Low Hesperian PCO2 constrained from in situ mineralogical analysis at Gale Crater, Mars

ESCRT-III: a versatile membrane remodeling machinery and its implications in cellular processes and diseases

The endosomal sorting complexes required for transport (ESCRT) machinery is an evolutionarily conserved cytosolic protein complex that plays a crucial role in membrane remodeling and scission events across eukaryotes. Initially discovered for its function in multivesicular body (MVB) formation, the ESCRT complex has since been implicated in a wide range of membrane-associated processes, including endocytosis, exocytosis, cytokinesis, and autophagy. Recent advances have elucidated the ESCRT assembly pathway and highlighted the distinct functions of the various ESCRT complexes and their associated partners. Among the ESCRT complexes, ESCRT-III stands out as a critical player in membrane remodeling, with its subunits assembled into higher-order multimers capable of bending and severing membranes. This review focuses on the ESCRT-III complex, exploring its diverse functions in cellular processes beyond MVB biogenesis. We delve into the molecular mechanisms underlying ESCRT-III-mediated membrane remodeling and highlight its emerging roles in processes such as viral budding, autophagosome closure, and cytokinetic abscission. We also discuss the implications of ESCRT-III dysregulation in neurodegenerative diseases. The versatile membrane remodeling capabilities of ESCRT-III across diverse cellular processes underscore its importance in maintaining proper cellular function. Furthermore, we highlight the promising potential of ESCRT-III as a therapeutic target for neurodegenerative diseases, offering insights into the treatments of the diseases and the technical applications in related research fields.

SUMOylation regulates Lem2 function in centromere clustering and silencing

Regulation by the small modifier SUMO is heavily dependent on spatial control of enzymes that mediate the attachment and removal of SUMO on substrate proteins. Here, we show that in the fission yeast Schizosaccharomyces pombe, delocalisation of the SUMO protease Ulp1 from the nuclear envelope results in centromeric defects that can be attributed to hyper-SUMOylation at the nuclear periphery. Unexpectedly, we find that although this localised hyper-SUMOylation impairs centromeric silencing, it can also enhance centromere clustering. Moreover, both effects are at least partially dependent on SUMOylation of the inner nuclear membrane protein Lem2. Lem2 has previously been implicated in diverse biological processes, including the promotion of both centromere clustering and silencing, but how these distinct activities are coordinated was unclear; our observations suggest a model whereby SUMOylation serves as a regulatory switch, modulating Lem2 interactions with competing partner proteins to balance its roles in alternative pathways. Our findings also reveal a previously unappreciated role for SUMOylation in promoting centromere clustering.

Anthropic Principle and the Hubble-Lemaître Constant

According to the weak formulation of the anthropic principle, all fundamental physical constants have just such values that they enabled the origin of life. In this survey paper, we demonstrate also that the current value of the Hubble–Lemaître constant essentially contributed to the existence of humankind. Life on Earth has existed continually for at least 3.5 Gyr, and this requires very stable conditions during this quite long time interval. Nevertheless, as the luminosity of the Sun increases, Earth has receded from the Sun by an appropriate speed such that it received an almost constant solar flux during the last 3.5 Gyr. We introduce several other examples illustrating that the solar system and also our galaxy expand by a speed comparable to the Hubble–Lemaître constant.

The Role of Meteorite Impacts in the Origin of Life

The conditions, timing, and setting for the origin of life on Earth and whether life exists elsewhere in our solar system and beyond represent some of the most fundamental scientific questions of our time. Although the bombardment of planets and satellites by asteroids and comets has long been viewed as a destructive process that would have presented a barrier to the emergence of life and frustrated or extinguished life, we provide a comprehensive synthesis of data and observations on the beneficial role of impacts in a wide range of prebiotic and biological processes. In the context of previously proposed environments for the origin of life on Earth, we discuss how meteorite impacts can generate both subaerial and submarine hydrothermal vents, abundant hydrothermal-sedimentary settings, and impact analogues for volcanic pumice rafts and splash pools. Impact events can also deliver and/or generate many of the necessary chemical ingredients for life and catalytic substrates such as clays as well. The role that impact cratering plays in fracturing planetary crusts and its effects on deep subsurface habitats for life are also discussed. In summary, we propose that meteorite impact events are a fundamental geobiological process in planetary evolution that played an important role in the origin of life on Earth. We conclude with the recommendation that impact craters should be considered prime sites in the search for evidence of past life on Mars. Furthermore, unlike other geological processes such as volcanism or plate tectonics, impact cratering is ubiquitous on planetary bodies throughout the Universe and is independent of size, composition, and distance from the host star. Impact events thus provide a mechanism with the potential to generate habitable planets, moons, and asteroids throughout the Solar System and beyond.

