Early Earth Environments Research Articles

Interactions of Clay Minerals with Biomolecules and Protocells Complex Structures in the Origin of Life: A Review

AbstractThe origin of life (OoL) has always been a mysterious and challenging topic that puzzles human beings. Clay minerals have unique properties and wide distribution in early Earth environments. They can not only adsorb biological small molecules to catalyze their polymerization, but play an active role in the formation and evolution of protocells. In this review, the research progress on the interactions of clay minerals with biomolecules and protocells complex structures in the field of the OoL based on chemical evolution theory is summarized. The types, structures and properties of clay minerals, biological molecules and protocell models related to the OoL are introduced in detail. The mechanism of interaction between clay minerals and biological molecules, the construction of protocells and the role of clay minerals in the formation, structure and stability of protocells are systematically described. Finally, the future research priorities and challenges in the field of OoL based on clay minerals, biomolecules and protocells are discussed. It is aspired that this review can further advance the exploration of the OoL from a new perspective, and can also bring some interesting findings and ideas to the interdisciplinary research of materials, biology, chemistry and other related disciplines.Clay minerals have a variety of interactions with small biomolecules, which can be used as structural and functional templates to promote the organic synthesis of biomolecules and the formation and evolution of protocells, playing a non‐negligible role in the field of the OoL.

Non-thermodynamic factors affect competition between thermophilic chemolithoautotrophs from deep-sea hydrothermal vents.

The deep subsurface is the largest reservoir of microbial biomass on Earth and serves as an analog for life on the early Earth and extraterrestrial environments. Methanogenesis and sulfur reduction are among the more common chemolithoautotrophic metabolisms found in hot anoxic hydrothermal vent environments. Competition between H2-oxidizing sulfur reducers and methanogens is primarily driven by the thermodynamic favorability of redox reactions with the former outcompeting methanogens. This study demonstrated that competition between the hydrothermal vent chemolithoautotrophs Methanocaldococcus jannaschii, Methanothermococcus thermolithotrophicus, and Desulfurobacterium thermolithotrophum is also influenced by other overlapping factors such as staggered optimal growth temperatures, stochasticity, and hydrology. By modeling all aspects of microbial competition coupled with field data, a better understanding is gained on how methanogens can outcompete thiosulfate reducers in hot anoxic environments and how the deep subsurface contributes to biogeochemical cycling.

The Rare Earth Element Distribution in Marine Carbonates as a Potential Proxy for Seawater pH on Early Earth

Understanding the marine environment of early Earth is crucial for understanding the evolution of climate and early life. However, the master variable of Archean and Proterozoic seawater, the pH, is poorly constrained, and published ideas about the pH range encompass ~7 pH units from mildly acidic to hyperalkaline. To better infer ancient seawater pH, we examine the possibility of a seawater pH proxy using rare earth elements (REEs) in marine carbonates. The principle is based on increasing concentrations of heavy rare earth elements in solution relative to the light REEs with decreasing pH due to REE complexation and scavenging. We calibrated such an REE pH proxy using pH variability in modern seawater and tested the proxy with ~100 REE measurements from 13 separate carbonate formations. We compared our pH estimates derived from the REE proxy to published pH estimates of Cenozoic and Neoproterozoic seawater that use the established pH proxy of boron isotopes (δ11B). REE-pH estimates agree with the Cenozoic and the Ediacaran δ11B-pH proxy based on the type of carbonate and boron isotopic composition at corresponding times. The uncertainty in our REE-pH proxy can probably be explained by model assumptions, noise from freshwater influence, siliciclastic input, and diagenesis. This proof-of-concept study demonstrates that the REE-pH method provides pH estimates comparable to boron isotope pH estimates within uncertainties, which potentially could constrain changes in Precambrian seawater pH to better understand the coevolution of life and early Earth’s environment.

