Sulfate was a trace constituent of Archean seawater

Simulated early Earth geochemistry fuels a hydrogen-dependent primordial metabolism

Molecular hydrogen is the electron donor for the ancient exergonic reductive acetyl-coenzyme A pathway (acetyl-CoA pathway), which is used by hydrogenotrophic methanogenic archaea. How the presence of iron-sulfides influenced the acetyl-CoA pathway under primordial early Earth geochemistry is still poorly understood. Here we show that the iron-sulfides mackinawite (FeS) and greigite (Fe3S4), which formed in chemical garden experiments simulating geochemical conditions of the early Archaean eon (4.0-3.6 billion years ago), produce abiotic H2 in sufficient quantities to support hydrogenotrophic growth of the hyperthermophilic methanogen Methanocaldococcus jannaschii. Abiotic H2 from iron-sulfide formation promoted CO2 fixation and methanogenesis and induced overexpression of genes encoding the acetyl-CoA pathway. We demonstrate that H2 from iron-sulfide precipitation under simulated early Earth hydrothermal geochemistry fuels a H2-dependent primordial metabolism.

Vertical distribution of methanotrophic archaea in an iron-rich groundwater discharge zone

Anaerobic oxidation of methane coupled to iron reduction (Fe-AOM) is a crucial process for methane removal in terrestrial environments. However, the occurrence of Fe-AOM in natural environments is rare, and the mechanisms behind the direct coupling of methane oxidation and iron reduction remain poorly understood. In this study, we investigated the environmental factors influencing the distribution of methanotrophic archaea in an iron-rich zone of a freshwater pond in Hiroshima Prefecture, Japan. High concentration of dissolved ferrous iron supplied by groundwater discharge led to considerable ferrihydrite precipitation. Pore water methane increased with sediment depth, while nitrate and sulfate concentrations were near detection limits throughout the sediment column. The coexistence of ferric iron and methane suggests the ongoing process of Fe-AOM. Tracer-based experiments using 14C showed potential Fe-AOM rates up to 110 pmol mL-1 day-1. Throughout the sediment core, except at the surface, PCR-based molecular ecological analyses of the 16S rRNA gene and functional genes for anaerobic oxidation of methane revealed abundant sequences belonging to the family "Candidatus Methanoperedenaceae". These geochemical and microbiological findings suggest that Fe-AOM plays a key role in biogeochemical cycles of iron and methane, positioning this environment as a modern analogue of early Earth conditions.

Fischer W. Reconciling Archean organic-rich mudrocks with low primary productivity before the Great Oxygenation Event

The organic carbon content of ancient rocks provides a fundamental record of the biosphere on early Earth. For over 50 y, the high organic content of Archean (>2.5 Ga) mudrocks has puzzled geologists and evolutionary biologists, because high biological primary productivity was unexpected for the nascent biosphere before the rise of O2. Here, we took a different approach to resolve this apparent paradox, by studying the accumulation rates of Archean organic-rich mudrocks. We evaluated the sedimentation rates of three sections of the Mount McRae Shale and Jeerinah Formation (2.68 to 2.48 Ga, Pilbara Craton, Australia) with new and recently published U-Pb zircon ages from intraformational ash beds. For comparison, we compiled Phanerozoic (<500 Ma) data from comparable depositional settings and developed an idealized model that considers the sedimentation rates for predicting rock organic content. We found that organic-rich Archean mudrocks were deposited under exceptionally low sedimentation rates (~1 m/Ma), in sharp contrast to organic-rich rocks from the Phanerozoic Eon (10 to 100 m/Ma). Constrained by observations, model results indicated that the Archean data reflect low primary productivity (~100-fold lower than during the Phanerozoic) and enhanced preservation under anoxic conditions, with the principal control on organic carbon content provided by dilution with inorganic sediment. Thus, the high organic carbon content which is typically attributed to high productivity instead reflects slow accumulation, high preservation, and minimal inorganic dilution-reconciling the geological evidence with a slow carbon cycle cadence during Archean time.

A Machine-Learning Approach to Biosignature Exploration on Early Earth and Mars Using Sulfur Isotope and Trace Element Data in Pyrite

We propose a novel approach to identify the origin of pyrite grains and distinguish biologically influenced sedimentary pyrite using combined in situ sulfur isotope (δ34S) and trace element (TE) analyses. To classify and predict the origin of individual pyrite grains, we applied multiple machine-learning algorithms to coupled δ34S and TE data from pyrite grains formed from diverse sedimentary, hydrothermal, and metasomatic processes across geologic time. Our unsupervised classification algorithm, K-means++ cluster analysis, yielded six classes based on the formation environment of the pyrite: sedimentary, low temperature hydrothermal, medium temperature, polymetallic hydrothermal, high temperature, and large euhedral. We tested three supervised models (random forest [RF], Naïve Bayes, k-nearest neighbors), and RF outperformed the others in predicting pyrite formation type, achieving a precision (area under the ROC curve) of 0.979 ± 0.005 and an overall average class accuracy of 0.878 ± 0.005. Moreover, we found that coupling TE and δ34S data significantly improved the performance of the RF model compared with using either TE or δ34S data alone. Our data provide a novel framework for exploring sedimentary rocks that have undergone multiple hydrothermal, magmatic, and metamorphic alterations. Most significant, however, is the demonstrated potential for distinguishing between biogenic and abiotic pyrite in samples from early Earth. This approach could also be applied to the search for potential biosignatures in samples returned from Mars.

Stochastic Character Mapping, Bayesian Model Selection, and Biosynthetic Pathways Shed New Light on the Evolution of Habitat Preference in Cyanobacteria

Cyanobacteria are the only prokaryotes to have evolved oxygenic photosynthesis paving the way for complex life. Studying the evolution and ecological niche of cyanobacteria and their ancestors is crucial for understanding the intricate dynamics of biosphere evolution. These organisms frequently deal with environmental stressors such as salinity and drought, and they employ compatible solutes as a mechanism to cope with these challenges. Compatible solutes are small molecules that help maintain cellular osmotic balance in high-salinity environments, such as marine waters. Their production plays a crucial role in salt tolerance, which, in turn, influences habitat preference. Among the 5 known compatible solutes produced by cyanobacteria (sucrose, trehalose, glucosylglycerol, glucosylglycerate, and glycine betaine), their synthesis varies between individual strains. In this study, we work in a Bayesian stochastic mapping framework, integrating multiple sources of information about compatible solute biosynthesis in order to predict the ancestral habitat preference of Cyanobacteria. Through extensive model selection analyses and statistical tests for correlation, we identify glucosylglycerol and glucosylglycerate as the most significantly correlated with habitat preference, while trehalose exhibits the weakest correlation. Additionally, glucosylglycerol, glucosylglycerate, and glycine betaine show high loss/gain rate ratios, indicating their potential role in adaptability, while sucrose and trehalose are less likely to be lost due to their additional cellular functions. Contrary to previous findings, our analyses predict that the last common ancestor of Cyanobacteria (living at around 3180 Ma) had a 97% probability of a high salinity habitat preference and was likely able to synthesize glucosylglycerol and glucosylglycerate. Nevertheless, cyanobacteria likely colonized low-salinity environments shortly after their origin, with an 89% probability of the first cyanobacterium with low-salinity habitat preference arising prior to the Great Oxygenation Event (2460 Ma). Stochastic mapping analyses provide evidence of cyanobacteria inhabiting early marine habitats, aiding in the interpretation of the geological record. Our age estimate of ~2590 Ma for the divergence of 2 major cyanobacterial clades (Macro- and Microcyanobacteria) suggests that these were likely significant contributors to primary productivity in marine habitats in the lead-up to the Great Oxygenation Event, and thus played a pivotal role in triggering the sudden increase in atmospheric oxygen.

Iron: Life's primeval transition metal

Modern life requires many different metal ions, which enable diverse biochemical functions. It is commonly assumed that metal ions' environmental availabilities controlled the evolution of early life. We argue that evolution can only explore the chemistry that life encounters, and fortuitous chemical interactions between metal ions and biological compounds can only be selected for if they first occur sufficiently frequently. We calculated maximal transition metal ion concentrations in the ancient ocean, determining that the amounts of biologically important transition metal ions were orders of magnitude lower than ferrous iron. Under such conditions, primitive bioligands would predominantly interact with Fe(II). While interactions with other metals in certain environments may have provided evolutionary opportunities, the biochemical capacities of Fe(II), Fe-S clusters, or the plentiful magnesium and calcium could have satisfied all functions needed by early life. Primitive organisms could have used Fe(II) exclusively for their transition metal ion requirements.