Semiarid climate and hyposaline lake on early Mars inferred from reconstructed water chemistry at Gale

Salinity, pH, and redox states are fundamental properties that characterize natural waters. These properties of surface waters on early Mars reflect palaeoenvironments, and thus provide clues on the palaeoclimate and habitability. Here we constrain these properties of pore water within lacustrine sediments of Gale Crater, Mars, using smectite interlayer compositions. Regardless of formation conditions of smectite, the pore water that last interacted with the sediments was of Na-Cl type with mild salinity (~0.1-0.5 mol/kg) and circumneutral pH. To interpret this, multiple scenarios for post-depositional alterations are considered. The estimated Na-Cl concentrations would reflect hyposaline, early lakes developed in 104-106-year-long semiarid climates. Assuming that post-depositional sulfate-rich fluids interacted with the sediments, the redox disequilibria in secondary minerals suggest infiltration of oxidizing fluids into reducing sediments. Assuming no interactions, the redox disequilibria could have been generated by interactions of upwelling groundwater with oxidized sediments in early post-depositional stages.

Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration

Here we review published studies on the abundance and diversity of terrestrial rock-hosted life, the environments it inhabits, the evolution of its metabolisms, and its fossil biomarkers to provide guidance in the search for life on Mars. Key findings are (1) much terrestrial deep subsurface metabolic activity relies on abiotic energy-yielding fluxes and in situ abiotic and biotic recycling of metabolic waste products rather than on buried organic products of photosynthesis; (2) subsurface microbial cell concentrations are highest at interfaces with pronounced chemical redox gradients or permeability variations and do not correlate with bulk host rock organic carbon; (3) metabolic pathways for chemolithoautotrophic microorganisms evolved earlier in Earth's history than those of surface-dwelling phototrophic microorganisms; (4) the emergence of the former occurred at a time when Mars was habitable, whereas the emergence of the latter occurred at a time when the martian surface was not continually habitable; (5) the terrestrial rock record has biomarkers of subsurface life at least back hundreds of millions of years and likely to 3.45 Ga with several examples of excellent preservation in rock types that are quite different from those preserving the photosphere-supported biosphere. These findings suggest that rock-hosted life would have been more likely to emerge and be preserved in a martian context. Consequently, we outline a Mars exploration strategy that targets subsurface life and scales spatially, focusing initially on identifying rocks with evidence for groundwater flow and low-temperature mineralization, then identifying redox and permeability interfaces preserved within rock outcrops, and finally focusing on finding minerals associated with redox reactions and associated traces of carbon and diagnostic chemical and isotopic biosignatures. Using this strategy on Earth yields ancient rock-hosted life, preserved in the fossil record and confirmable via a suite of morphologic, organic, mineralogical, and isotopic fingerprints at micrometer scale. We expect an emphasis on rock-hosted life and this scale-dependent strategy to be crucial in the search for life on Mars.

Products of the iron cycle on the early Earth

Traditional models for pre-GOE oceans commonly view iron as a critical link to multiple biogeochemical cycles, and an important source of electrons to primary producers. However, an accurate and detailed understanding of the ancient iron cycle has been limited by: (1) our ability to constrain primary depositional processes through observations of the ancient sedimentary rock record, and (2) a quantitative understanding of the aqueous geochemistry of ferrous iron. Recent advances in high resolution petrography and experimental geochemistry, however, have contributed to a new understanding of certain aspects of the early Fe cycle. Most importantly, high resolution petrographic studies of late Archean/early Paleoproterozoic iron formation have documented the prolific deposition of Fe(II)-silicate-rich chemical muds from a dominantly anoxic ocean. At the same time, recent experimental work has shed new light on processes likely to have controlled steady state Fe concentrations in Archean oceans. These studies suggest that spontaneous precipitation of Fe(II)-carbonate was probably rare in Archean oceans, and that Fe(II)-carbonate would have more commonly precipitated on the surfaces of suitable mineral substrates within clastic and chemical sediments, consistent with petrographic observations. In addition, although experimental investigations suggest that maximum Fe concentrations in Archean oceans would have been limited by authigenic Fe(II)-silicate production (rather than Fe(II)-carbonate), the rock record indicates that this process was rarely operative. Instead, sedimentology, stratigraphy, and geochemical modelling suggest that much of the precursor sediment to late Archean iron formation was produced as hydrothermal effluent interacted with seawater in close proximity to seafloor vents. Together, these observations help define a new topology for the ancient Fe cycle. In this view, hydrothermal effluent-seawater mixing would have strongly attenuated the flux of dissolved Fe²⁺ to Archean oceans, and early diagenetic siderite formation may have balanced globally averaged riverine and hydrothermal Fe²⁺ input fluxes. In contrast to previous models, this emerging picture of the early Fe cycle suggests that Fe played only a negligible role in supporting anoxygenic phototrophs, reinforcing the concept that electron donors were in comparatively limited supply before the advent of oxygenic photosynthesis.