Slow Freeze‐Thaw Cycles Enhanced Hybridization of Kilobase DNA with Long Complementary Sticky Ends

AbstractThe creation of large information molecules may have played an essential role in the origins of life. In this study, we conducted slow freeze‐thaw (F/T) experiments to test the possibility of enhanced hybridization between the complementary sticky ends attached to kilobase‐sized DNA fragments at sub‐nanomolar concentrations. DNA fragments of 2‐ and 3‐kilobase pairs (kbp) with 50‐base complementary sticky ends that can form 5 kbp‐sized hybridization products were mixed. While simple incubation provided little hybridization product, significantly effective hybridization was observed after freezing and thawing at a controlled time rate (<0.3 K min−1), even with small DNA concentrations (<1 nM). Furthermore, slow thawing had a more effect on hybridization than slow freezing. The reaction efficiency was reduced by rapid thawing instead of slow thawing, suggesting that the eutectic phase concentration played an important role in hybridization. A slow F/T cycle was effective even for the hybridization reaction between two 10 kbp DNA fragments, which yielded a 20 kbp product at sub‐nanomolar concentrations. Repeating the slow F/T cycle significantly improved the reaction efficiency. The possible role of the F/T cycles in early Earth environments is discussed here.

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.

Prebiotic membrane structures mimic the morphology of alleged early traces of life on Earth

Elucidating compositions of the first cell membranes requires experiments with molecules and chemical conditions representative of early Earth. The molecules used are described as ‘prebiotically plausible’, i.e., they could have formed through abiotic reactions before the emergence of biology. Similarly, the chemical properties of solutions in which these membranes are formed (e.g., pH, temperature, ionic strength) must represent early Earth environments. Here, using confocal and transmission electron microscopy combined with population morphometry, we show that prebiotically plausible molecules, in solutions representative of Hadean submarine alkaline hydrothermal vents, form microstructures with substantial morphological diversity. The microstructures hold the potential for use as analogues of prebiotic processes in the rock record. Additionally, many of the structures are morphologically similar to purported early microfossils, highlighting limitations of morphological interpretation in these studies. Detailed analyses of abiotic microstructures are essential for understanding the earliest life on Earth, and for interpretation of potential biosignatures from extra-terrestrial bodies.

The Precambrian continental record: A window into early Earth environments

The Precambrian was characterized by unique palaeoenvironmental conditions in the Earth’s atmosphere, biosphere and geosphere. This study presents a global quantitative analysis of Precambrian sedimentary successions of aeolian, alluvial, fluvial, lacustrine and glacigenic origins, examined in the broader context of Earth evolution. In the Precambrian, an apparent scarcity of aeolian successions is observed. This may be linked to: (1) differences in atmospheric density, which controlled wind erosion and sedimentation; (2) different astronomical configurations, which may have influenced tides and atmospheric circulation, thereby affecting sand availability and the width of subtropical zones; (3) potentially hotter and more humid climates, restricting dry-sand availability; (4) a lack of vascular vegetation that could prevent reworking of aeolian deposits; (5) poor preservation potential; (6) misinterpretation of the Precambrian record. Mixed aeolian-alluvial strata are more abundant, perhaps because their preservation in the geological record was favoured by water tables sustained by incursions of alluvial systems into otherwise aeolian dominated environments. Aeolian deposits were preferentially accumulated during phases of supercontinental breakup, where rapidly subsiding rift basins provided accommodation suitable for preservation. Other than in the Neoproterozoic record, where glacigenic deposits dominate, alluvial strata are the most common and thickest type of continental deposit in the Precambrian. Precambrian braided alluvial systems were more widespread than in the Phanerozoic. Major alluvial systems formed preferentially during phases of supercontinent assembly, whereby alluvial systems drained major orogens, and long drainage pathways developed from supercontinent interiors to coastlines. In the Paleoproterozoic, ephemeral, saline to partly arid lakes developed extensively in the desertic interior of Columbia. Glacial deposits preferentially formed in the breakup phase of supercontinental cycles; this supports theories invoking enhanced chemical weathering of uplifted rift shoulders as a driver of carbon dioxide sequestration, global cooling, and glaciation. Overall, the number of identified continental successions increases towards the Precambrian-Phanerozoic boundary. This may be an artefact of an increasingly more complete stratigraphic record as time progresses. However, the abundance of continental successions varies on a quasi-periodic cycle of 500 – 700 Myr, with peaks coinciding with the tenure and breakup of Precambrian supercontinents.

