Microbial stowaways: inimitable survivors or hopeless pioneers?

Abstract The resiliency of prokaryotic life has provided colonization across the globe and in the recesses of Earth's most extreme environments. Horizontal gene transfer provides access to a global bank of genetic resources that creates diversity and allows real-time adaptive potential to the clonal prokaryotic world. We assess the likelihood that this Earth-based strategy could provide survival and adaptive potential, in the case of microbial stowaways off Earth. Key Words: Bacillus-Horizontal gene transfer-Bacteria-Earth Mars-Evolution. Astrobiology 12, 710-715.     

Spectral signatures of photosynthesis. I. Review of Earth organisms

Why do plants reflect in the green and have a "red edge" in the red, and should extrasolar photosynthesis be the same? We provide (1) a brief review of how photosynthesis works, (2) an overview of the diversity of photosynthetic organisms, their light harvesting systems, and environmental ranges, (3) a synthesis of photosynthetic surface spectral signatures, and (4) evolutionary rationales for photosynthetic surface reflectance spectra with regard to utilization of photon energy and the planetary light environment. We found the "near-infrared (NIR) end" of the red edge to trend from blue-shifted to reddest for (in order) snow algae, temperate algae, lichens, mosses, aquatic plants, and finally terrestrial vascular plants. The red edge is weak or sloping in lichens. Purple bacteria exhibit possibly a sloping edge in the NIR. More studies are needed on pigment-protein complexes, membrane composition, and measurements of bacteria before firm conclusions can be drawn about the role of the NIR reflectance. Pigment absorbance features are strongly correlated with features of atmospheric spectral transmittance: P680 in Photosystem II with the peak surface incident photon flux density at approximately 685 nm, just before an oxygen band at 687.5 nm; the NIR end of the red edge with water absorbance bands and the oxygen A-band at 761 nm; and bacteriochlorophyll reaction center wavelengths with local maxima in atmospheric and water transmittance spectra. Given the surface incident photon flux density spectrum and resonance transfer in light harvesting, we propose some rules with regard to where photosynthetic pigments will peak in absorbance: (1) the wavelength of peak incident photon flux; (2) the longest available wavelength for core antenna or reaction center pigments; and (3) the shortest wavelengths within an atmospheric window for accessory pigments. That plants absorb less green light may not be an inefficient legacy of evolutionary history, but may actually satisfy the above criteria.     

Travel, Sex, and Food: What's Speciation Got to Do with It?

Abstract We discuss the potential interactions among travel (dispersal and gene flow), bacterial "sex" (mainly as horizontal gene transfer), and food (metabolic plasticity and responses to nutrient availability) in shaping microbial communities. With regard to our work at a unique desert oasis, the Cuatro Ciénegas Basin in Coahuila, Mexico, we propose that diversification and low phosphorus availability, in combination with mechanisms for nutrient recycling and community cohesion, result in enhanced speciation through reproductive as well as geographic isolation. We also discuss these mechanisms in the broader sense of ecology and evolution. Of special relevance to astrobiology and central to evolutionary biology, we ask why there are so many species on Earth and provide a working hypothesis and a conceptual framework within which to consider the question. Key Words: Microbial ecology-Microbial mats-Evolution-Horizontal gene transfer-Metabolism. Astrobiology 12, 634-640.     

The cuatro ciénegas basin in coahuila, Mexico: an astrobiological precambrian park

Abstract The Cuatro Ciénegas Basin (CCB) is a rare oasis in the Chihuahuan Desert in the state of Coahuila, Mexico. It has a biological endemism similar to that of the Galapagos Islands, and its spring-fed ecosystems have very low nutrient content (nitrogen or phosphorous) and are dominated by diverse microbialites. Thus, it has proven to be a distinctive opportunity for the field of astrobiology, as the CCB can be seen as a proxy for an earlier time in Earth's history, in particular the late Precambrian, the biological frontier when prokaryotic life yielded at least partial dominance to eukaryotes and multicellular life. It is a kind of ecological time machine that provides abundant opportunities for collaborative investigations by geochemists, geologists, ecologists, and population biologists in the study of the evolutionary processes that structured Earth-based life, especially in the microbial realm. The CCB is an object of investigation for the identification of biosignatures of past and present biota that can be used in our search for extraterrestrial life. In this review, we summarize CCB research efforts that began with microbial ecology and population biology projects and have since been expanded into broader efforts that involve biogeochemistry, comparative genomics, and assessments of biosignatures. We also propose that, in the future, the CCB is sanctioned as a "Precambrian Park" for astrobiology. Key Words: Microbial mats-Stromatolites-Early Earth-Extremophilic microorganisms-Microbial ecology. Astrobiology 12, 641-647.     

