Explore the vast range of titles available.

Recent studies have shown that biosignatures of ancient microbial life exist in mineral coatings in deep bedrock fractures of Precambrian cratons, but such surveys have been few and far between. Here, we report results from southwestern Sweden in an area of 1.6–1.5 Ga Paleoproterozoic rocks heavily reworked by the 1.14–0.96 Ga Sveconorwegian orogeny, a terrane previously scarcely explored for ancient microbial biosignatures. Calcite‐pyrite‐adularia‐illite‐coated fractures were analyzed for stable isotopes via Secondary Ion Mass Spectrometry (δ13C, δ18O, δ34S) and in situ Rb/Sr geochronology via Laser‐ablation inductively coupled plasma mass spectrometry. The Rb/Sr ages for calcite‐adularia and calcite‐illite show that several fluid flow events can be discerned (797 ± 18–769 ± 7, 391 ± 5–387 ± 6, 356 ± 5–347 ± 4, and 301 ± 7 Ma). The δ13C, δ18O and 87Sr/86Sr values of different calcite growth zones further confirmed episodic fluid flow. Pyrite δ34S values down to −49.9‰V‐CDT, together with systematically increased δ34S from crystal core to rim, suggest formation following microbial sulfate reduction under semi‐closed conditions. Assemblages involving MSR‐related pyrite generally have Devonian to Permian Rb/Sr ages, indicating an association to extension‐related fracturing and fluid mixing during foreland‐basin formation linked to Caledonian orogeny in the northwest. An assemblage with an age of 301 ± 7 Ma is potentially related to Oslo Rift extension, whereas the Neo‐Proterozoic ages relate to post‐Sveconorwegian extensional tectonics. Remnants of short‐chained fatty acids in the youngest calcite coatings further indicate a biogenic origin, while the absence of organic molecules in older calcite is in line with thermal degradation, potentially related to heating during Caledonian foreland basin burial. Plain Language Summary This study investigates mineral coatings in Proterozoic basement fractures of Southwestern Sweden, within the Precambrian Fennoscandian shield, to gain insights into ancient microbial life and paleo‐fluid flow. Isotopic signatures of these mineral coatings suggest that microbial sulfate reducers have been present in the system as also indicated by preserved organic molecules. Microanalytical geochronology determinations reveal that the fracture system has been activated several times in the Neoproterozoic, Devonian‐Early Carboniferous, and Late Carboniferous/Early Permian. These activations are associated with extension events following the Sveconorwegian and Caledonian orogenies as well as formation of the Oslo Rift. The signs of microbial activity are related to the youngest of these events, post‐dating burial in the Caledonian foreland basin, when bedrock temperatures became habitable. Key Points Rb/Sr geochronology of calcite veins detects several generations of fracture activation and fluid flow in Paleoproterozoic basement rocks Substantial δ34S variation and preserved fatty acids suggest that microbial sulfate reduction has been active in the fracture system Microbial activity relates to Paleozoic fracture activation in relation to Caledonian orogeny/foreland basin and later extensional events

The exergonic reaction of FeS with H 2 S to form FeS 2 (pyrite) and H 2 was postulated to have operated as an early form of energy metabolism on primordial Earth. Since the Archean, sedimentary pyrite formation has played a major role in the global iron and sulfur cycles, with direct impact on the redox chemistry of the atmosphere. However, the mechanism of sedimentary pyrite formation is still being debated. We present microbial enrichment cultures which grew with FeS, H 2 S, and CO 2 as their sole substrates to produce FeS 2 and CH 4 . Cultures grew over periods of 3 to 8 mo to cell densities of up to 2 to 9 × 10 6 cells per mL −1 . Transformation of FeS with H 2 S to FeS 2 was followed by 57 Fe Mössbauer spectroscopy and showed a clear biological temperature profile with maximum activity at 28 °C and decreasing activities toward 4 °C and 60 °C. CH 4 was formed concomitantly with FeS 2 and exhibited the same temperature dependence. Addition of either penicillin or 2-bromoethanesulfonate inhibited both FeS 2 and CH 4 production, indicating a coupling of overall pyrite formation to methanogenesis. This hypothesis was supported by a 16S rRNA gene-based phylogenetic analysis, which identified at least one archaeal and five bacterial species. The archaeon was closely related to the hydrogenotrophic methanogen Methanospirillum stamsii , while the bacteria were most closely related to sulfate-reducing Deltaproteobacteria, as well as uncultured Firmicutes and Actinobacteria. Our results show that pyrite formation can be mediated at ambient temperature through a microbially catalyzed redox process, which may serve as a model for a postulated primordial iron−sulfur world.