Co-evolution of early Earth environments and microbial life

Two records of Earth history capture the evolution of life and its co-evolving ecosystems with interpretable fidelity: the geobiological and geochemical traces preserved in rocks and the evolutionary histories captured within genomes. The earliest vestiges of life are recognized mostly in isotopic fingerprints of specific microbial metabolisms, whereas fossils and organic biomarkers become important later. Molecular biology provides lineages that can be overlayed on geologic and geochemical records of evolving life. All these data lie within a framework of biospheric evolution that is primarily characterized by the transition from an oxygen-poor to an oxygen-rich world. In this Review, we explore the history of microbial life on Earth and the degree to which it shaped, and was shaped by, fundamental transitions in the chemical properties of the oceans, continents and atmosphere. We examine the diversity and evolution of early metabolic processes, their couplings with biogeochemical cycles and their links to the oxygenation of the early biosphere. We discuss the distinction between the beginnings of metabolisms and their subsequent proliferation and their capacity to shape surface environments on a planetary scale. The evolution of microbial life and its ecological impacts directly mirror the Earth's chemical and physical evolution through cause-and-effect relationships.

Microbial community and extracellular polymeric substance dynamics in arid-zone temporary pan ecosystems

Microbial extracellular polymeric substances (EPS) are an important component in sediment ecology. However, most research is highly skewed towards the northern hemisphere and in more permanent systems. This paper investigates EPS (i.e., carbohydrates and proteins) dynamics in arid Austral zone temporary pans sediments. Colorimetric methods and sequence-based metagenomics techniques were employed in a series of small temporary pan ecosystems characterised by alternating wet and dry hydroperiods. Microbial community patterns of distribution were evaluated between seasons (hot-wet and cool-dry) and across depths (and inferred inundation period) based on estimated elevation. Carbohydrates generally occurred in relatively higher proportions than proteins; the carbohydrate:protein ratio was 2.8:1 and 1.6:1 for the dry and wet season respectively, suggesting that EPS found in these systems was largely diatom produced. The wet- hydroperiods (Carbohydrate mean 102 μg g-1; Protein mean 65 μg g-1) supported more EPS production as compared to the dry- hydroperiods (Carbohydrate mean 73 μg g-1; Protein mean 26 μg g-1). A total of 15,042 Unique Amplicon Sequence Variants (ASVs) were allocated to 51 bacterial phyla and 1127 genera. The most abundant genera had commonality in high temperature tolerance, with Firmicutes, Actinobacteria and Proteobacteria in high abundances. Microbial communities were more distinct between seasons compared to within seasons which further suggested that the observed metagenome functions could be seasonally driven. This study's findings implied that there were high levels of denitrification by mostly nitric oxide reductase and nitrite reductase enzymes. EPS production was high in the hot-wet season as compared to relatively lower rates of nitrification in the cool-dry season by ammonia monooxygenases. Both EPS quantities and metagenome functions were highly associated with availability of water, with high rates being mainly associated with wet- hydroperiods compared to dry- hydroperiods. These data suggest that extended dry periods threaten microbially mediated processes in temporary wetlands, with implications to loss of biodiversity by desiccation.

A multi-scale assessment of the impact of salinity on the desorption of chromate from hematite: Sea level rise implications

The solubility and transport of Cr(VI) is primarily controlled by adsorption-desorption reactions at the surfaces of soil minerals such as iron oxides. Environmental properties such as pH, ionic strength, and ion competition are expected to affect the mobility and fate of Cr(VI). Sea level rise (SLR), and consequent seawater intrusion, is creating a new biogeochemical soil environment at coastal margins, potentially impacting Cr(VI) retention at contaminated sites. We employed in-situ ATR-FTIR spectroscopy and DFT calculations to investigate at the molecular level the adsorption of Cr(VI) on the hematite surface and its desorption by sulfate, as a function of pH and ionic strength. We further used a batch experiment to assess Cr(VI) desorption at varying artificial seawater (ASW) concentrations. IR results demonstrate the complexity of Cr(VI) adsorption, showing a combination of monodentate inner-sphere complexation at high pH and dichromate outer-sphere (∼75%) at low pH. The Cr(VI)-complexes exhibited desorption induced by increasing pH values (58% of desorption) and sulfate competition (∼40% desorption). ASW desorbed ∼20% more Cr(VI), even at just 1% concentration. Our findings provide insight into Cr(VI)-adsorption complexation that controls the retention and remobilization of Cr(VI) on Fe-oxide minerals. The results point to an elevated risk of Cr(VI) mobilization in contaminated soils affected by SLR.

Evaluating the multiple sulfur isotope signature of Eoarchean rocks from the Isua Supracrustal Belt (Southwest-Greenland) by MC-ICP-MS: Volcanic nutrient sources for early life

On the anoxic Archean Earth, prior to the onset of oxidative weathering, electron acceptors were relatively scarce, perhaps limiting microbial productivity. An important metabolite may have been sulfate produced during the photolysis of volcanogenic SO2 gas. Multiple sulfur isotope data can be used to track this sulfur source, and indeed this record indicates SO2 photolysis dating back to at least 3.7 Ga, that is, as far back as proposed evidence of life on Earth. However, measurements of multiple sulfur isotopes in some key strata from that time can be challenging due to low sulfur concentrations. Some studies have overcome this challenge with NanoSIMS or optimized gas-source mass spectrometry techniques, but those instruments are not readily accessible. Here, we applied an aqua regia leaching protocol to extract small amounts of sulfur from whole rocks for analyses of multiple sulfur isotopes by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Measurements of standards and replicates demonstrate good precision and accuracy. We applied this technique to meta-sedimentary rocks with putative biosignatures from the Eoarchean Isua Supracrustal Belt (ISB, >3.7 Ga) and found positive ∆33S (1.40-1.80‰) in four meta-turbidites and negative ∆33S (-0.80‰ and -0.66‰) in two meta-carbonates. Two meta-basalts do not display significant mass-independent fractionation (MIF, -0.01‰ and 0.16‰). In situ Re-Os dating on a molybdenite vein hosted in the meta-turbidites identifies an early ca. 3.7 Ga hydrothermal phase, and in situ Rb-Sr dating of micas in the meta-carbonates suggests metamorphism affected the rocks at ca. 2.2 and 1.7 Ga. We discuss alteration mechanisms and conclude that there is most likely a primary MIF-bearing phase in these meta-sediments. Our new method is therefore a useful addition to the geochemical toolbox, and it confirms that organisms at that time, if present, may indeed have been fed by volcanic nutrients.

Chapter 4: A Geological and Chemical Context for the Origins of Life on Early Earth

Within the first billion years of Earth's history, the planet transformed from a hot, barren, and inhospitable landscape to an environment conducive to the emergence and persistence of life. This chapter will review the state of knowledge concerning early Earth's (Hadean/Eoarchean) geochemical environment, including the origin and composition of the planet's moon, crust, oceans, atmosphere, and organic content. It will also discuss abiotic geochemical cycling of the CHONPS elements and how these species could have been converted to biologically relevant building blocks, polymers, and chemical networks. Proposed environments for abiogenesis events are also described and evaluated. An understanding of the geochemical processes under which life may have emerged can better inform our assessment of the habitability of other worlds, the potential complexity that abiotic chemistry can achieve (which has implications for putative biosignatures), and the possibility for biochemistries that are vastly different from those on Earth.

Chapter 1: The Astrobiology Primer 3.0

The Astrobiology Primer 3.0 (ABP3.0) is a concise introduction to the field of astrobiology for students and others who are new to the field of astrobiology. It provides an entry into the broader materials in this supplementary issue of Astrobiology and an overview of the investigations and driving hypotheses that make up this interdisciplinary field. The content of this chapter was adapted from the other 10 articles in this supplementary issue and thus represents the contribution of all the authors who worked on these introductory articles. The content of this chapter is not exhaustive and represents the topics that the authors found to be the most important and compelling in a dynamic and changing field.

