye or low power magnification. We also want hind. This may be equally true for the subsurface of some additional unique characteristic…either geo- Mars and other planets. Where surface conditions are chemical, isotopic, crystallographic, unusual elemental particularly hostile (as on Mars), the subsurface may ratios, or oxidation states. This is a great deal to ask offer the ONLY access to recognizably preserved from a single biosignature suite, but we have several biosignatures or extant lifeforms [7,8]. examples that promise just such an array of indicators. Mineral/Microbial Biosignatures: We are inves- Crisco and Moonmilk: We have identified several tigating specific subsurface mineral/microbial associa- cave deposit types that are found in macroscopically tions in a variety of chemically and physically distinct detectable quantities (sometimes entire passages are Earth caves [2,3,4,5,6]. Our goal is to discover detect- covered) and that derive from biological activity able macroscopic, microscopic, and chemical charac- showing microscopically visible characteristics and teristics that are unequivocally biogenic. Because we with accompanying chemical signatures. One gummy are working in fragile, non-renewable and frequently calcite material, dubbed “Crisco” by cavers, occurs in legally-protected environments, we emphasize the de- great abundance on the surfaces of particular passages velopment of non-invasive and minimal impact ana- in Spider Cave, NM (Fig. 1). lytical techniques for life detection and characteriza- tion. Moreover, we are conducting these procedures under very difficult field conditions that share many of the limitations common to extraterrestrial robotic and human missions [9,10]. Thus, the cave/subsurface model of extraterrestrial life search strategies addresses the most important goals of astrobiological inquiry from both scientific and operational points of view The Ideal Extraterrestrial Biosignature Suite: We have numerous examples of individual biosigna- tures from various cave environments, ranging from obvious morphological structures to characteristic bio- genic mud textures [2,3,4,5,6]. However, to be espe- cially useful in the astrobiological context, biosigna- tures must be 1) detectable at both macroscopic and Figure 1. The Walls Are Alive! M. Spilde sampling microscopic scales, 2) independently verifiable by non- active microbe-mineral deposit, Spider Cave, NM. morphological means, and 3) relatively independent of Forming a sticky paste that clings like clay-rich mud to the specific details of the life chemistries of the re- every surface, examination with a JEOL 5800LV sponsible organisms. This is a tall order. We are Scanning Electron Microscope reveals filaments of searching for the most robust combinations of these bimodal size distribution and putative bacteria that Lunar and Planetary Science XXXII (2001) 2015.pdf CAVE BIOSIGNITURES: P. J. Boston et al. may be forming pits in the calcite (Fig. 2). EDS analy- chemically in some cases. For example, in long dried sis (Energy Dispersion Spectroscopy) shows elemental out cave pools, finger-shaped deposits remain although abundances consistent only with calcite. However, no living entitites or even organic materials are present upon dissolution with weak acid (HCl), a transparent [3]. Characteristic micritic textures, unmistakable mor- glob of organic material remains. Staining of intact phological microbial fossils, and δ13C values consistent Crisco with DNA-binding fluorescent dyes (e.g. with microbial metabolism of carbon are exquisitely Acridine Orange and DAPI) shows that the calcite is preserved and easily detectable (Fig.4). precipitated on the surface of filaments that are or re- cently were living. Crisco is one example of a class of cave deposits collectively known as “moonmilks”. These vary in chemical composition (calcite to hydro- magnesite), but form distinctive white, gooey mudlike deposits, often over extensive areas. When calcitic moonmilk is dried out and apparently no longer active, it also presents a characteristic appearance that is eas- ily identifiable macroscopically and with SEM (Fig. 3). Laboratory work by Chafetz and Guidry [11] has shown unique calcite bundles produced only by micro- bial activity that are reminiscent of crystals that we see in moonmilks. Figure 3. Dried, biologically inactive calcite moon- milk, Barrancas Cave, NM. Scale bar = 2 µm. Figure 2. Biologically active calcite moonmilk (Crisco), Spider Cave, NM. Arrows show possible bacterial components. Scale bar = 2 µm. Stringy Manganese: Our attention is not restricted Figure 4. Fossil microbial mat textures from long simply to calcite. Stringy and fabric-like manganese dry cave pool, Hidden Cave, NM. Scale bar = 2 µm. deposits occur in Lechuguilla Cave, NM and we are Other candidate biosignature suites from caves currently verifying which features are attributable to (bacterial/gypsum crystal snottite formations, long- microbes [4]. This material also forms distinctive mac- dead lithified u-loops, silicified microbial mats with roscopic deposits and is much more stable over time subsequent pyritization in H2S systems, and others) than analogous iron minerals produced bacterially. will be discussed in detail. Mats on Walls, Ceilings, and Lining Springs: References: [1] Amy, P.S. and Haldeman, D.L. (1997) Microbiol. We have found microbial mats of many different types of Terrestr. Subsurf. [2] Hose, L. D. et al. (1999) Chem.Geol., 169, in caves [2,3,6]. Often these mats appear to be nothing 399-423. [3] Melim L.A. et al. (2001) Geomicrobiol. J., in press. [4] more than unusual mud textures. When examined with Spilde, M.N. et al., (2001) LPS XXXI, this volume. [5] Northup, D.E. SEM, epifluorescence, and analyzed for chemical and et al. (2000) J. Cave & Karst Studies, 62(2), 149-160. [6] Boston, molecular genetic markers, however, we find that these P.J. (1999) 4th Intl. Symp. Subsurf. Microbiol. Abs., 36. [7] Boston, are actually microbial mats containing huge numbers P.J. et al. (1992) Icarus 95, 300-308. [8] Boston, P.J. (2000), Geo- of organisms, mineral particles, and held together by times 45(8), 14-17. [9] Boston, P.J. (2000), NASA Tech. Mem. 2000- extracellular slimes. Dried, inactive examples of these 208577, 9-17. [10] Boston, P.J. (1999) Life on Mars, JBIS 105-112. materials are still distinguishable both physically and [11] Chafetz, H.S. and Guidry, S.A. (1999) Sed. Geol. 126, 57-74..