2000 Annual Science Report
NASA Johnson Space Center Reporting | JUL 1999 – JUN 2000
Biomarkers in Terrestrial Samples
The importance of terrestrial samples to Astrobiology cannot be overestimated. Only by understanding the evidence for life preserved in the rocks and soil of Earth can we assess indications of possible life elsewhere in the universe. We currently have four main thrusts of this research: Modern Life in Extreme Environments, Ancient Life, Natural contamination of fossiliferous and non-fossiliferous lithologies, and Experiments.
Modern Life in Extreme Environments
We are studying samples from hot springs, caves, mines and endolithic environments to document the presence of microbial life and its physical and chemical biomarkers.
We are studying rocks that contain the earliest physical evidence of life on Earth, in order to document the retention of microbial forms and other biosignatures in the geologic record. Furthermore, we are investigating the variety of environments in which the most ancient life is preserved. This will hopefully aid interpretation of where and possibly how life started.
Natural contamination of fossiliferous and non-fossiliferous lithologies
Fossiliferous rocks can become naturally contaminated through microbes living in cracks in both fossiliferous and non-fossiliferous lithologies, as well as microbes living between grains in harsh environments. We are looking at fossilized microbes in cracks in Early Archaean, carbonaceous, fossiliferous cherts, as well as in serpentinized deep sea ultramafics. Fossilized endolithic microorganisms in Arctic environments are also being investigated. Impact craters comprise another environment under investigation for fossilized endolithic organisms.
We are conducting irradiation and shock experiments on terrestrial rocks to understand the effects of these stresses on samples from other planets. Some of the material being shocked is fossiliferous. Fossiliferous Early Archaean cherts are being experimentally metamorphosed to determine the effects of amphibolite/lower granulite metamorphism on the survival of carbonaceous microfossils. Films formed from prebiotic molecules are being artificially created as a comparison for biogenic films (biofilms) in order to distinguish differences between abiotic and biotic polymer films.
Biomarkers in Terrestrial Samples (dm)
Physical Biomarkers in Carbonate Hot Springs
Physical evidence of life (physical biomarkers) from the deposits of carbonate hot springs were documented at the scale of microorganisms â?? submillimeter to submicrometer. The four moderate-temperature (57â??C to 72â??C), neutral pH springs reported on in this study, support diverse communities of bacteria adapted to specific physical and chemical conditions. Some of the microbes coexist with travertine deposits in endolithic communities. In other cases, the microbes are rapidly coated and destroyed by precipitates but leave distinctive mineral fabrics. Some microbes adapted to carbonate hot springs produce extracellular polymeric substance which forms a three-dimensional matrix with living cells, and cell remains, known as a biofilm. Silicon and iron oxides often coat the biofilm, leading to long-term preservation. Submicrometer mineralized spheres composed of calcium fluoride or silica are common in carbonate hot spring deposits. Sphere formation is biologically mediated but the spheres themselves are apparently not fossils or microbes. Additionally, some microbes selectively weather mineral surfaces in distinctive patterns. Hot spring deposits have been cited as prime locations for exobiological exploration of Mars. The presence of preserved microscopic physical biomarkers at all four sites supports a strategy of searching for evidence of life in hot spring deposits on Mars.
Early Archaean Fossil Bacteria and Biofilms in Hydrothermally-Influenced Sediments from the Barberton Greenstone Belt, South Africa
Electron microscopy (SEM) imaging of HF etched, 3.3-3.5 GA cherts from the Onverwacht Group, South Africa, reveals small spherical (1 mm diameter) and rod-shaped structures (2-3.8 mm in length), which are interpreted as possible fossil coccoid and bacillar bacteria (prokaryotes), respectively, preserved by mineral replacement. Other possibly biogenic structures include smaller rod-shaped bacteriomorphs (<2 mm in length) and bacteriomorph molds. The identification of these structures as fossil bacteria is based on size, shape, cell division, distribution in colonies and occurrence in biolaminated sediments. The exceptionally fine conservation has preserved textures such as wrinkled outer walls on the coccoid fossils, while the bacillar fossils are turgid. Carbon isotope analyses support the presence of bacteria in these cherts with d13C value around â?? 27 per mil. The cherts are characterized by fine, wavy laminae created by granular to smooth or ropy-textured films coating bedding planes, interpreted as probable bacterial biofilms, which have been pseudomorphed by minerals. Although most of the Onverwacht Group was deposited in relatively deep water (>900 m), textures in the sediments in which these biogenic structures occur suggest that they were probably deposited in a shallow water environment which was subjected to intermittent subaerial exposure. Pervasive hydrothermal activity is evidenced by oxygen isotope studies as well as the penecontemporaneous silicification of all rock types by low temperature (£ 220â??C) hydrothermal solutions.