Dissolved silica affects the bulk iron redox state and recrystallization of minerals generated by photoferrotrophy in a simulated Archean ocean

Chemical sedimentary deposits called Banded Iron Formations (BIFs) are one of the best surviving records of ancient marine (bio)geochemistry. Many BIF precursor sediments precipitated from ferruginous, silica-rich waters prior to the Great Oxidation Event at ~2.43 Ga. Reconstructing the mineralogy of BIF precursor phases is key to understanding the coevolution of seawater chemistry and early life. Many models of BIF deposition invoke the activity of Fe(II)-oxidizing photoautotrophic bacteria as a mechanism for precipitating mixed-valence Fe(II,III) and/or fully oxidized Fe(III) minerals in the absence of molecular oxygen. Although the identity of phases produced by ancient photoferrotrophs remains debated, laboratory experiments provide a means to explore what their mineral byproducts might have been. Few studies have thoroughly characterized precipitates produced by photoferrotrophs in settings representative of Archean oceans, including investigating how residual Fe(II)aq can affect the mineralogy of expected solid phases. The concentration of dissolved silica (Si) is also an important variable to consider, as silicate species may influence the identity and reactivity of Fe(III)-bearing phases. To address these uncertainties, we cultured Rhodopseudomonas palustris TIE-1 as a photoferrotroph in synthetic Archean seawater with an initial [Fe(II)aq ] of 1 mM and [Si] spanning 0-1.5 mM. Ferrihydrite was the dominant precipitate across all Si concentrations, even with substantial Fe(II) remaining in solution. Consistent with other studies of microbial iron oxidation, no Fe-silicates were observed across the silica gradient, although Si coprecipitated with ferrihydrite via surface adsorption. More crystalline phases such as lepidocrocite and goethite were only detected at low [Si] and are likely products of Fe(II)-catalyzed ferrihydrite transformation. Finally, we observed a substantial fraction of Fe(II) in precipitates, with the proportion of Fe(II) increasing as a function of [Si]. These experimental results suggest that photoferrotrophy in a Fe(II)-buffered ocean may have exported Fe(II,III)-oxide/silica admixtures to BIF sediments, providing a more chemically diverse substrate than previously hypothesized.

Metabolic features that select for Bathyarchaeia in modern ferruginous lacustrine subsurface sediments

Ferruginous conditions prevailed through Earth's early oceans history, yet our understanding of biogeochemical cycles in anoxic iron-rich, sulfate-poor sediments remains elusive in terms of redox processes and organic matter remineralization. Using comprehensive geochemistry, cell counts, and metagenomic data, we investigated the taxonomic and functional distribution of the microbial subsurface biosphere in Lake Towuti, a stratified ferruginous analogue. Below the zone in which pore water becomes depleted in electron acceptors, cell densities exponentially decreased while microbial assemblages shifted from iron- and sulfate-reducing bacterial populations to fermentative anaerobes and methanogens, mostly selecting Bathyarchaeia below the sulfate reduction zone. Bathyarchaeia encode metabolic machinery to cycle and assimilate polysulfides via sulfhydrogenase, sulfide dehydrogenase, and heterodisulfide reductase, using dissimilatory sulfite reductase subunit E and rubredoxin as carriers. Their metagenome-assembled genomes showed that carbon fixation could proceed through the complete methyl-branch Wood-Ljungdahl pathway, conducting (homo)acetogenesis in the absence of methyl coenzyme M reductase. Further, their partial carbonyl-branch, assumed to act in tetrahydrofolate interconversions of C1 and C2 compounds, could support close interactions with methylotrophic methanogens in the fermentation zone. Thus, Bathyarchaeia appeared capable of coupling sulfur-redox reactions with fermentative processes, using electron bifurcation in a redox-conserving (homo)acetogenic Wood-Ljungdahl pathway, and revealing geochemical ferruginous conditions at the transition between the sulfate reduction and fermentation zone as their preferential niche.

Sedimentary parameters control the sulfur isotope composition of marine pyrite

Reconstructions of coupled carbon, oxygen, and sulfur cycles rely heavily on sedimentary pyrite sulfur isotope compositions (δ34Spyr). With a model of sediment diagenesis, paired with global datasets of sedimentary parameters, we show that the wide range of δ34Spyr (~100 per mil) in modern marine sediments arises from geographic patterns in the relative rates of diffusion, burial, and microbial reduction of sulfate. By contrast, the microbial sulfur isotope fractionation remains large and relatively uniform. Over Earth history, the effect of increasing seawater sulfate and oxygen concentrations on sulfate and sulfide transport and reaction may explain the corresponding increase observed in the δ34S offset between sulfate and pyrite. More subtle variations may be related to changes in depositional environments associated with sea level fluctuations and supercontinent cycles.

Are Large Sulfur Isotope Variations Biosignatures in an Ancient, Impact-Induced Hydrothermal Mars Analog?

Discrepancies have emerged concerning the application of sulfur stable isotope ratios as a biosignature in impact crater paleolakes. The first in situ δ34S data from Mars at Gale crater display a ∼75‰ range that has been attributed to an abiotic mechanism. Yet biogeochemical studies of ancient environments on Earth generally interpret δ34S fractionations >21‰ as indicative of a biological origin, and studies of δ34S at analog impact crater lakes on Earth have followed the same approach. We performed analyses (including δ34S, total organic carbon wt%, and scanning electron microscope imaging) on multiple lithologies from the Nördlinger Ries impact crater, focusing on hydrothermally altered impact breccias and associated sedimentary lake-fill sequences to determine whether the δ34S properties define a biosignature. The differences in δ34S between the host lithologies may have resulted from thermochemical sulfate reduction, microbial sulfate reduction, hydrothermal equilibrium fractionation, or any combination thereof. Despite abundant samples and instrumental precision currently exclusive to Earth-bound analyses, assertions of biogenicity from δ34S variations >21‰ at the Miocene Ries impact crater are tenuous. This discourages the use of δ34S as a biosignature in similar environments without independent checks that include the full geologic, biogeochemical, and textural context, as well as a comprehensive acknowledgment of alternative hypotheses.

Chemical conditions on Hycean worlds

Traditionally, the search for life on exoplanets has been predominantly focused on rocky exoplanets. The recently proposed Hycean worlds have the potential to significantly expand and accelerate the search for life elsewhere. Hycean worlds are a class of habitable sub-Neptunes with planet-wide oceans and H2-rich atmospheres. Their broad range of possible sizes and temperatures lead to a wide habitable zone and high potential for discovery and atmospheric characterization using transit spectroscopy. Over a dozen candidate Hycean planets are already known to be transiting nearby M dwarfs, making them promising targets for atmospheric characterization with the James Webb Space Telescope (JWST). In this work, we investigate possible chemical conditions on a canonical Hycean world, focusing on (a) the present and primordial molecular composition of the atmosphere, and (b) the inventory of bioessential elements for the origin and sustenance of life in the ocean. Based on photochemical and kinetic modeling for a range of conditions, we discuss the possible chemical evolution and observable present-day composition of its atmosphere. In particular, for reduced primordial conditions the early atmospheric evolution passes through a phase that is rich in organic molecules that could provide important feedstock for prebiotic chemistry. We investigate avenues for delivering bioessential metals to the ocean, considering the challenging lack of weathering from a rocky surface and the ocean separated from the rocky core by a thick icy mantle. Based on ocean depths from internal structure modelling and elemental estimates for the early Earth's oceans, we estimate the requirements for bioessential metals in such a planet. We find that the requirements can be met for plausible assumptions about impact history and atmospheric sedimentation, and supplemented by other steady state sources. We discuss the observational prospects for atmospheric characterisation of Hycean worlds with JWST and future directions of this new paradigm in the search for life on exoplanets.