Olivine-catalyzed glycolaldehyde and sugar synthesis under aqueous conditions: Application to prebiotic chemistry

The presence of minerals in the prebiotic environment likely shaped the evolution of organic matter, thereby contributing to the emergence of prebiotic systems. Records of such systems are lacking and the interactions between abiotic organic matter and primary minerals remain poorly understood. Here, we demonstrate the ability of olivine silicates, in simulated early Earth or planetary aqueous environments, to catalyse glycolaldehyde formation from only formaldehyde, and help producing sugars that are essential components for life, through the formose reaction. By combining comprehensive gas chromatography analyses on experimental samples with quantum chemical simulations, we provide a mechanism for an olivine-catalyzed glycolaldehyde formation. Our findings suggest that olivine plays a triple role in the formose chemical network: maintaining an alkaline pH, enabling the initiation step towards the formation of glycoladehyde (which is typically the most challenging step) and promoting the autocatalytic cycle. These results open-up new scenarios on the impact of primary minerals on the evolution of chemical pathways in aqueous environments that were probably essential for the emergence of the first biomolecules.

Taxonomic and carbon metabolic diversification of Bathyarchaeia during its coevolution history with early Earth surface environment

Bathyarchaeia, as one of the most abundant microorganisms on Earth, play vital roles in the global carbon cycle. However, our understanding of their origin, evolution, and ecological functions remains poorly constrained. Here, we present the largest dataset of Bathyarchaeia metagenome assembled genome to date and reclassify Bathyarchaeia into eight order-level units corresponding to the former subgroup system. Highly diversified and versatile carbon metabolisms were found among different orders, particularly atypical C1 metabolic pathways, indicating that Bathyarchaeia represent overlooked important methylotrophs. Molecular dating results indicate that Bathyarchaeia diverged at ~3.3 billion years, followed by three major diversifications at ~3.0, ~2.5, and ~1.8 to 1.7 billion years, likely driven by continental emergence, growth, and intensive submarine volcanism, respectively. The lignin-degrading Bathyarchaeia clade emerged at ~300 million years perhaps contributed to the sharply decreased carbon sequestration rate during the Late Carboniferous period. The evolutionary history of Bathyarchaeia potentially has been shaped by geological forces, which, in turn, affected Earth’s surface environment.

Metal Catalysis Acting on Nitriles in Early Earth Hydrothermal Systems.

Hydrothermal systems are areas in which heated fluids and organic molecules rush through basaltic material rich in metals and minerals. By studying malononitrile and acetonitrile, we examine the effects of metal and mineral nanoparticles on nitrile compounds in anoxic, hydrothermal conditions representing a prebiotic environment of early Earth. Polymerization, reduction, cyclization, and a phenomenon colloquially known as 'chemical gardening' (structure building via reprecipitation of metal compounds or complexing with organics) are all potential outcomes with the addition of metals and minerals. Reduction occurs with the addition of rhodium (Rh) or iron (II) sulfide (FeS), with positive identification of ethanol and ethylamine forming from acetonitrile reduction. We find that polymerization and insoluble product formation were associated with oxide minerals, metallic nickel (Ni), and metallic cobalt (Co) acting as catalysts. Oxide minerals strongly promoted polymerization into insoluble, tar-like products of nitriles. FeS, iron-nickel alloy (FeNi), and rhodium are unique cases that appear to act as reagents by actively participating in chemical gardening without returning to their initial state. Further, FeS tentatively had a phase change into the mineral parabutlerite. This research aims to identify metals and metal minerals that could best serve nitrile catalysis and reactions on early Earth.

Fungal-induced fossil biomineralization

Exceptional preservation of fossils has often been attributed to the actions of bacteria that aid in the preservation of soft tissues that normally decay rapidly. However, it is well known that fungi play a major role in organic matter decomposition, biogeochemical cycling of elements, and metal-mineral transformations in modern ecosystems. Although the fungal fossil record can be traced back over a billion years, there are only a few recorded examples of fungal roles in fossilization. In this research, we have carried out a detailed geobiological investigation on early Pleistocene hyena coprolites (fossilized dung) in an attempt to ascertain possible fungal involvement in their formation. Using an advanced microscopic and mineralogical approach, we found that numerous hydroxyapatite nanofibers (25-34nm on average), interwoven to form spheroidal structures, constituted the matrix of the coprolites in addition to food remains. These structures were found to be extremely similar in texture and mineral composition to biominerals produced during laboratory culture of a common saprophytic and geoactive fungus, Aspergillus niger, in the presence of a solid source of calcium (Ca) and phosphorus (P). This observation, and our other data obtained, strongly suggests that fungal metabolism can provide a mechanism that can result in fossil biomineralization, and we hypothesize, therefore, that this may have contributed to the formation of well-preserved fossils (Lagerstätten) in the geological record. The characteristic polycrystalline nanofibers may also have served as a potential biosignature for fungal life in early Earth and extraterrestrial environments.

