RESUMO
Dryness is one of the main environmental challenges to microbial survival. Understanding the threshold of microbial tolerance to extreme dryness is relevant to better constrain the environmental limits of life on Earth and critically evaluate long-term habitability models of Mars. Biomolecular proxies for microbial adaptation and growth were measured in Mars-like hyperarid surface soils in the Atacama Desert that experience only a few millimeters of precipitation per decade, and in biologically active soils a few hundred kilometers away that experience two- to fivefold more precipitation. Diversity and abundance of lipids and other biomolecules decreased with increasing dryness. Cyclopropane fatty acids (CFAs), which are indicative of adaptive response to environmental stress and growth in bacteria, were only detected in the wetter surface soils. The ratio of trans to cis isomers of an unsaturated fatty acid, another bacterial stress indicator, decreased with increasingly dry conditions. Aspartic acid racemization ratios increased from 0.01 in the wetter soils to 0.1 in the driest soils, which is indicative of racemization rates comparable to de novo biosynthesis over long timescales (â¼10,000 years). The content and integrity of stress proteins profiled by immunoassays were additional indicators that biomass in the driest soils is not recycled at significant levels. Together, our results point to minimal or no in situ microbial growth in the driest surface soils of the Atacama, and any metabolic activity is likely to be basal for cellular repair and maintenance only. Our data add to a growing body of evidence that the driest Atacama surface soils represent a threshold for long-term habitability (i.e., growth and reproduction). These results place constraints on the potential for extant life on the surface of Mars, which is 100-1000 times drier than the driest regions in the Atacama. Key Words: Atacama Desert-Dryness-Growth-Habitability-Biomarker-Mars. Astrobiology 18, 955-966.
Assuntos
Bactérias/metabolismo , Clima Desértico , Exobiologia/métodos , Marte , Microbiologia do Solo , Bactérias/isolamento & purificação , Biomarcadores/análise , Biomarcadores/metabolismo , Biomassa , Chile , Meio Ambiente ExtraterrenoRESUMO
Here we consider that the corrosion of polished bared metal coupons can be used as a passive sensor to detect or identify the lower limit of water availability suitable for biological activity in Atacama Desert soils or solid substrates. For this purpose, carbon steel coupons were deposited at selected sites along a west-east transect and removed at predetermined times for morphological inspection. The advantage of this procedure is that the attributes of the oxide layer (corrosion extent, morphology and oxide phases) can be considered as a fingerprint of the atmospheric moisture history at a given time interval. Two types of coupons were used, long rectangular shaped ones that were half-buried in a vertical position, and square shaped ones that were deposited on the soil surface. The morphological attributes observed by SEM inspection were found to correlate to the so-called humectation time which is determined from local meteorological parameters. The main finding was that the decreasing trend of atmospheric moisture along the transect was closely related to corrosion behaviour and water soil penetration. For instance, at the coastal site oxide phases formed on the coupon surface rapidly evolve into well-crystallized species, while at the driest inland site Lomas Bayas only amorphous oxide was observed on the coupons.
Assuntos
Clima Desértico , Monitoramento Ambiental/instrumentação , Umidade , Aço/química , Chile , Corrosão , Microbiologia do Solo , TemperaturaRESUMO
Traces of life are nearly ubiquitous on Earth. However, a central unresolved question is whether these traces always indicate an active microbial community or whether, in extreme environments, such as hyperarid deserts, they instead reflect just dormant or dead cells. Although microbial biomass and diversity decrease with increasing aridity in the Atacama Desert, we provide multiple lines of evidence for the presence of an at times metabolically active, microbial community in one of the driest places on Earth. We base this observation on four major lines of evidence: (i) a physico-chemical characterization of the soil habitability after an exceptional rain event, (ii) identified biomolecules indicative of potentially active cells [e.g., presence of ATP, phospholipid fatty acids (PLFAs), metabolites, and enzymatic activity], (iii) measurements of in situ replication rates of genomes of uncultivated bacteria reconstructed from selected samples, and (iv) microbial community patterns specific to soil parameters and depths. We infer that the microbial populations have undergone selection and adaptation in response to their specific soil microenvironment and in particular to the degree of aridity. Collectively, our results highlight that even the hyperarid Atacama Desert can provide a habitable environment for microorganisms that allows them to become metabolically active following an episodic increase in moisture and that once it decreases, so does the activity of the microbiota. These results have implications for the prospect of life on other planets such as Mars, which has transitioned from an earlier wetter environment to today's extreme hyperaridity.
Assuntos
Bactérias/isolamento & purificação , Ecossistema , Microbiologia do Solo , Bactérias/classificação , Bactérias/genética , Biodiversidade , Clima Desértico , Solo/química , América do SulRESUMO
The hyperarid core of the Atacama Desert is one of the driest and most inhospitable places on Earth, where life is most commonly found in the interior of rocks (i.e., endolithic habitats). Due to the extreme dryness, microbial activity in these habitats is expected to be low; however, the rate of carbon cycling within these microbial communities remains unknown. We address this issue by characterizing the isotopic composition ((13)C and (14)C) of phospholipid fatty acids (PLFA) and glycolipid fatty acids (GLFA) in colonized rocks from four different sites inside the hyperarid core. δ(13)C results suggest that autotrophy and/or quantitative conversion of organic matter to CO2 are the dominant processes occurring with the rock. Most Δ(14)C signatures of PLFA and GLFA were consistent with modern atmospheric CO2, indicating that endoliths are using atmospheric carbon as a primary carbon source and are also cycling carbon quickly. However, at one site the PLFA contained (14)C from atmospheric nuclear weapons testing that occurred during the 1950s and 1960s, indicating a decadal rate of carbon cycling. At the driest site (Yungay), based on the relative abundance and (14)C content of GLFA and PLFA, there was evidence of possible preservation. Hence, in low-moisture conditions, glycolipids may persist while phospholipids are preferentially hydrolyzed.