Exploring the Effects of Residence Time on the Utility of Stable Isotopes and S/C Ratios as Proxies for Ocean Connectivity

Various geochemical proxies have been developed to determine if ancient sedimentary strata were deposited in marine or nonmarine environments. A critical parameter for proxy reliability is the residence time of aqueous species in seawater, which is rarely considered for proxies relying on stable isotopes and elemental abundance ratios. Differences in residence time may affect our ability to track geologically short-lived alternations between marine and nonmarine conditions. To test this effect for sulfur and nitrogen isotopes and sulfur/carbon ratios, we investigated a stratigraphic section in the Miocene Oberpullendorf Basin in Austria. Here, previous work revealed typical seawater-like rare earth element and yttrium (REY) systematics transitioning to nonmarine-like systematics. This shift was interpreted as a brief transition from an open marine depositional setting to a restricted embayment with a reduced level of exchange with the open ocean and possibly freshwater influence. Our isotopic results show no discernible response in carbonate-associated sulfate sulfur isotopes and carbon/sulfur abundance ratios during the interval of marine restriction inferred from the REY data, but nitrogen isotopes show a decrease by several permil. This observation is consistent with the much longer residence time of sulfate in seawater compared with REY and nitrate. Hence, this case study illustrates that the residence time is a key factor for the utility of seawater proxies. In some cases, it may make geochemical parameters more sensitive to marine water influx than paleontological observations, as in the Oberpullendorf Basin. Particular care is warranted in deep time, when marine residence times likely differ markedly from the modern.

The evolution and spread of sulfur cycling enzymes reflect the redox state of the early Earth

The biogeochemical sulfur cycle plays a central role in fueling microbial metabolisms, regulating the Earth's redox state, and affecting climate. However, geochemical reconstructions of the ancient sulfur cycle are confounded by ambiguous isotopic signals. We use phylogenetic reconciliation to ascertain the timing of ancient sulfur cycling gene events across the tree of life. Our results suggest that metabolisms using sulfide oxidation emerged in the Archean, but those involving thiosulfate emerged only after the Great Oxidation Event. Our data reveal that observed geochemical signatures resulted not from the expansion of a single type of organism but were instead associated with genomic innovation across the biosphere. Moreover, our results provide the first indication of organic sulfur cycling from the Mid-Proterozoic onwards, with implications for climate regulation and atmospheric biosignatures. Overall, our results provide insights into how the biological sulfur cycle evolved in tandem with the redox state of the early Earth.

The adsorption of arsenate and p-arsanilic acid onto ferrihydrite and subsequent desorption by sulfate and artificial seawater: Future implications of sea level rise

Sea level rise (SLR) is estimated to impact 25% of the world's population along coastal areas leading to an increase in saltwater intrusion. Consequently, changes in the soil biogeochemistry of currently non-saline and/or well-drained soils due to saltwater intrusion are of major concern. Saltwater intrusion is expected to affect farmland across large broiler producer regions, where large amounts of manure containing organic arsenicals were applied over the past decades. To determine how SLR may impact the speciation and mobility of adsorbed inorganic and organic As, we used in situ real-time attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) to determine the adsorption and desorption mechanisms of As(V) and 4-aminophenylarsonic (p-ASA, a poultry feed additive) on ferrihydrite (Fh) in the presence of sulfate at varying pH. The adsorption of As(V) and p-ASA increased at lower pH, with As(V) showing IR features consistent with the formation of inner-sphere of As-Fh surface complexes, while p-ASA also formed others structures as H-bonded As-surface complexes, likely mediated by outer-sphere complexes, based on our FTIR and batch experiments data. No observable As(V) or p-ASA desorption from the Fh surface was promoted by sulfate, however sulfate adsorption on the Fh surface was remarkably larger for p-ASA than for As(V). Complimentary, we carried out batch studies of As(V) and p-ASA desorption by Fh, using artificial seawater (ASW) at varying concentrations. The 1% ASW desorbed ∼10% of initially sorbed p-ASA, while at 100% ASW desorbed ∼40%. However, <1% of As(V) was desorbed by 1% ASW solution and only ∼7.9% were desorbed at 100% ASW. The spectroscopic data support the more extensive desorption of p-ASA compared to As(V) observed in batch experiments, suggesting that organoarsenicals may be easily desorbed and, after conversion to inorganic forms, pose a risk to water supplies.

Recent Advances in Biotechnologies for the Treatment of Environmental Pollutants Based on Reactive Sulfur Species

The definition of reactive sulfur species (RSS) is inspired by the reactivity and variable chemical valence of sulfur. Sulfur is an essential element for life and is a part of global geochemical cycles. Wastewater treatment bioreactors can be divided into two major categories: sulfur reduction and sulfur oxidation. We review the origins of the definition of RSS and related biotechnological processes in environmental management. Sulfate reduction, sulfide oxidation, and sulfur-based redox reactions are key to driving the coupled global carbon, nitrogen, and sulfur co-cycles. This shows the coupling of the sulfur cycle with the carbon and nitrogen cycles and provides insights into the global material-chemical cycle. We also review the biological classification and RSS metabolic mechanisms of functional microorganisms involved in the biological processes, such as sulfate-reducing and sulfur-oxidizing bacteria. Developments in molecular biology and genomic technologies have allowed us to obtain detailed information on these bacteria. The importance of RSS in environmental technologies requires further consideration.

What Controls the Sulfur Isotope Fractionation during Dissimilatory Sulfate Reduction?

Sulfate often behaves conservatively in the oxygenated environments but serves as an electron acceptor for microbial respiration in a wide range of natural and engineered systems where oxygen is depleted. As a ubiquitous anaerobic dissimilatory pathway, therefore, microbial reduction of sulfate to sulfide has been of continuing interest in the field of microbiology, ecology, biochemistry, and geochemistry. Stable isotopes of sulfur are an effective tool for tracking this catabolic process as microorganisms discriminate strongly against heavy isotopes when cleaving the sulfur-oxygen bond. Along with its high preservation potential in environmental archives, a wide variation in the sulfur isotope effects can provide insights into the physiology of sulfate reducing microorganisms across temporal and spatial barriers. A vast array of parameters, including phylogeny, temperature, respiration rate, and availability of sulfate, electron donor, and other essential nutrients, has been explored as a possible determinant of the magnitude of isotope fractionation, and there is now a broad consensus that the relative availability of sulfate and electron donors primarily controls the magnitude of fractionation. As the ratio shifts toward sulfate, the sulfur isotope fractionation increases. The results of conceptual models, centered on the reversibility of each enzymatic step in the dissimilatory sulfate reduction pathway, are in qualitative agreement with the observations, although the underlying intracellular mechanisms that translate the external stimuli into the isotopic phenotype remain largely unexplored experimentally. This minireview offers a snapshot of our current understanding of the sulfur isotope effects during dissimilatory sulfate reduction as well as their potential quantitative applications. It emphasizes the importance of sulfate respiration as a model system for the isotopic investigation of other respiratory pathways that utilize oxyanions as terminal electron acceptors.

Multiple sulphur isotope record of Paleoarchean sedimentary rocks across the Onverwacht Group, Barberton Greenstone Belt, South Africa

This study presents multiple sulphur isotope (³² S, ³³ S, ³⁴ S, ³⁶ S) data on pyrites from silicified volcano-sedimentary rocks of the Paleoarchean Onverwacht Group of the Barberton greenstone belt, South Africa. These rocks include seafloor cherts and felsic conglomerates that were deposited in shallow marine environments preserving a record of atmospheric and biogeochemical conditions on the early Earth. A strong variation in mass independent sulphur isotope fractionation (MIF-S) anomalies is found in the cherts, with Δ³³ S ranging between -0.26‰ and 3.42‰. We explore possible depositional and preservational factors that could explain some of this variation seen in MIF-S. Evidence for microbial activity is recorded by the c. 3.45 Ga Hooggenoeg Formation Chert (HC4) preserving a contribution of microbial sulphate reduction (-Δ³³ S and -δ³⁴ S), and a c. 3.33 Ga Kromberg Formation Chert (KC5) recording a possible contribution of microbial elemental sulphur disproportionation (+Δ³³ S and -δ³⁴ S). Pyrites from a rhyo-dacitic conglomerate of the Noisy Formation do not plot along a previously proposed global Felsic Volcanic Array, and this excludes short-lived pulses of intense felsic volcanic gas emissions as the dominant control on Archean MIF-S. Rather, we suggest that the MIF-S signals measured reflect dilution during marine deposition, early diagenetic modification, and mixing with volcanic/hydrothermal S sources. Given the expanded stratigraphic interval (3.47-3.22 Ga) now sampled from across the Barberton Supergroup, we conclude that large MIF-S exceeding >4‰ is atypical of Paleoarchean near-surface environments on the Kaapvaal Craton.

