Alexei Yu. Rozanov

Meteorite-catalyzed syntheses of nucleosides and of other prebiotic compounds from formamide under proton irradiation

Significance Modern research on the origin of life started with Urey–Miller’s 1953 report on the spontaneous formation of amino acids upon application of electric discharge on a model of the pristine Earth atmosphere. Formamide provides a chemically sound starting material for the syntheses of prebiotic compounds; its role in prebiotics is becoming recognized. Kiloparsecs-wide clouds of formamide were observed in the interstellar space. The energy sources for the syntheses explored so far were largely thermal, and the catalysts used were mostly terrestrial. In the presence of meteorites and with high-energy protons, we observe the production of unprecedented panels of nucleobases, sugars, and, most notably, nucleosides. Carboxylic acids and amino acids complete the recipe. These findings extend prebiotic plausible scenarios well beyond our planet. Liquid formamide has been irradiated by high-energy proton beams in the presence of powdered meteorites, and the products of the catalyzed resulting syntheses were analyzed by mass spectrometry. Relative to the controls (no radiation, or no formamide, or no catalyst), an extremely rich, variegate, and prebiotically relevant panel of compounds was observed. The meteorites tested were representative of the four major classes: iron, stony iron, chondrites, and achondrites. The products obtained were amino acids, carboxylic acids, nucleobases, sugars, and, most notably, four nucleosides: cytidine, uridine, adenosine, and thymidine. In accordance with theoretical studies, the detection of HCN oligomers suggests the occurrence of mechanisms based on the generation of radical cyanide species (CN·) for the synthesis of nucleobases. Given that many of the compounds obtained are key components of extant organisms, these observations contribute to outline plausible exogenous high-energy–based prebiotic scenarios and their possible boundary conditions, as discussed.

Microfossils, biominerals, and chemical biomarkers in meteorites

The discovery of biominerals, chemical biomarkers and evidence of microfossils in the Mars meteorite (ALH84001) stimulated research into biomarkers, microbial extremophiles and provided impetus to the newly emerging fields of Astrobiology and Bacterial Paleontology. The debate following the ALH84001 results has highlighted the importance of developing methodologies for recognition of mineral and elemental bioindicators, chemical biomarkers and microfossils in terrestrial rocks and meteorites prior to sample return missions to comets, asteroids, and Mars. Comparative studies of living and fossil micro-organisms and biomarkers are vital to developing expertise needed to recognize indigenous biosignatures and recent contaminants. This paper reviews elemental and mineral bioindicators, chemical biomarkers and keropgen in terrestrial rocks and meteorites. Electron Microscopy images of hyperthermophilic nanobacteria, sulfur and sulfate reducing bacteria, and mineralized microfossils and kerogen found in-situ in carbonaceous meteorite rock matrix are presented.

Indigenous microfossils in carbonaceous meteorites

Indigenous embedded microbial filaments, bacterial cells and other microfossils were found in the Orgueil, Ivuna (CI1), Murchison, and Bells (CM2) carbonaceous meteorites. Biominerals, biofilms, framboids, magnetite platelets, and curious elemental iron ovoids covered with minute fibrils and carbon sheaths were also found. The S-4100 Hitachi Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive X-ray Analysis (EDAX) were used for in situ investigations of freshly fractured interior meteorite surfaces. EDAX x-ray spectra shows the microfossils bear signatures of the meteorite matrix and possess elemental ratios indicating they are indigenous and not recent microbial contaminants. Many of the well-preserved biogenic remains in the meteorites are encased within carbon-rich, sometimes electron transparent, sheaths. Their size, morphology and ultra microstructure are comparable to microfossils known from the phosphorites of Khubsughul, Mongolia and to some of the living cyanobacteria and other sulfur- and sulfate-reducing bacteria known from the halophilic Microcoleus mats of Sivash Lagoon, Crimea and from Mono Lake in California.

Microfossils in CI and CO Carbonaceous Meteorites

We have obtained x-ray spectral data and secondary/backscatter electron images of a suite of complex forms that we interpret as microfossils in several CI (Alais, Orgueil, and Tagish Lake) and CO3 (Rainbow and Dar al Gani 749) carbonaceous meteorites using the Field Emission and Environmental Scanning Electron Microscopes. Many of these embedded and lithified or carbonized forms are similar to photoautrophs (cyanobacteria or purple nonsulfur bacteria) or extinct phytoplankton (acritarchs and hystrichospheres) that are not considered likely post-arrival contaminants and therefore we interpret them as indigenous microfossils. We discuss the meteorites and provide images of several biogenic forms found embedded in freshly fractured meteorite matrix.

Framboidal structures in Earth rocks and in astromaterials

Framboidal structures are common in both Earth rocks and in meteorites - carbonaceous chondrites. The main methods of formation of these structures are discussed. The role of biologic factors in formation of framboids is elevated. Comparison of crystal forms comprising framboids formed in laboratory conditions and in nature is provided. On the basis of investigations of framboidal structures the proposition that pyritoidal form of crystals is typical for the formation of biogenic framboidal structures.