The global geographic distribution of subseafloor sedimentary microbes and the cause(s) of that distribution are largely unexplored. Here, we show that total microbial cell abundance in subseafloor sediment varies between sites by ca. five orders of magnitude. This variation is strongly correlated with mean sedimentation rate and distance from land. Based on these correlations, we estimate global subseafloor sedimentary microbial abundance to be 2.9⋅10 ²⁹ cells [corresponding to 4.1 petagram (Pg) C and ∼0.6% of Earth’s total living biomass]. This estimate of subseafloor sedimentary microbial abundance is roughly equal to previous estimates of total microbial abundance in seawater and total microbial abundance in soil. It is much lower than previous estimates of subseafloor sedimentary microbial abundance. In consequence, we estimate Earth’s total number of microbes and total living biomass to be, respectively, 50–78% and 10–45% lower than previous estimates.

The Lost City hydrothermal field is a dramatic example of the biological potential of serpentinization. Microbial life is prevalent throughout the Lost City chimneys, powered by the hydrogen gas and organic molecules produced by serpentinization and its associated geochemical reactions. Microbial life in the serpentinite subsurface below the Lost City chimneys, however, is unlikely to be as dense or active. The marine serpentinite subsurface poses serious challenges for microbial activity, including low porosities, the combination of stressors of elevated temperature, high pH and a lack of bioavailable ∑CO 2 . A better understanding of the biological opportunities and challenges in serpentinizing systems would provide important insights into the total habitable volume of Earth's crust and for the potential of the origin and persistence of life in Earth's subsurface environments. Furthermore, the limitations to life in serpentinizing subsurface environments on Earth have significant implications for the habitability of subsurface environments on ocean worlds such as Europa and Enceladus. Here, we review the requirements and limitations of life in serpentinizing systems, informed by our research at the Lost City and the underwater mountain on which it resides, the Atlantis Massif. This article is part of a discussion meeting issue ‘Serpentinite in the Earth System’.

No other environment hosts as many microbial cells as the marine sedimentary biosphere. While the majority of these cells are expected to be alive, they are speculated to be persisting in a state of maintenance without net growth due to extreme starvation. Here, we report evidence for in situ growth of anaerobic ammonium-oxidizing (anammox) bacteria in ∼80,000-y-old subsurface sediments from the Arctic Mid-Ocean Ridge. The growth is confined to the nitrate–ammonium transition zone (NATZ), a widespread geochemical transition zone where most of the upward ammonium flux from deep anoxic sediments is being consumed. In this zone the anammox bacteria abundances, assessed by quantification of marker genes, consistently displayed a four order of magnitude increase relative to adjacent layers in four cores. This subsurface cell increase coincides with a markedly higher power supply driven mainly by intensified anammox reaction rates, thereby providing a quantitative link between microbial proliferation and energy availability. The reconstructed draft genome of the dominant anammox bacterium showed an index of replication (iRep) of 1.32, suggesting that 32% of this population was actively replicating. The genome belongs to a Scalindua species which we name Candidatus Scalindua sediminis, so far exclusively found in marine sediments. It has the capacity to utilize urea and cyanate and a mixotrophic lifestyle. Our results demonstrate that specific microbial groups are not only able to survive unfavorable conditions over geological timescales, but can proliferate in situ when encountering ideal conditions with significant consequences for biogeochemical nitrogen cycling.

Low‐temperature hydrothermal fluids in crustal rocks of the Clarion‐Clipperton‐Zone (East Pacific) supply dissolved oxygen into the sediment from below. Diffusive upward transport led to formation of an inverse oxygen gradient zone in the overlying sediments. The resultant oxic/suboxic transition zone could provide suitable conditions for a deep, mirrored habitat for microaerophilic magnetotactic bacteria that were so far only found in the shallow oxygen gradient zone beneath the sediment‐water interface. Previously, the presence of such deep‐living MTB was only inferred from paleo‐ and rock‐magnetic proxies, but here it is evidenced by electron microscopy showing intact multi‐stranded, large (>120 nm diameter) magnetofossil chains from the deep past oxic/suboxic transition zone. Sediment magnetic parameters indicate localized zones of biogenic magnetite accumulation affirming the existence of living MTB at around 8 m sediment depth. This finding is the first evidence for MTB in bottom‐up oxygenated sediments near the sediment/crust interface. Plain Language Summary Biogeochemical processes in sediments at the ocean floor consume oxygen dissolved in pore waters, resulting in anoxic conditions in deeper sediment layers. However, in young, tectonically active parts as in the eastern Pacific oceanic crust, seawater can enter and flow through the permeable rocks that form the crystalline base of the seafloor and thereby supply oxygen into the overlying sediment column from below. This process was shown to create an oxic zone within deep sediments that may be populated by so‐called “magnetotactic” bacteria, which require small amounts of oxygen and therefore usually live just beneath the sediment surface. It was not thought possible so far that such mobile bacteria could also live in deeper subsurface sediments. These bacteria produce magnetic particles, which they use as a compass. Known as “magnetofossils,” these tiny particles can be preserved in marine sediments for millions of years. We could detect fresh “magnetofossils” in very old deep sediments with specialized microscopes and magnetic measurements. These deep‐living bacteria thus inhabit a “mirror‐world,” where their necessary oxygen supply comes from below, not from above. Their compass should therefore be oriented in the opposite vertical direction to their shallow‐living analogs. Key Points Fresh magnetosome chains of magnetotactic bacteria were detected in deep oxygen‐gradient zone near (<15 m) the sediment/crust interface Selective magnetic parameters reveal microbial activity in past oxic/suboxic transition zones of bottom‐up oxygenated sediments Oxygen‐rich low‐temperature hydrothermal fluids in young oceanic crust can support microaerophilic life in the overlying bottom sediment