Transport-limited reactions in microbial systems

Predicting microbial metabolic rates and emergent biogeochemical fluxes remains challenging due to the many unknown population dynamical, physiological and reaction-kinetic parameters and uncertainties in species composition. Here, we show that the need for these parameters can be eliminated when population dynamics and reaction kinetics operate at much shorter time scales than physical mixing processes. Such scenarios are widespread in poorly mixed water columns and sediments. In this 'fast-reaction-transport' (FRT) limit, all that is required for predictions are chemical boundary conditions, the physical mixing processes and reaction stoichiometries, while no knowledge of species composition, physiology or population/reaction kinetic parameters is needed. Using time-series data spanning years 2001-2014 and depths 180-900 m across the permanently anoxic Cariaco Basin, we demonstrate that the FRT approach can accurately predict the dynamics of major electron donors and acceptors (Pearson r ≥ 0.9 in all cases). Hence, many microbial processes in this system are largely transport limited and thus predictable regardless of species composition, population dynamics and kinetics. Our approach enables predictions for many systems in which microbial community dynamics and kinetics are unknown. Our findings also reveal a mechanism for the frequently observed decoupling between function and taxonomy in microbial systems.

Unravelling microbial drivers of the sulfate-reduction process inside landfill using metagenomics

Hydrogen sulfide (H₂S) is one of the common landfill odor. This research demonstrates that the sulfate transformation behavior is significantly enhanced during the landfill process, accompanied by a shift in microbial structure. The relative abundance of dissimilatory sulfate reduction (DSR) and thiosulfate oxidation by SOX (sulfur-oxidation) complex gradually decreases through the landfill processes while the assimilatory sulfate reduction (ASR) demonstrates the opposite behavior. The major module for landfill sulfate reduction is ASR, accounting for 31.72% ± 2.84% of sulfate metabolism. Based on the functional genes for the sulfate pathway, the drivers for sulfate biotransformation in landfills were determined and further identified their contribution in the sulfate metabolism during landfill processes. Pseudomonas, Methylocaldum, Bacillus, Methylocystis and Hyphomicrobium were the top 5 contributors for ASR pathway, and only one genus Pseudomonas was found for DSR pathway. Among the 26 high-quality metagenome-assembled genomes of sulfate functional species, 24 were considered novel species for sulfuric metabolism. Overall, this study provides unique insight into the sulfate transformation process related to the H₂S odor control in landfill management.

Phosphorylation and sulfation share a common biosynthetic pathway, but extend biochemical and evolutionary diversity of biological macromolecules in distinct ways

Phosphate and sulfate groups are integral to energy metabolism and introduce negative charges into biological macromolecules. One purpose of such modifications is to elicit precise binding/activation of protein partners. The physico-chemical properties of the two groups, while superficially similar, differ in one important respect—the valency of the central (phosphorus or sulfur) atom. This dictates the distinct properties of their respective esters, di-esters and hence their charges, interactions with metal ions and their solubility. These, in turn, determine the contrasting roles for which each group has evolved in biological systems. Biosynthetic links exist between the two modifications; the sulfate donor 3′-phosphoadenosine-5′-phosphosulfate being formed from adenosine triphosphate (ATP) and adenosine phosphosulfate, while the latter is generated from sulfate anions and ATP. Furthermore, phosphorylation, by a xylosyl kinase (Fam20B, glycosaminoglycan xylosylkinase) of the xylose residue of the tetrasaccharide linker region that connects nascent glycosaminoglycan (GAG) chains to their parent proteoglycans, substantially accelerates their biosynthesis. Following observations that GAG chains can enter the cell nucleus, it is hypothesized that sulfated GAGs could influence events in the nucleus, which would complete a feedback loop uniting the complementary anionic modifications of phosphorylation and sulfation through complex, inter-connected signalling networks and warrants further exploration.

Anoxic photochemical weathering of pyrite on Archean continents

Sulfur is an essential element of life that is assimilated by Earth's biosphere through the chemical breakdown of pyrite. On the early Earth, pyrite weathering by atmospheric oxygen was severely limited, and low marine sulfate concentrations persisted for much of the Archean eon. Here, we show an anoxic photochemical mechanism of pyrite weathering that could have provided substantial amounts of sulfate to the oceans as continents formed in the late Archean. Pyrite grains suspended in anoxic ferrous iron solutions produced millimolar sulfate concentrations when irradiated with ultraviolet light. The Fe²⁺_(aq) was photooxidized, which, in turn, led to the chemical oxidation of pyritic sulfur. Additional experiments conducted with 2.68 Ga shale demonstrated that photochemically derived ferric iron oxidizes and dissolves sedimentary pyrite during chemical weathering. The results suggest that before the rise of atmospheric oxygen, oxidative pyrite weathering on Archean continents was controlled by the exposure of land to sunlight.

The Dziani Dzaha Lake: A long-awaited modern analogue for superheavy pyrites

Sedimentary records of superheavy pyrites in Phanerozoic and Proterozoic successions (i.e., extremely positive δ³⁴ S_pyrite values together with higher δ³⁴ S_pyrite than coeval δ³⁴ S_CAS ) are mostly interpreted as resulting either from secondary postdepositional processes or from multiple redox reactions between sulfate and sulfide in stratified sulfate-poor environments. We report here the first observation of strongly positive δ³⁴ S values for both dissolved sulfate and sulfide (average δ³⁴ S_diss.sulfate value of 34.6‰ and δ³⁴ S_diss.sulfide values of 36.7‰) compared to the present-day seawater δ³⁴ S_diss ._sulfate (~21‰), with a negative apparent fractionation between sulfate and sulfide (∆³⁴ S_{diss.sulfate-diss.sulfide} ~ -2.1 ± 1.4‰), in the sulfate-poor (<3 mm) modern thalassohaline lacustrine system Dziani Dzaha (Mayotte, Indian Ocean). Overall, surface sediments faithfully record the water column isotopic signatures including a mainly negative ∆³⁴ S_{sed.sulfate-sed.sulfide} (-4.98 ± 4.5‰), corresponding to the definition of superheavy pyrite documented in the rock record. We propose that in the Dziani Dzaha this superheavy pyrite signature results from a two-stage evolution of the sulfur biogeochemical cycle. In a first stage, the sulfur cycle would have been dominated by sulfate from initially sulfate-rich marine waters. Overtime, Raleigh distillation by microbial sulfate reduction coupled with sulfide burial in the sediment would have progressively enriched in ³⁴ S the water column residual sulfate. In a second still active stage, quantitative sulfate reduction not only occurs below the halocline during stratified periods but also in the whole water column during fully anoxic episodes. Sulfates are then regenerated by partial oxidation of sulfides as the oxic-anoxic interface moves downward. These results demonstrate that the atypical superheavy pyrite isotope signature does not necessarily require postdepositional or secondary oxidative processes and can result from primary processes in restricted sulfate-poor and highly productive environments analogous to the Dziani Dzaha.

An overview of experimental simulations of microbial activity in early Earth

Microbial activity has shaped the evolution of the ocean and atmosphere throughout the Earth history. Thus, experimental simulations of microbial metabolism under the environment conditions of the early Earth can provide vital information regarding biogeochemical cycles and the interaction and coevolution between life and environment, with important implications for extraterrestrial exploration. In this review, we discuss the current scope and knowledge of experimental simulations of microbial activity in environments representative of those of early Earth, with perspectives on future studies. Inclusive experimental simulations involving multiple species, and cultivation experiments with more constraints on environmental conditions similar to early Earth would significantly advance our understanding of the biogeochemical cycles of the geological past.