The Nature of Fossil bacteria: A Guide to the Search for Extraterrestrial Life
In an attempt to establish reliable criteria for the identification of potential fossil life in extraterrestrial materials, the fossilizable characteristics of bacteria, namely, size, shape, cell wall texture, association, and colony formation, are described, and an overview is given of the ways in which fossil bacteria are preserved (as compressions in fine-grained sediments; preservation in amber; permineralized by silica; replacement by minerals such as silica, pyrite, Fe/Mn oxides, calcite, phosphate, and siderite; or as molds in minerals). The problem of confounding minerally replaced bacteria with non-biological structures having a bacterial morphology is addressed. Examples of fossilized bacteria from the Early Archaean through to the Recent are used to illustrate the various modes of preservation and the morphology of fossil bacteria.
Biofilms as Biomarkers in Terrestrial and Extraterrestrial Materials
Organic polymeric substances are a fundamental component of microbial biofilms. Microorganisms, especially bacteria, secrete extracellular polymeric substances (EPS) to form slime layers in which they reproduce. In the sedimentary environment biofilms commonly contain the products of bacterial degradation as well as allocthonous and autochthonous mineral components. They are complex structures which serve as protection for the colonies of microorganisms living in them and also act as nutrient traps. Biofilms are almost ubiquitous wherever there is an interface and moisture (liquid/liquid, liquid/solid, liquid/gas, solid/gas). In sedimentary rocks they are commonly recognized as stromatolites. The EPS and cell components of the microbial biofilms contain many cation-chleation sites which are implicated in the mineralization of the films. EPS, biofilms, and their related components, thus, have strong preservation potential in the rock record. Fossilized microbial polymeric substances (FPS) and biofilms appear to retain the same morphological characteristics as the unfossilized material and have been recognized in rock formations dating back to Early Archaean (3.5 Ga). We describe FPS and biofilms from hot spring, deep sea, volcanic lake, and shallow marine/littoral environments ranging up to 3.5 Ga in age. FPS and biofilms are more common than fossil bacteria themselves, especially in the older part of the terrestrial record. The widespread distribution of microbial biofilms and their great survival potential makes their fossilized remains a useful biomarker as a proxy for live with obvious application to the search for life in extraterrestrial materials.
Gamma Sterilization of Mars Rocks and Minerals
Rock and soil samples from Mars are due to be returned to Earth within a decade. Martian samples initially will be tested for evidence of life and biological hazard under strict biological containment. Wider distribution of samples for organic and inorganic analysis may occur only if neither evidence of life nor hazard is detected, or if the samples are first sterilized. We subjected a range of Mars analog rocks and minerals to high doses of gamma radiation in order to determine the effects of gamma sterilization on the samples’ isotopic, chemical, and physical properties. Gamma photons from 60Co (1.17 and 1.33 MeV) in doses as high as 3 × 107 rads did not induce radioactivity in the samples and produced no measurable changes in their isotopic and chemical compositions. This level of irradiation also produced no measurable changes in the crystallographic structure of any sample, the surface areas of soil analogs, or the fluid inclusion homogenization temperature of quartz. The only detectable effects of irradiation were dose-dependent changes in the visible and near-infrared spectral region (e.g., discoloration and darkening of quartz and halite and an increase in albedo of carbonates) and increases in the thermoluminescence of quartz and plagioclase. If samples returned from Mars require biological sterilization, gamma irradiation provides a feasible option.
PROJECT MEMBERS:David McKay
Mary Sue Bell
RELATED OBJECTIVES:Objective 6.0
Define how ecophysiological processes structure microbial communities, influence their adaptation and evolution, and affect their detection on other planets.
Identify the environmental limits for life by examining biological adaptations to extremes in environmental conditions.
Refine planetary protection guidelines and develop protection technology for human and robotic missions.