The Earth’s deep biosphere hosts some of its most ancient chemolithotrophic lineages. The history of habitation in this environment is thus of interest for understanding the origin and evolution of life. The oldest rocks on Earth, formed about 4 billion years ago, are in continental cratons that have experienced complex histories due to burial and exhumation. Isolated fracture-hosted fluids in these cratons may have residence times older than a billion years, but understanding the history of their microbial communities requires assessing the evolution of habitable conditions. Here, we present a thermochronological perspective on the habitability of Precambrian cratons through time. We show that rocks now in the upper few kilometers of cratons have been uninhabitable (>∼122 °C) for most of their lifetime or have experienced high-temperature episodes, such that the longest record of habitability does not stretch much beyond a billion years. In several cratons, habitable conditions date back only 50 to 300 million years, in agreement with dated biosignatures. The thermochronologic approach outlined here provides context for prospecting and interpreting the little-explored geologic record of the deep biosphere of Earth’s cratons, when and where microbial communities may have thrived, and candidate areas for the oldest records of chemolithotrophic microbes.

Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts expelled from mud volcanoes above the Izu–Bonin–Mariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni–Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122 °C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ∼10,000 m below the seafloor. This is deeper than the 122 °C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, Mid-Atlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth’s largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth’s history such as the late heavy bombardment and global mass extinctions.

Bathyarchaeia, among the most ancient and abundant microbial lineages on Earth, dominate diverse anoxic subsurface ecosystems and play a pivotal role in global carbon cycling. This review synthesizes current understanding of their physiological, metabolic, and evolutionary foundations underlying their ecological significance and environmental effects over geological timescales. Despite their global distribution in the deep biosphere, the phylogenetic diversity and total cellular abundance of Bathyarchaeia remain substantially underestimated. As uncultivated metabolic generalists, Bathyarchaeia exhibit remarkable metabolic versatility, including anaerobic organic degradation, dark carbon fixation, and putative methane and alkane metabolism. Specifically, genus Baizosediminiarchaeum has been demonstrated to adopt organomixotrophy by coupling anaerobic lignin degradation with inorganic carbon assimilation. These metabolic strategies likely enable them to thrive in energy-limited subsurface environments with dynamic geochemical fluctuations. The early evolutionary history of Bathyarchaeia appears closely linked to major geological events, including tectonic activity and plant evolution, whereas more recent lineage expansions reflect physiological adaptations to host-associated and anthropogenically influenced environments, highlighting their ongoing co-evolution with Earth’s modern environments. Overall, we propose carbon metabolic innovation as the central driver behind the ecological and evolutionary significance of Bathyarchaeia, putatively linking microbial ecological functions to planetary biogeochemical processes. Future efforts in isolation and cultivation remain essential for elucidating their unknown physiological and metabolic mechanisms. In parallel, advances in ecological modeling and the development of lineage-specific lipid biomarkers hold great promise for quantifying their contributions to global carbon budgets and reconstructing paleoenvironmental and paleoclimate conditions.

The deep biosphere is one of the least understood ecosystems on Earth. Although most microbiological studies in this system have focused on prokaryotes and neglected microeukaryotes, recent discoveries have revealed existence of fossil and active fungi in marine sediments and sub-seafloor basalts, with proposed importance for the subsurface energy cycle. However, studies of fungi in deep continental crystalline rocks are surprisingly few. Consequently, the characteristics and processes of fungi and fungus-prokaryote interactions in this vast environment remain enigmatic. Here we report the first findings of partly organically preserved and partly mineralized fungi at great depth in fractured crystalline rock (−740 m). Based on environmental parameters and mineralogy the fungi are interpreted as anaerobic. Synchrotron-based techniques and stable isotope microanalysis confirm a coupling between the fungi and sulfate reducing bacteria. The cryptoendolithic fungi have significantly weathered neighboring zeolite crystals and thus have implications for storage of toxic wastes using zeolite barriers. Deep subsurface microorganisms play an important role in nutrient cycling, yet little is known about deep continental fungal communities. Here, the authors show organically preserved and partly mineralized fungi at 740 m depth, and find evidence of an anaerobic fungi and sulfate reducing bacteria consortium.