Eutrophication leads to the formation of a sulfide-rich deep-water layer in Lake Sevan, Armenia

Lake Sevan is a meso-eutrophic water body, which was severely impacted by anthropogenic level decrease, pollution and eutrophication during the last century. Starting in the 1970s, these processes resulted in the formation of an oxygen-depleted hypolimnion during summer-autumn stratification of the lake. In this work, we demonstrate for the first time that eutrophication of the lake leads not only to the full depletion of oxygen and nitrate in the hypolimnion but as well to the presence of sulfate-reducing microorganisms and toxic hydrogen sulfide. Concentrations of hydrogen sulfide in the hypolimnion of Major and Minor Sevan in October were as high as 9 and 39 μM, respectively. In October 2019, 66 % of lake's bottom was covered by sulfidic waters, while the fraction of sulfidic water volume reached 19 %. Values of δ³⁴S for hypolimnetic sulfide are lower by only 7-12 ‰ compared to epilimnetic sulfate, while δ³³S values of sulfide are similar to the δ³³S values of sulfate. These isotopic fingerprints are not consistent with microbial sulfate reduction as the sole source of hydrogen sulfide in the hypolimnion. We attribute the formation of a sulfidic deep-water layer to a combination of microbial sulfate reduction in the water column and diffusion of hydrogen sulfide from the sediments.

Study of Ferrocyanide Adsorption onto Different Minerals as Prebiotic Chemistry Assays

Considered one of the most promising building blocks of life on primitive Earth, cyanide and its complexes are likely to have played an important role in the emergence of life on the planet. Investigation into cyanide on Earth has primarily considered high concentrations, but the cyanide concentration in the oceans of prebiotic Earth was exceptionally low. Thus, Bernal's hypothesis has allowed investigators to work around this problem. We observed, however, that cyanide does not adsorb onto several minerals; therefore, ferrocyanide could be used as a cyanide source when adsorbed onto mineral surfaces to promote the synthesis of molecules of biological significance. When adsorbed onto bentonite, a mineral that has Fe³⁺ atoms in its interlayers, the formation of Prussian blue analog complexes occurs through endothermic reaction and with increased entropy. The adsorption of ferrocyanide onto kaolinite indicates an exothermic and outer-sphere interaction, which results in degeneracy breakdown for C ≡ N stretch energy into two new bands of FTIR-ATR spectrum. Magnetite, which has iron atoms in its structure, and ferrocyanide interactions have been observed by outer-sphere coordination as well as the formation of Prussian blue analogs, as confirmed by the appearance of a new doublet in the Mössbauer spectra and a broadband close to 750 nm at UV-visible spectroscopy. Magnetite and kaolinite experiments presented relevant results only when performed in seawater, which suggests the importance of seawater composition for prebiotic experiments. These obtained results prove that ferrocyanide interacts with minerals differently according to structure and composition and show that this complex, like the Prussian blue analogs, may have played a crucial role as a source of cyanide on primitive Earth.

Mercury abundance and isotopic composition indicate subaerial volcanism prior to the end-Archean "whiff" of oxygen

Earth's early atmosphere witnessed multiple transient episodes of oxygenation before the Great Oxidation Event 2.4 billion years ago (Ga) [e.g., A. D. Anbar et al., Science 317, 1903-1906 (2007); M. C. Koehler, R. Buick, M. E. Barley, Precambrian Res. 320, 281-290 (2019)], but the triggers for these short-lived events are so far unknown. Here, we use mercury (Hg) abundance and stable isotope composition to investigate atmospheric evolution and its driving mechanisms across the well-studied "whiff" of O₂ recorded in the ∼2.5-Ga Mt. McRae Shale from the Pilbara Craton in Western Australia [A. D. Anbar et al., Science 317, 1903-1906 (2007)]. Our data from the oxygenated interval show strong Hg enrichment paired with slightly negative ∆¹⁹⁹Hg and near-zero ∆²⁰⁰Hg, suggestive of increased oxidative weathering. In contrast, slightly older beds, which were evidently deposited under an anoxic atmosphere in ferruginous waters [C. T. Reinhard, R. Raiswell, C. Scott, A. D. Anbar, T. W. Lyons, Science 326, 713-716 (2009)], show Hg enrichment coupled with positive ∆¹⁹⁹Hg and slightly negative ∆²⁰⁰Hg values. This pattern is consistent with photochemical reactions associated with subaerial volcanism under intense UV radiation. Our results therefore suggest that the whiff of O₂ was preceded by subaerial volcanism. The transient interval of O₂ accumulation may thus have been triggered by diminished volcanic O₂ sinks, followed by enhanced nutrient supply to the ocean from weathering of volcanic rocks causing increased biological productivity.

Oxygenation, Life, and the Planetary System during Earth's Middle History: An Overview

The long history of life on Earth has unfolded as a cause-and-effect relationship with the evolving amount of oxygen (O₂) in the oceans and atmosphere. Oxygen deficiency characterized our planet's first 2 billion years, yet evidence for biological O₂ production and local enrichments in the surface ocean appear long before the first accumulations of O₂ in the atmosphere roughly 2.4 to 2.3 billion years ago. Much has been written about this fundamental transition and the related balance between biological O₂ production and sinks coupled to deep Earth processes that could buffer against the accumulation of biogenic O₂. However, the relationship between complex life (eukaryotes, including animals) and later oxygenation is less clear. Some data suggest O₂ was higher but still mostly low for another billion and a half years before increasing again around 800 million years ago, potentially setting a challenging course for complex life during its initial development and ecological expansion. The apparent rise in O₂ around 800 million years ago is coincident with major developments in complex life. Multiple geochemical and paleontological records point to a major biogeochemical transition at that time, but whether rising and still dynamic biospheric oxygen triggered or merely followed from innovations in eukaryotic ecology, including the emergence of animals, is still debated. This paper focuses on the geochemical records of Earth's middle history, roughly 1.8 to 0.5 billion years ago, as a backdrop for exploring possible cause-and-effect relationships with biological evolution and the primary controls that may have set its pace, including solid Earth/tectonic processes, nutrient limitation, and their possible linkages. A richer mechanistic understanding of the interplay between coevolving life and Earth surface environments can provide a template for understanding and remotely searching for sustained habitability and even life on distant exoplanets.

Mass-Transfer-Limited Biodegradation at Low Concentrations-Evidence from Reactive Transport Modeling of Isotope Profiles in a Bench-Scale Aquifer

Organic contaminant degradation by suspended bacteria in chemostats has shown that isotope fractionation decreases dramatically when pollutant concentrations fall below the (half-saturation) Monod constant. This masked isotope fractionation implies that membrane transfer is slow relative to the enzyme turnover at μg L^-1 substrate levels. Analogous evidence of mass transfer as a bottleneck for biodegradation in aquifer settings, where microbes are attached to the sediment, is lacking. A quasi-two-dimensional flow-through sediment microcosm/tank system enabled us to study the aerobic degradation of 2,6-dichlorobenzamide (BAM), while collecting sufficient samples at the outlet for compound-specific isotope analysis. By feeding an anoxic BAM solution through the center inlet port and dissolved oxygen (DO) above and below, strong transverse concentration cross-gradients of BAM and DO yielded zones of low (μg L^-1) steady-state concentrations. We were able to simulate the profiles of concentrations and isotope ratios of the contaminant plume using a reactive transport model that accounted for a mass-transfer limitation into bacterial cells, where apparent isotope enrichment factors *ε decreased strongly below concentrations around 600 μg/L BAM. For the biodegradation of organic micropollutants, mass transfer into the cell emerges as a bottleneck, specifically at low (μg L^-1) concentrations. Neglecting this effect when interpreting isotope ratios at field sites may lead to a significant underestimation of biodegradation.

Organic matter mineralization in modern and ancient ferruginous sediments

Deposition of ferruginous sediment was widespread during the Archaean and Proterozoic Eons, playing an important role in global biogeochemical cycling. Knowledge of organic matter mineralization in such sediment, however, remains mostly conceptual, as modern ferruginous analogs are largely unstudied. Here we show that in sediment of ferruginous Lake Towuti, Indonesia, methanogenesis dominates organic matter mineralization despite highly abundant reactive ferric iron phases like goethite that persist throughout the sediment. Ferric iron can thus be buried over geologic timescales even in the presence of labile organic carbon. Coexistence of ferric iron with millimolar concentrations of methane further demonstrates lack of iron-dependent methane oxidation. With negligible methane oxidation, methane diffuses from the sediment into overlying waters where it can be oxidized with oxygen or escape to the atmosphere. In low-oxygen ferruginous Archaean and Proterozoic oceans, therefore, sedimentary methane production was likely favored with strong potential to influence Earth's early climate.

