RESUMO
The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.
Assuntos
Ciclo do Carbono , Extinção Biológica , Erupções Vulcânicas , Dióxido de Carbono/análise , Aquecimento Global , México , Modelos TeóricosRESUMO
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
Microbialites are organosedimentary structures that are formed through the interaction of benthic microbial communities and sediments and include mineral precipitation. These lithifying microbial mat structures include stromatolites and thrombolites. Exuma Sound in the Bahamas, and Hamelin Pool in Shark Bay, Western Australia, are two locations where significant stands of modern microbialites exist. Although prokaryotic diversity in these structures is reasonably well documented, little is known about the eukaryotic component of these communities and their potential to influence sedimentary fabrics through grazing, binding and burrowing activities. Accordingly, comparisons of eukaryotic communities in modern stromatolitic and thrombolitic mats can potentially provide insight into the coexistence of both laminated and clotted mat structures in close proximity to one another. Here we examine this possibility by comparing eukaryotic diversity based on Sanger and high-throughput pyrosequencing of small subunit ribosomal RNA (18S rRNA) genes. Analyses were based on total RNA extracts as template to minimize input from inactive or deceased organisms. Results identified diverse eukaryotic communities particularly stramenopiles, Alveolata, Metazoa, Amoebozoa and Rhizaria within different mat types at both locations, as well as abundant and diverse signatures of eukaryotes with <80% sequence similarity to sequences in GenBank. This suggests the presence of significant novel eukaryotic diversity, particularly in hypersaline Hamelin Pool. There was evidence of vertical structuring of protist populations and foraminiferal diversity was highest in bioturbated/clotted thrombolite mats of Highborne Cay.
Assuntos
Biodiversidade , Eucariotos/fisiologia , Animais , Austrália , Bahamas , Baías/microbiologia , Eucariotos/genética , Foraminíferos/genética , Sedimentos Geológicos/microbiologia , RNA Ribossômico 18S/genéticaRESUMO
Microbialites, which are organosedimentary structures formed by microbial communities through binding and trapping and/or in situ precipitation, have a wide array of distinctive morphologies and long geologic record. The origin of morphological variability is hotly debated; elucidating the cause or causes of microfabric differences could provide insights into ecosystem functioning and biogeochemistry during much of Earth's history. Although rare today, morphologically distinct, co-occurring extant microbialites provide the opportunity to examine and compare microbial communities that may be responsible for establishing and modifying microbialite microfabrics. Highborne Cay, Bahamas, has extant laminated (i.e., stromatolites) and clotted (i.e., thrombolites) marine microbialites in close proximity, allowing focused questions about how community composition relates to physical attributes. Considerable knowledge exists about prokaryotic composition of microbialite mats (i.e., stromatolitic and thrombolitic mats), but little is known about their eukaryotic communities, especially regarding heterotrophic taxa. Thus, the heterotrophic eukaryotic communities of Highborne stromatolites and thrombolites were studied. Here, we show that diverse foraminiferal communities inhabit microbialite mat surfaces and subsurfaces; thecate foraminifera are relatively abundant in all microbialite types, especially thrombolitic mats; foraminifera stabilize grains in mats; and thecate reticulopod activities can impact stromatolitic mat lamination. Accordingly, and in light of foraminiferal impacts on modern microbialites, our results indicate that the microbialite fossil record may reflect the impact of the radiation of these protists.
Assuntos
Ecossistema , Foraminíferos/crescimento & desenvolvimento , Sedimentos Geológicos/química , Sedimentos Geológicos/microbiologia , Bahamas , Monitoramento Ambiental , Foraminíferos/classificação , Foraminíferos/genética , Fósseis , Microscopia Confocal , Dados de Sequência Molecular , Densidade Demográfica , RNA Ribossômico 18S/genética , Água do Mar/química , Água do Mar/microbiologia , Análise de Sequência de DNA , Especificidade da Espécie , Microtomografia por Raio-XRESUMO
Bacterial hopanoids are ubiquitous in Earth surface environments. They hold promise as environmental and ecological biomarkers, if the phylogeny and physiological drivers of hopanoid biosynthesis can be linked with the distribution of hopanoids observed across a breadth of samples. Here we survey the diversity of hopanoid cyclases from a land-sea gradient across the island of San Salvador, in the easternmost part of the Bahamas. The distribution of lipids was determined for the same sites, for the first time overlaying quantification of bacteriohopanepolyols with sqhC phylogeny. The results are similar to previous reports: environmental sqhCs average < 65% translated amino acid identity to their closest named relatives, and sequences from putative Proteobacteria dominate. Additionally, a new and apparently ubiquitous group of marine hopanoid producers is identified; it has no identifiable close relatives. The greatest diversity of hopanoid lipids occurs in soil, but hopanoids represent a minor fraction of total soil-derived lipids. Marine samples contain fewer identifiable hopanoids, but they are more abundant as a fraction of the total extractable lipids. In soil, the dominant compounds are 35-aminobacteriohopane-32,33,34-triol and adenosylhopane. In an upper estuarine sample, bacteriohopanetetrol and 32,35-anhydrobacteriohopanetetrol dominate; while in lower estuarine and open marine samples, the most abundant are bacteriohopanetetrol and bacteriohopaneribonolactone. Cyclitol ethers are trace components in the soil, absent in the estuary, and of moderate abundance in the open marine setting, suggesting a dominant marine source. Conversely, aminotriol and aminotetrol decrease in abundance or disappear completely from land to ocean, while 2-methyldiplopterol shows the opposite trend. Small quantities of 2-methylbacteriohopanepolyols are detectable in all samples. The overall hopanoid distributions may correlate to the major phylogenetic families of hopanoid producers or to the environments in which they are found.