Astrobiological Complexity with Probabilistic Cellular Automata

Search for extraterrestrial life and intelligence constitutes one of the major endeavors in science, but has yet been quantitatively modeled only rarely and in a cursory and superficial fashion. We argue that probabilistic cellular automata (PCA) represent the best quantitative framework for modeling astrobiological history of the Milky Way and its Galactic Habitable Zone. The relevant astrobiological parameters are to be modeled as the elements of the input probability matrix for the PCA kernel. With the underlying simplicity of the cellular automata constructs, this approach enables a quick analysis of large and ambiguous input parameters' space. We perform a simple clustering analysis of typical astrobiological histories and discuss the relevant boundary conditions of practical importance for planning and guiding actual empirical astrobiological and SETI projects. In addition to showing how the present framework is adaptable to more complex situations and updated observational databases from current and near-future space missions, we demonstrate how numerical results could offer a cautious rationale for continuation of practical SETI searches.Comment: 37 pages, 11 figures, 2 tables; added journal reference belo

Targeted Sister Chromatid Cohesion by Sir2

The protein complex known as cohesin binds pericentric regions and other sites of eukaryotic genomes to mediate cohesion of sister chromatids. In budding yeast Saccharomyces cerevisiae, cohesin also binds silent chromatin, a repressive chromatin structure that functionally resembles heterochromatin of higher eukaryotes. We developed a protein-targeting assay to investigate the mechanistic basis for cohesion of silent chromatin domains. Individual silencing factors were tethered to sites where pairing of sister chromatids could be evaluated by fluorescence microscopy. We report that the evolutionarily conserved Sir2 histone deacetylase, an essential silent chromatin component, was both necessary and sufficient for cohesion. The cohesin genes were required, but the Sir2 deacetylase activity and other silencing factors were not. Binding of cohesin to silent chromatin was achieved with a small carboxyl terminal fragment of Sir2. Taken together, these data define a unique role for Sir2 in cohesion of silent chromatin that is distinct from the enzyme's role as a histone deacetylase

The responses of an anaerobic microorganism, Yersinia intermedia MASE-LG-1 to individual and combined simulated Martian stresses

The limits of life of aerobic microorganisms are well understood, but the responses of anaerobic microorganisms to individual and combined extreme stressors are less well known. Motivated by an interest in understanding the survivability of anaerobic microorganisms under Martian conditions, we investigated the responses of a new isolate, Yersinia intermedia MASE-LG-1 to individual and combined stresses associated with the Martian surface. This organism belongs to an adaptable and persistent genus of anaerobic microorganisms found in many environments worldwide. The effects of desiccation, low pressure, ionizing radiation, varying temperature, osmotic pressure, and oxidizing chemical compounds were investigated. The strain showed a high tolerance to desiccation, with a decline of survivability by four orders of magnitude during a storage time of 85 days. Exposure to X-rays resulted in dose-dependent inactivation for exposure up to 600 Gy while applied doses above 750 Gy led to complete inactivation. The effects of the combination of desiccation and irradiation were additive and the survivability was influenced by the order in which they were imposed. Ionizing irradiation and subsequent desiccation was more deleterious than vice versa. By contrast, the presence of perchlorates was not found to significantly affect the survival of the Yersinia strain after ionizing radiation. These data show that the organism has the capacity to survive and grow in physical and chemical stresses, imposed individually or in combination that are associated with Martian environment. Eventually it lost its viability showing that many of the most adaptable anaerobic organisms on Earth would be killed on Mars today

Biosignatures in Deep Time

International audienceLife on the early Earth inhabited a planet whose environment was vastly different from the Earth of today. An anaerobic and hot early Earth was the birthplace of the first living cells but wide-spread small-scale physico-chemical diversity provided opportunities for a variety of specialists: alkalophiles, acidophiles, halophiles etc. The earliest record of life has been lost due to plate tectonic recycling and the oldest preserved terranes (~3.9–3.7 Ga) are heavily altered by metamorphism, although they may contain traces of fossil life. As of ~3.5 Ga, ancient sediments are so well-preserved that a broad diversity of micro-environments and fossil traces of life can be studied, providing a surprising window into communities of microbes that had already reached the evolutionary stage of photosynthesis. From the wide variety of traces of ancient life that have been reported from the Archaean geological record in Greenland, Canada, South Africa and Western Australia, we examine a few particularly pertinent examples. Biosignatures in the rock record include microfossils, microbial mats, stromatolites, microbially induced sedimentary structures, biominerals, biologically indicative isotopic ratios and fractionations, elemental distributions, organochemical patterns and other geochemical peculiarities best explained by biological mediation. Due to dynamic geological reprocessing over the billions of years since these fossils entered the rock record, identifications of very ancient traces of life have been subject to criticism, hence the often complex arguments regarding their biogenicity. We here highlight a range of unambiguously bona fide and widely supported examples of fossil biosignatures. Fossil biosignatures have great promise as analogues of life that might be detected on other planets. In this respect, the study of the early Earth is particularly pertinent to the search for life on Mars, given the planetary- and microbial-scale similarities that prevailed on both planets during their early histories, together with the lack of subsequent geological reprocessing on Mars, which may make it an ideal repository for a near-pristine fossil record