Biomarkers and microfossils in the Murchison, Rainbow, and Tagish Lake meteorites

During the past six years, we have conducted extensive scanning electron and optical microscopy investigations and x-ray analysis to determine the morphology, life cycle processes, and elemental distributions in living and fossil cyanobacteria, bacteria, archaea, fungi, and algae sampled from terrestrial environments relevant to Astrobiology. Biominerals, pseudomorphs and microfossils have been studied for diverse microbial groups in various states of preservation in many types of rocks (e.g., oil shales, graphites, shungites, bauxites, limestones, pyrites, phosphorites, and hydrothermal vent chimneys). Results of these studies have been applied to the search for biosignatures in carbonaceous chondrites, stony, and nickel iron meteorites. We review important biomarkers found in terrestrial rocks and meteorites and present additional evidence for the existence of indigenous bacterial microfossils in-situ in freshly fractured surfaces of the Murchison, Rainbow and Tagish Lake carbonaceous meteorites. We provide secondary and backscatter electron images and spectral data obtained with Field Emission and Environmental Scanning Electron Microscopes of biominerals and microfossils. We discuss techniques for discriminating indigenous microfossils from recent terrestrial contaminants. Images are provided of framboidal magnetites in oil shales and meteorites and images and 2D x-ray maps are shown of bacterial microfossils embedded in the mineral matrix of the Murchison, Rainbow and Tagish Lake Carbonaceous Meteorites. These microfossils exhibit characteristics that preclude their interpretation as post-arrival contaminants and we interpret them as indigenous biogenic remains.

Proton irradiation: a key to the challenge of N-glycosidic bond formation in a prebiotic context

The formation of nucleosides in abiotic conditions is a major hurdle in origin-of-life studies. We have determined the pathway of a general reaction leading to the one-pot synthesis of ribo- and 2′-deoxy-ribonucleosides from sugars and purine nucleobases under proton irradiation in the presence of a chondrite meteorite. These conditions simulate the presumptive conditions in space or on an early Earth fluxed by slow protons from the solar wind, potentially mimicking a plausible prebiotic scenario. The reaction (i) requires neither pre-activated precursors nor intermediate purification/concentration steps, (ii) is based on a defined radical mechanism, and (iii) is characterized by stereoselectivity, regioselectivity and (poly)glycosylation. The yield is enhanced by formamide and meteorite relative to the control reaction.

Fossil microorganisms in Archaean deposits of Northern Karelia

Newly found biomorphic microstructures from the Upper Archaean (lopian) rocks from Northern Karelia are described. The presence of various microorganisms of a bacterial nature and even cyanobacteria (and possibly eukaryotic forms) is suggested. The necessity of employing methods of electron microscopy, as well as traditional methods, while studying the very early manifestations of life in Archaean and Early Proterozoic is noted.

Astrobiology: traces of life in the cosmos

The discovery of traces of life in the ancient Mars meteorite triggered the development of the rapidly emerging field of Astrobiology. Astrobiologists are seeking to develop conclusive methods to recognize biosignatures and microfossils of bacteria and other microbiota as well as to understand the spatial, temporal, environmental and chemical limitations of microbial extremophiles. Recent discoveries have revealed the great distribution and diversity of microbial extremophiles on Earth and profoundly increased the probability that life may exist elsewhere in the Cosmos. The rapidly emerging science of Bacterial Paleontology has provided important new information critical to the recognition of fossil bacteria on Earth and in Astromaterials. We have recently conducted independent scanning electron microscopy and x-ray analysis investigations in the US and Russia in order to better understand the morphology and chemical composition of microfossils in ancient terrestrial rocks and carbonaceous meteorites. In this paper, we review some aspects of microbial extremophiles of Earth as modals for life on other bodies of the Solar System. We consider several of the important chemical, mineral and morphological biomarkers that provide definitive evidence of biogenic activity in ancient rocks and meteorites. We present Environmental Scanning Electron Microscope images of microfossils found in-situ in freshly fractured meteorite surfaces and describe Energy Dispersive Spectroscopy and Link microprobe analysis of the chemical elements in the mineralized and/or kerogenous microfossils and meteorite rock matrix. We also discuss technqiues and methods that may be used to help discriminate indigenous microfosils from recent terrestrial contaminants. We will also present new data from our in-situ investigations of living cyanobacteria and bacteria and freshly broken surfaces of terrestrial rocks and meteorites. Comparative analysis of these images are interpreted as providing dramatic evidence of indigenous microfossils of magnetotactic bacteria, cyanobacteria, and acritarchs in the Nogoya, Efremovka, Orgueil, Murchison and Tagish Lake Meteorites. Many of the forms in carbonaceous meteorites are large and complex providing strong evidence of biogenicity. Many of the forms found in carbonaceous meteorites are strikingly similar to microfossils of bacteria, cyanobacteria and fungi we know from the Cambrian phosphorites of Khubsugul, Mongolia and high carbon Phanerozoic and Precambrian rocks of the Siberian and Russian Platforms. Some meteorite microfossil assemblages are consistent with known characteristics of distinct microbial life cycles and reproductive stages of Nostocacean cyanobacteria. We also recognize assemblages consistent with microbial ecosystems we studied in permafrost and cryoconite communities of Antarctica, Alaska and Siberia and microfossil ecosystems from the Cambrian of Mongolia.

Phosphate biomineralization of cambrian microorganisms

As part of a long term study of biological markers, we are documenting a variety of features which reflect the previous presence of living organisms. As we study meteorites and samples returned form Mars, our main clue to recognizing possible microbial material may be the presence of biomarkers rather than the organisms themselves. One class of biomarkers consists of biominerals which have either been precipitated directly by microorganisms, or whose precipitation has been influenced by the organisms. Such microbe-mediated mineral formation may include important clues to the size, shape, and environment of the microorganisms. The process of fossilization or mineralization can cause major changes in morphologies and textures of the original organisms. The study of fossilized terrestrial organisms can help provide insight into the interpretation of mineral biomarkers. The study of fossilized terrestrial organisms can help provide insight into the interpretation of mineral biomarkers. This paper describes the results of investigations of microfossils in Cambrian phosphate-rich rocks that were found in Khubsugul, Northern Mongolia.