Pseudodesulfovibrio cashew sp. Nov., a Novel Deep-Sea Sulfate-Reducing Bacterium, Linking Heavy Metal Resistance and Sulfur Cycle

Sulfur cycling is primarily driven by sulfate reduction mediated by sulfate-reducing bacteria (SRB) in marine sediments. The dissimilatory sulfate reduction drives the production of enormous quantities of reduced sulfide and thereby the formation of highly insoluble metal sulfides in marine sediments. Here, a novel sulfate-reducing bacterium designated Pseudodesulfovibrio cashew SRB007 was isolated and purified from the deep-sea cold seep and proposed to represent a novel species in the genus of Pseudodesulfovibrio. A detailed description of the phenotypic traits, phylogenetic status and central metabolisms of strain SRB007 allowed the reconstruction of the metabolic potential and lifestyle of a novel member of deep-sea SRB. Notably, P. cashew SRB007 showed a strong ability to resist and remove different heavy metal ions including Co2+, Ni2+, Cd2+ and Hg2+. The dissimilatory sulfate reduction was demonstrated to contribute to the prominent removal capability of P. cashew SRB007 against different heavy metals via the formation of insoluble metal sulfides.

A Reconstructed Subaerial Hot Spring Field in the ∼3.5 Billion-Year-Old Dresser Formation, North Pole Dome, Pilbara Craton, Western Australia

Recent discoveries of geyserite and siliceous sinter with textural biosignatures in the ∼3.5 Ga Dresser Formation of the Pilbara Craton, Western Australia, extended the record of inhabited subaerial hot springs on Earth by ∼3 billion years, back to the time when siliceous sinter deposits are known to have formed on Mars (e.g., at Columbia Hills, Gusev Crater). Here, we present more detailed lithostratigraphic, petrographic and geochemical data collected from 100 measured sections across a ∼14 km strike length in the Dresser Formation. The data indicate deposition of a wide range of hot spring and associated deposits in a restricted interval that directly overlies a hydrothermally influenced volcanic caldera lake facies, with shoreline stromatolites. Hot spring deposits show abrupt lateral facies changes and include associated channelized clastic deposits that support fluvial, subaerial hot spring deposition. All Dresser hot spring and associated lithofacies have direct analogs with proximal, middle, and distal apron hot spring facies that are characteristic of those from New Zealand, Yellowstone National Park, USA, and Argentina. Rare earth element and yttrium geochemistry shows that the Dresser geyserite shares identical patterns with Phanerozoic hot spring sinters. This geochemical data further supports textural and contextual evidence that indicate the Dresser geyserite formed as a subaerial hot spring sinter. Further, the Dresser hot spring deposits are temporally associated with a diverse suite of textural biosignatures that indicate a thriving microbial community existed within in a Paleoarchean hot spring field. The results presented here underscore the importance of continued study of the early geological record for astrobiological research. In particular these findings reinforce the long-standing hypothesis that hydrothermal systems are optimal places to search for past life on Mars.

Bistability in the redox chemistry of sediments and oceans

Changes in ocean redox chemistry are frequently observed in Earth’s history and have fundamental implications for the evolution of marine life. These transitions are commonly ascribed to large changes in the supply of iron, sulfur, or organic carbon in the deeper ocean. We propose that small variations in carbon input flux can drive nonreversible redox changes of the ocean interior and other anoxic systems, such as marine sediments. Nonlinear interactions in the iron and sulfur cycles create tipping points where regime shifts can occur between alternative stable states that are either iron dominated or sulfide dominated. The recognition that the biogeochemistry of sediments and oceans embeds intrinsic bistability provides a conceptual framework for understanding past and present anoxic marine systems.

For most of Earth’s history, the ocean’s interior was pervasively anoxic and showed occasional shifts in ocean redox chemistry between iron-buffered and sulfide-buffered states. These redox transitions are most often explained by large changes in external inputs, such as a strongly altered delivery of iron and sulfate to the ocean, or major shifts in marine productivity. Here, we propose that redox shifts can also arise from small perturbations that are amplified by nonlinear positive feedbacks within the internal iron and sulfur cycling of the ocean. Combining observational evidence with biogeochemical modeling, we show that both sedimentary and aquatic systems display intrinsic iron–sulfur bistability, which is tightly linked to the formation of reduced iron–sulfide minerals. The possibility of tipping points in the redox state of sediments and oceans, which allow large and nonreversible geochemical shifts to arise from relatively small changes in organic carbon input, has important implications for the interpretation of the geological rock record and the causes and consequences of major evolutionary transitions in the history of Earth’s biosphere.

Versatile cyanobacteria control the timing and extent of sulfide production in a Proterozoic analog microbial mat

Cyanobacterial mats were hotspots of biogeochemical cycling during the Precambrian. However, mechanisms that controlled O2 release by these ecosystems are poorly understood. In an analog to Proterozoic coastal ecosystems, the Frasassi sulfidic springs mats, we studied the regulation of oxygenic and sulfide-driven anoxygenic photosynthesis (OP and AP) in versatile cyanobacteria, and interactions with sulfur reducing bacteria (SRB). Using microsensors and stable isotope probing we found that dissolved organic carbon (DOC) released by OP fuels sulfide production, likely by a specialized SRB population. Increased sulfide fluxes were only stimulated after the cyanobacteria switched from AP to OP. O2 production triggered migration of large sulfur-oxidizing bacteria from the surface to underneath the cyanobacterial layer. The resultant sulfide shield tempered AP and allowed OP to occur for a longer duration over a diel cycle. The lack of cyanobacterial DOC supply to SRB during AP therefore maximized O2 export. This mechanism is unique to benthic ecosystems because transitions between metabolisms occur on the same time scale as solute transport to functionally distinct layers, with the rearrangement of the system by migration of microorganisms exaggerating the effect. Overall, cyanobacterial versatility disrupts the synergistic relationship between sulfide production and AP, and thus enhances diel O2 production.

Triple iron isotope constraints on the role of ocean iron sinks in early atmospheric oxygenation

The role that iron played in the oxygenation of Earth's surface is equivocal. Iron could have consumed molecular oxygen when Fe³⁺-oxyhydroxides formed in the oceans, or it could have promoted atmospheric oxidation by means of pyrite burial. Through high-precision iron isotopic measurements of Archean-Paleoproterozoic sediments and laboratory grown pyrites, we show that the triple iron isotopic composition of Neoarchean-Paleoproterozoic pyrites requires both extensive marine iron oxidation and sulfide-limited pyritization. Using an isotopic fractionation model informed by these data, we constrain the relative sizes of sedimentary Fe³⁺-oxyhydroxide and pyrite sinks for Neoarchean marine iron. We show that pyrite burial could have resulted in molecular oxygen export exceeding local Fe²⁺ oxidation sinks, thereby contributing to early episodes of transient oxygenation of Archean surface environments.

Using modern low-oxygen marine ecosystems to understand the nitrogen cycle of the Paleo- and Mesoproterozoic oceans

During the productive Paleoproterozoic (2.4-1.8 Ga) and less productive Mesoproterozoic (1.8-1.0 Ga), the ocean was suboxic to anoxic and multicellular organisms had not yet evolved. Here, we link geologic information about the Proterozoic ocean to microbial processes in modern low-oxygen systems. High iron concentrations and rates of Fe cycling in the Proterozoic are the largest differences from modern oxygen-deficient zones. In anoxic waters, which composed most of the Paleoproterozoic and ~40% of the Mesoproterozoic ocean, nitrogen cycling dominated. Rates of N₂ production by denitrification and anammox were likely linked to sinking organic matter fluxes and in situ primary productivity under anoxic conditions. Additionally autotrophic denitrifiers could have used reduced iron or methane. 50% of the Mesoproterozoic ocean may have been suboxic, promoting nitrification and metal oxidation in the suboxic water and N₂ O and N₂ production by partial and complete denitrification in anoxic zones in organic aggregates. Sulfidic conditions may have composed ~10% of the Mesoproterozoic ocean focused along continental margins. Due to low nitrate concentrations in offshore regions, anammox bacteria likely dominated N₂ production immediately above sulfidic zones, but in coastal regions, higher nitrate concentrations probably promoted complete S-oxidizing autotrophic denitrification at the sulfide interface.