Fe-oxide concretions formed by interacting carbonate and acidic waters on Earth and Mars

Spherical Fe-oxide concretions on Earth, especially in Utah, USA, have been investigated as an analog of hematite spherules found in Meridiani Planum on Mars to support interpretations of water-rock interactions in early Mars. Although several formation mechanisms have been proposed for the Fe-oxide concretions on Earth, it is still unclear whether these mechanisms are viable because a precise formation process and precursor of the concretions are missing. This paper presents evidence that Fe-oxide concretions in Utah and newly found Fe-oxide concretions in Mongolia had spherical calcite concretions as precursors. Different formation stages of calcite and Fe-oxide concretions observed, both in Utah and Mongolia, indicate that calcite concretions initially formed within eolian sandstone strata and were dissolved by infiltrating Fe-rich acidic waters to form spherical FeO(OH) crusts due to pH buffering. The similarity between these Fe-oxide concretions on Earth and the hematite spherule occurrences in Meridiani Planum, combined with evidence of acid sulfate water influences on Mars, suggest that the hematite spherules also formed from dissolution of preexisting carbonate spherules possibly formed under a dense carbon dioxide early martian atmosphere.

Magnetite Authigenesis and the Warming of Early Mars

The Curiosity rover has documented lacustrine sediments at Gale Crater, but how liquid water became physically stable on the early Martian surface is a matter of significant debate. To constrain the composition of the early Martian atmosphere during sediment deposition, we experimentally investigated the nucleation and growth kinetics of authigenic Fe-minerals in Gale Crater mudstones. Experiments show that pH variations within anoxic basaltic waters trigger a series of mineral transformations that rapidly generate magnetite and H2(aq). Magnetite continues to form through this mechanism despite high PCO2 and supersaturation with respect to Fe-carbonate minerals. Reactive transport simulations that incorporate these experimental data show that groundwater infiltration into a lake equilibrated with a CO2-rich atmosphere can trigger the production of both magnetite and H2(aq) in the mudstones. H2(aq), generated at concentrations that would readily exsolve from solution, is capable of increasing annual mean surface temperatures above freezing in CO2-dominated atmospheres. We therefore suggest that magnetite authigenesis could have provided a short-term feedback for stabilizing liquid water, as well as a principal feedstock for biologically relevant chemical reactions, at the early Martian surface.

A Field Guide to Finding Fossils on Mars

The Martian surface is cold, dry, exposed to biologically harmful radiation and apparently barren today. Nevertheless, there is clear geological evidence for warmer, wetter intervals in the past that could have supported life at or near the surface. This evidence has motivated National Aeronautics and Space Administration and European Space Agency to prioritize the search for any remains or traces of organisms from early Mars in forthcoming missions. Informed by (1) stratigraphic, mineralogical and geochemical data collected by previous and current missions, (2) Earth's fossil record, and (3) experimental studies of organic decay and preservation, we here consider whether, how, and where fossils and isotopic biosignatures could have been preserved in the depositional environments and mineralizing media thought to have been present in habitable settings on early Mars. We conclude that Noachian-Hesperian Fe-bearing clay-rich fluvio-lacustrine siliciclastic deposits, especially where enriched in silica, currently represent the most promising and best understood astropaleontological targets. Siliceous sinters would also be an excellent target, but their presence on Mars awaits confirmation. More work is needed to improve our understanding of fossil preservation in the context of other environments specific to Mars, particularly within evaporative salts and pore/fracture-filling subsurface minerals.

Compatibility of Amino Acids in Ice Ih: Implications for the Origin of Life

Icy environments may have been common on early Earth due to the faint young sun. Previous studies have proposed that the formation of large icy bodies in the early ocean could concentrate the building blocks of life in eutectic fluids and, therefore, facilitate the polymerization of monomers. This hypothesis is based on the untested assumption that organic molecules are virtually incompatible in ice Ih (hexagonal ice). In this study, we conducted freezing experiments to explore the partitioning behavior of selected amino acids (AAs; glycine, l-alanine, l-proline, and l-phenylalanine) between ice Ih and aqueous solutions analogous to seawater. We allowed ice crystals to grow slowly from a few seeds in equilibrium with the solution and used Raman spectroscopy to analyze in situ the relative concentrations of AAs in the ice and aqueous solution. During freezing, there was no precipitation of AA crystals, indicating that the concentrations in solution never reached their solubility limit, even when the droplet was mostly frozen. Analyses of the Raman spectra of the ice and eutectic solution suggested that considerable amounts of AAs existed in the ice phase with partition coefficients varying between 0.2 and 0.5. These observations imply little incompatibility of AAs in ice Ih during the freezing of the solutions, rendering the concentration hypothesis in a eutectic system unwarranted. However, incorporation into ice Ih could protect AAs from decomposition or racemization and significantly improve the efficiency of extraterrestrial transport of small organics. Therefore, this study supports the hypothesis of extraterrestrial delivery of organic molecules in icy comets and asteroids to the primitive Earth as suggested by an increasing number of independent observations. Key Words: Ice Ih-Partition coefficient-Amino acids-Polymerization-Extraterrestrial transport of organics. Astrobiology 18, 381-392.