Pb isotopes reveal intracrustal exhalative hydrothermalism during deposition of the ∼3.43 Ga Strelley Pool Formation

Hydrothermal systems are widely considered to have supported development of Earth's early microbial ecosystems by providing energy and nutrients to surface and subsurface environments. We studied a remarkably extensive and well-preserved hydrothermal dike complex active during deposition of the ∼3.43 Ga Strelley Pool Formation (Warrawoona Group, Pilbara Region, Western Australia). We report field observations, mineralogy, Pb isotope ratios, and selected trace metal concentrations for black, silica-rich dikes at the study site. The majority of the samples fall into two distinct arrays in the common Pb isotope diagram. One group of samples, with many very unradiogenic isotope ratios, defines a slope whose apparent age is roughly coeval with deposition of the Strelley Pool Formation. The other group, containing samples with elevated 206Pb/204Pb ratios and generally low 208Pb/204Pb ratios, has a moderately well-defined slope of apparent Neoarchean age. The original Pb isotope ratios of this latter group were likely disturbed by a combination of Pb isotope memory resetting, Pb-loss, and U-addition. Using the trace metal concentrations of samples associated with Paleoarchean Pb isotope memory, we identify a crustal rather than a mantle source for metals carried in the hydrothermal system. An intracrustal (as opposed to an oceanic ridge) setting is also consistent with local and regional geology. Combined 208/207/206/204Pb systematics of the dike complex reveal a striking similarity to unconformable Pb found in older Pilbara Craton sediment-hosted metal deposits of the Doolena Gap Formation, Apex Basalt, Coppin Gap, and the Dresser Formation. The unusual Pb isotope systematics, plotting far above conformable Pb isotope evolution curves, is currently poorly understood. We discuss the use of Pb isotopes to constrain the timing of geological events including the age of initial deposition, and the loss and resetting of original geochemical signatures. We propose that improving our understanding of early Earth environments influenced by hydrothermalism requires combining geochemical fingerprints with proxies, such as Pb isotopes, which are very sensitive to potential contribution from crustal sources.

Goldilocks and RNA: where Mg2+ concentration is just right.

Magnesium, the most abundant divalent cation in cells, catalyzes RNA cleavage but also promotes RNA folding. Because folding can protect RNA from cleavage, we predicted a 'Goldilocks landscape', with local maximum in RNA lifetime at Mg2+ concentrations required for folding. Here, we use simulation and experiment to discover an innate and sophisticated mechanism of control of RNA lifetime. By simulation we characterized RNA Goldilocks landscapes and their dependence on cleavage and folding parameters. Experiments with yeast tRNAPhe and the Tetrahymena ribozyme P4-P6 domain show that structured RNAs can inhabit Goldilocks peaks. The Goldilocks peaks are tunable by differences in folded and unfolded cleavage rate constants, Mg2+ binding cooperativity, and Mg2+ affinity. Different folding and cleavage parameters produce Goldilocks landscapes with a variety of features. Goldilocks behavior allows ultrafine control of RNA chemical lifetime, whereas non-folding RNAs do not display Goldilocks peaks of protection. In sum, the effects of Mg2+ on RNA persistence are expected to be pleomorphic, both protecting and degrading RNA. In evolutionary context, Goldilocks behavior may have been a selectable trait of RNA in an early Earth environment containing Mg2+ and other metals.

Oxygen Effect on the Ultraviolet-C Photochemistry of Lactic Acid.

Lactic acid, a small α-hydroxyacid, is ubiquitous in both indoor and outdoor environments. Recently, the photochemistry of lactic acid has garnered interest among the abiotic organic chemistry community as it would have been present in abiotic settings and photoactive with the high-energy solar radiation that would have been available in the low oxygen early Earth environment. Additionally, we propose that the photochemistry of lactic acid is relevant to modern Earth during indoor ultraviolet-C (UVC) sterilization procedures as lactic acid is emitted by humans and is thus prevalent in indoor environments where UVC sterilization is increasingly being used. Here, we study the oxygen effect on the gas phase photolysis of lactic acid using Fourier-transform infrared (FTIR) spectroscopy and isotopically labeled oxygen (18O2). We find that the major products of gas phase lactic acid photolysis are CO2, CO, acetaldehyde, and acetic acid. Furthermore, these products are the same with or without added oxygen, but the partial pressures of produced CO2, CO, and acetaldehyde increase with the amount of added oxygen. Notably, the added oxygen is primarily incorporated into produced CO2 and CO, while little or none is incorporated into acetaldehyde. We combine the results presented here with those in the literature to propose a mechanism for the gas phase photolysis of lactic acid and the role of oxygen in this mechanism. Finally, we compare the output of a krypton-chloride excimer lamp (λ = 222 nm), one of the lamps proposed for UVC sterilization procedures, to the absorption of lactic acid. We show that lactic acid would be photoactive during UVC sterilization procedures, and we use the gas phase results presented here and aqueous lactic acid photolysis results previously published to assess potential byproducts from lactic acid reactions during UVC sterilization procedures.