Crustal thickening and endogenic oxidation of magmatic sulfur

Porphyry ore deposits, Earth's most important resources of copper, molybdenum, and rhenium, are strongly associated with felsic magmas showing signs of high-pressure differentiation and are usually found in places with thickened crust (>45 kilometers). This pattern is well-known, but unexplained, and remains an outstanding problem in our understanding of porphyry ore deposit formation. We approach this problem by investigating the oxidation state of magmatic sulfur, which controls the behavior of ore-forming metals during magma differentiation and magmatic-hydrothermal transition. We use sulfur in apatite to reconstruct the sulfur oxidation state in the Gangdese batholith, southern Tibet. We find that magma sulfate content increased abruptly after India-Eurasia collision. Apatite sulfur content and the calculated magma S6+/ΣS ratio correlate with whole-rock dysprosium/ytterbium ratio, suggesting that residual garnet, favored in thickened crust, exerts a first-order control on sulfur oxidation in magmatic orogens. Our findings link sulfur oxidation to internal petrogenic processes and imply an intrinsic relationship of magma oxidation with synmagmatic crustal thickening.

In situ S-isotope compositions of sulfate and sulfide from the 3.2 Ga Moodies Group, South Africa: A record of oxidative sulfur cycling

Sulfate minerals are rare in the Archean rock record and largely restricted to the occurrence of barite (BaSO₄ ). The origin of this barite remains controversially debated. The mass-independent fractionation of sulfur isotopes in these and other Archean sedimentary rocks suggests that photolysis of volcanic aerosols in an oxygen-poor atmosphere played an important role in their formation. Here, we report on the multiple sulfur isotopic composition of sedimentary anhydrite in the ca. 3.22 Ga Moodies Group of the Barberton Greenstone Belt, southern Africa. Anhydrite occurs, together with barite and pyrite, in regionally traceable beds that formed in fluvial settings. Variable abundances of barite versus anhydrite reflect changes in sulfate enrichment by evaporitic concentration across orders of magnitude in an arid, nearshore terrestrial environment, periodically replenished by influxes of seawater. The multiple S-isotope compositions of anhydrite and pyrite are consistent with microbial sulfate reduction. S-isotope signatures in barite suggest an additional oxidative sulfate source probably derived from continental weathering of sulfide possibly enhanced by microbial sulfur oxidation. Although depositional environments of Moodies sulfate minerals differ strongly from marine barite deposits, their sulfur isotopic composition is similar and most likely reflects a primary isotopic signature. The data indicate that a constant input of small portions of oxidized sulfur from the continents into the ocean may have contributed to the observed long-term increase in Δ³³ S_sulfate values through the Paleoarchean.

Iron-biomineralizing organelle in magnetotactic bacteria: function, synthesis and preservation in ancient rock samples

Magnetotactic bacteria (MTB) are ubiquitous aquatic microorganisms that incorporate iron from their environment to synthesize intracellular nanoparticles of magnetite (Fe₃ O₄ ) or greigite (Fe₃ S₄ ) in a genetically controlled manner. Magnetite and greigite magnetic phases allow MTB to swim towards redox transition zones where they thrive. MTB may represent some of the oldest microorganisms capable of synthesizing minerals on Earth and have been proposed to significantly impact the iron biogeochemical cycle by immobilizing soluble iron into crystals that subsequently fossilize in sedimentary rocks. In the present article, we describe the distribution of MTB in the environment and discuss the possible function of the magnetite and greigite nanoparticles. We then provide an overview of the chemical mechanisms leading to iron mineralization in MTB. Finally, we update the methods used for the detection of MTB crystals in sedimentary rocks and present their occurrences in the geological record.

Mantle data imply a decline of oxidizable volcanic gases could have triggered the Great Oxidation

Aerobic lifeforms, including humans, thrive because of abundant atmospheric O₂, but for much of Earth history O₂ levels were low. Even after evidence for oxygenic photosynthesis appeared, the atmosphere remained anoxic for hundreds of millions of years until the ~2.4 Ga Great Oxidation Event. The delay of atmospheric oxygenation and its timing remain poorly understood. Two recent studies reveal that the mantle gradually oxidized from the Archean onwards, leading to speculation that such oxidation enabled atmospheric oxygenation. But whether this mechanism works has not been quantitatively examined. Here, we show that these data imply that reducing Archean volcanic gases could have prevented atmospheric O₂ from accumulating until ~2.5 Ga with ≥95% probability. For two decades, mantle oxidation has been dismissed as a key driver of the evolution of O₂ and aerobic life. Our findings warrant a reconsideration for Earth and Earth-like exoplanets.

The early Earth’s atmosphere had very low oxygen levels for hundreds of millions of years, until the 2.4 Ga Great Oxidation Event, which remains poorly understood. Here, the authors show that reducing Archean volcanic gases could have prevented atmospheric O₂ from accumulating, and therefore mantle oxidation was likely very important in setting the evolution of O₂ and aerobic life.

Co-evolution of primitive methane-cycling ecosystems and early Earth's atmosphere and climate

The history of the Earth has been marked by major ecological transitions, driven by metabolic innovation, that radically reshaped the composition of the oceans and atmosphere. The nature and magnitude of the earliest transitions, hundreds of million years before photosynthesis evolved, remain poorly understood. Using a novel ecosystem-planetary model, we find that pre-photosynthetic methane-cycling microbial ecosystems are much less productive than previously thought. In spite of their low productivity, the evolution of methanogenic metabolisms strongly modifies the atmospheric composition, leading to a warmer but less resilient climate. As the abiotic carbon cycle responds, further metabolic evolution (anaerobic methanotrophy) may feed back to the atmosphere and destabilize the climate, triggering a transient global glaciation. Although early metabolic evolution may cause strong climatic instability, a low CO:CH4 atmospheric ratio emerges as a robust signature of simple methane-cycling ecosystems on a globally reduced planet such as the late Hadean/early Archean Earth.

Sulfur Chemistry May Have Paved the Way for Evolution of Antioxidants

The first organisms on the young Earth, just 1-1.5 billion years old, were likely chemolithoautotrophic anaerobes, thriving in an anoxic world rich in water, CO₂, and N₂. It is generally assumed that, until the accumulation of O₂ in the atmosphere, life was exempted from the oxidative stress that reactive oxygen species (ROS) impose on hydrocarbon-based life. Therefore, it is perplexing to note that life on the early Earth already carried antioxidants such as superoxide dismutase enzymes, catalase, and peroxiredoxins, the function of which is to counteract all forms of ROS, including H₂O₂. Phylogenetic investigations suggest that the presence of these enzymes in the last universal common ancestor, far predating the great oxygenation event (GOE) sometime between 2.3 and 2.7 billion years ago, is thought to be due to the appearance of oxygen-producing microorganisms and the subsequent need to respond to the appearance of ROS. Since the metabolic enzymes that counteract ROS have been found in all domains of life, they are considered of primitive origin. Two questions arise: (1) Could there be a nonbiological source of ROS that predates the oxygenic microbial activity? (2) Could sulfur, the homologue of oxygen, have played that role? Reactive sulfur species (RSS) may have triggered the evolution of antioxidants such that the ROS antioxidants started out as "antisulfur" enzymes developed to cope with, and take advantage of, various forms of RSS that were abundantly present on the early Earth.

In Situ Fe and S isotope analyses in pyrite from the 3.2 Ga Mendon Formation (Barberton Greenstone Belt, South Africa): Evidence for early microbial iron reduction

On the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7-2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28- to 3.26-Gyr-old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ⁵⁶ Fe values of -1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass-independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ³³ S values > +3‰, while the other groups display more variable and closer to 0‰ Δ³³ S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ³³ S values in the pyrite core and similar trace element compositions. We therefore suggest that two of the pyrite groups have experienced late fluid circulations that have led to partial recrystallization and dilution of S isotope mass-independent signature but not modification of the Fe isotope record. Considering the mineralogy and geochemistry of the pyrites and associated organic material, we conclude that this iron isotope systematic derives from microbial respiration of iron oxides during early diagenesis. Our data extend the geological record of dissimilatory iron reduction (DIR) back more than 560 million years (Myr) and confirm that micro-organisms closely related to the last common ancestor had the ability to reduce Fe(III).