Spectral signatures of photosynthesis. II. Coevolution with other stars and the atmosphere on extrasolar worlds

As photosynthesis on Earth produces the primary signatures of life that can be detected astronomically at the global scale, a strong focus of the search for extrasolar life will be photosynthesis, particularly photosynthesis that has evolved with a different parent star. We take previously simulated planetary atmospheric compositions for Earth-like planets around observed F2V and K2V, modeled M1V and M5V stars, and around the active M4.5V star AD Leo; our scenarios use Earth's atmospheric composition as well as very low O2 content in case anoxygenic photosynthesis dominates. With a line-by-line radiative transfer model, we calculate the incident spectral photon flux densities at the surface of the planet and under water. We identify bands of available photosynthetically relevant radiation and find that photosynthetic pigments on planets around F2V stars may peak in absorbance in the blue, K2V in the red-orange, and M stars in the near-infrared, in bands at 0.93-1.1 microm, 1.1-1.4 microm, 1.5-1.8 microm, and 1.8-2.5 microm. However, underwater organisms will be restricted to wavelengths shorter than 1.4 microm and more likely below 1.1 microm. M star planets without oxygenic photosynthesis will have photon fluxes above 1.6 microm curtailed by methane. Longer-wavelength, multi-photo-system series would reduce the quantum yield but could allow for oxygenic photosystems at longer wavelengths. A wavelength of 1.1 microm is a possible upper cutoff for electronic transitions versus only vibrational energy; however, this cutoff is not strict, since such energetics depend on molecular configuration. M star planets could be a half to a tenth as productive as Earth in the visible, but exceed Earth if useful photons extend to 1.1 microm for anoxygenic photosynthesis. Under water, organisms would still be able to survive ultraviolet flares from young M stars and acquire adequate light for growth.     

等离子喷涂Al_2O_3涂层的电击穿机理

采用大气等离子喷涂技术在铜基体上沉积了Al2O3涂层。采用XRD和SEM对涂层微观结构进行了表征。通过探讨孔隙率和吸潮行为对绝缘性能的影响,分析了等离子喷涂Al2O3涂层结构与电绝缘失效机理的关系。结果表明:等离子喷涂Al2O3涂层较致密,界面结合较好。随涂层厚度不同其孔隙率在5%~7%范围变化。等离子喷涂Al2O3涂层结构中的孔洞是电绝缘失效的主要部位且呈典型电晕击穿形貌。电晕击穿诱发的裂纹沿击穿方向扩展形成击穿隧道。击穿方向与电极极性无关而由击穿孔洞位置决定。涂层厚度与涂层击穿强度呈现倒数关系。吸潮会诱发导电通路形成降低Al2O3涂层抗击穿能力。     

Spatially resolved genomic, stable isotopic, and lipid analyses of a modern freshwater microbialite from cuatro ciénegas, Mexico

Abstract Microbialites are biologically mediated carbonate deposits found in diverse environments worldwide. To explore the organisms and processes involved in microbialite formation, this study integrated genomic, lipid, and both organic and inorganic stable isotopic analyses to examine five discrete depth horizons spanning the surface 25?mm of a modern freshwater microbialite from Cuatro Ciénegas, Mexico. Distinct bacterial communities and geochemical signatures were observed in each microbialite layer. Photoautotrophic organisms accounted for approximately 65% of the sequences in the surface community and produced biomass with distinctive lipid biomarker and isotopic (δ(13)C) signatures. This photoautotrophic biomass was efficiently degraded in the deeper layers by heterotrophic organisms, primarily sulfate-reducing proteobacteria. Two spatially distinct zones of carbonate precipitation were observed within the microbialite, with the first zone corresponding to the phototroph-dominated portion of the microbialite and the second zone associated with the presence of sulfate-reducing heterotrophs. The coupling of photoautotrophic production, heterotrophic decomposition, and remineralization of organic matter led to the incorporation of a characteristic biogenic signature into the inorganic CaCO(3) matrix. Overall, spatially resolved multidisciplinary analyses of the microbialite enabled correlations to be made between the distribution of specific organisms, precipitation of carbonate, and preservation of unique lipid and isotopic geochemical signatures. These findings are critical for understanding the formation of modern microbialites and have implications for the interpretation of ancient microbialite records. Key Words: Microbial ecology-Microbe-mineral interactions-Microbial mats-Stromatolites-Genomics. Astrobiology 12, 685-698.