Frontiers in Prebiotic Chemistry and Early Earth Environments.

The Prebiotic Chemistry and Early Earth Environments (PCE3) Consortium is a community of researchers seeking to understand the origins of life on Earth and in the universe. PCE3 is one of five Research Coordination Networks (RCNs) within NASA’s Astrobiology Program. Here we report on the inaugural PCE3 workshop, intended to cross-pollinate, transfer information, promote cooperation, break down disciplinary barriers, identify new directions, and foster collaborations. This workshop, entitled, “Building a New Foundation”, was designed to propagate current knowledge, identify possibilities for multidisciplinary collaboration, and ultimately define paths for future collaborations. Presentations addressed the likely conditions on early Earth in ways that could be incorporated into prebiotic chemistry experiments and conceptual models to improve their plausibility and accuracy. Additionally, the discussions that followed among workshop participants helped to identify within each subdiscipline particularly impactful new research directions. At its core, the foundational knowledge base presented in this workshop should underpin future workshops and enable collaborations that bridge the many disciplines that are part of PCE3.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11084-022-09622-x.

Changes in Magnetic Properties of Magnetite Nanoparticles Upon Microbial Iron Reduction

AbstractThe magnetic signals of magnetite nanoparticles (NPs) preserved in rocks, soils, and sediments are effective index for paleoenvironmental reconstruction. It has been demonstrated that magnetite NPs can serve as a terminal electron sink for the microbial respiration (i.e., microbial iron reduction). The magnetic properties of magnetite NPs may be altered by microbial iron reduction, which is a critical but often overlooked process in paleomagnetism. In this study, three magnetite NPs with different particle sizes were reduced by a dissimilatory iron‐reducing bacterium (Shewanella oneidensis MR‐1) under a non‐growth condition mimicking that of the early Earth and modern oligotrophic environment. The changes in magnetic, chemical as well as crystallographic properties of the magnetite NPs during the microbial reduction process were examined. Our results showed that the bioreduction rate of magnetite NPs was mainly controlled by their particle size and redox state. In addition, the microbial iron reduction could affect both the crystallographic and magnetic properties of three types of magnetite NPs used herein. After bioreduction, the crystal lattice parameters and magnetic susceptibility of the magnetite NPs increased, while their remanence recording capability and coercivity decreased (i.e., “softer” magnetism). Furthermore, bioreduced magnetite NPs had a larger remanence loss near the Verwey transition region with low‐temperature magnetic analysis. These results indicate that the microbial reduction of magnetite NPs deserves attention when sedimentary magnetites are used in paleoenvironment reconstruction.

Identifying Impact Melt from the Smythii Basin: Toward an Improved Chronology for Lunar Basin Formation

Abstract At c. 820 km in diameter, the Smythii impact basin is one of the large lunar basins (>200 km diameter) thought to have formed during the pre-Nectarian period. We combine Lunar Reconnaissance Orbiter imagery, topography, and Moon Mineralogy Mapper compositional data to interpret the surface and subsurface geology of the Smythii basin with the goal of identifying datable impact melt for investigation by a future lunar lander. Surface outcrops exposed on the central peak of the Schubert C crater are identified as uplifted deposits of Smythii impact melt, and a mission concept is presented for sampling these exposures in order to establish the absolute age of the Smythii basin using radioisotopic geochronology. This mission concept is in line with one of the current top-tier priorities for lunar science: determining the age of large basins and thus constraining the impact flux during the Moon's first billion years, which is a proxy record for the role of impacts on the surface environment and habitability of early Earth and the inner solar system during this interval.

Planetary Minerals Catalyze Conversion of a Polycyclic Aromatic Hydrocarbon to a Prebiotic Quinone: Implications for Origins of Life.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in astrochemical environments and are disbursed into planetary environments via meteorites and extraterrestrial infall where they may interact with mineral phases to produce quinones important for origins of life. In this study, we assessed the potential of the phyllosilicates montmorillonite (MONT) and kaolinite (KAO), and the enhanced Mojave Mars Simulant (MMS) to convert the PAH anthracene (ANTH) to the biologically important 9,10-anthraquinone (ANTHQ). All studied mineral substrates mediate conversion over the temperature range assessed (25-500°C). Apparent rate curves for conversion were sigmoidal for MONT and KAO, but quadratic for MMS. Conversion efficiency maxima for ANTHQ were 3.06% ± 0.42%, 1.15% ± 0.13%, and 0.56% ± 0.039% for MONT, KAO, and MMS, respectively. We hypothesized that differential substrate binding and compound loss account for the apparent conversion kinetics observed. Apparent loss rate curves for ANTH and ANTHQ were exponential for all substrates, suggesting a pathway for wide distribution of both compounds in warmer prebiotic environments. These findings improve upon our previously reported ANTHQ conversion efficiency on MONT and provide support for a plausible scenario in which PAH-mineral interactions could have produced prebiotically relevant quinones in early Earth environments.

Phot0, a plausible primeval pigment on Earth and rocky exoplanets

In view of the existing controversy around the origin of the photosynthesis and, therefore, the first photosynthetic pigments, our work focuses on the theoretical study of a hypothetical first pigment, simpler than those existing today, that collects energy from solar radiation on Earth-like exoplanets. Our theoretical results show that there could exist geochemical conditions that allow the abiotic formation of a primeval pigment that might become sufficiently abundant in the early stages of habitable rocky exoplanets. These conditions would place this pigment before the appearance of life in a very young planet, thanks to chemical routes instead of biochemical transformations. Thus, our results may refute the currently accepted hypothesis that the complex biomolecules that allowed the photosynthesis to be carried out were synthesized through complex and evolved metabolic pathways. In addition, we show that the proposed primeval pigment, which we call Phot0, is also a precursor of the more evolved pigments known today on Earth and demonstrate, for the first time, an abiotic chemical route leading to tetrapyrroles not involving pyrrole derivatives. Our proposal places simple and very abundant raw materials in never-before-proposed geochemical conditions that lead to the formation of biomolecules of biological interest.

Sedimentary pyrite sulfur isotope compositions preserve signatures of the surface microbial mat environment in sediments underlying low-oxygen cyanobacterial mats.

The sedimentary pyrite sulfur isotope (δ34 S) record is an archive of ancient microbial sulfur cycling and environmental conditions. Interpretations of pyrite δ34 S signatures in sediments deposited in microbial mat ecosystems are based on studies of modern microbial mat porewater sulfide δ34 S geochemistry. Pyrite δ34 S values often capture δ34 S signatures of porewater sulfide at the location of pyrite formation. However, microbial mats are dynamic environments in which biogeochemical cycling shifts vertically on diurnal cycles. Therefore, there is a need to study how the location of pyrite formation impacts pyrite δ34 S patterns in these dynamic systems. Here, we present diurnal porewater sulfide δ34 S trends and δ34 S values of pyrite and iron monosulfides from Middle Island Sinkhole, Lake Huron. The sediment-water interface of this sinkhole hosts a low-oxygen cyanobacterial mat ecosystem, which serves as a useful location to explore preservation of sedimentary pyrite δ34 S signatures in early Earth environments. Porewater sulfide δ34 S values vary by up to ~25‰ throughout the day due to light-driven changes in surface microbial community activity that propagate downwards, affecting porewater geochemistry as deep as 7.5cm in the sediment. Progressive consumption of the sulfate reservoir drives δ34 S variability, instead of variations in average cell-specific sulfate reduction rates and/or sulfide oxidation at different depths in the sediment. The δ34 S values of pyrite are similar to porewater sulfide δ34 S values near the mat surface. We suggest that oxidative sulfur cycling and other microbial activity promote pyrite formation in and immediately adjacent to the microbial mat and that iron geochemistry limits further pyrite formation with depth in the sediment. These results imply that primary δ34 S signatures of pyrite deposited in organic-rich, iron-poor microbial mat environments capture information about microbial sulfur cycling and environmental conditions at the mat surface and are only minimally affected by deeper sedimentary processes during early diagenesis.