Microbial biosignatures in ancient deep‐sea hydrothermal sulfides

Deep‐sea hydrothermal systems provide ideal conditions for prebiotic reactions and ancient metabolic pathways and, therefore, might have played a pivotal role in the emergence of life. To understand this role better, it is paramount to examine fundamental interactions between hydrothermal processes, non‐living matter, and microbial life in deep time. However, the distribution and diversity of microbial communities in ancient deep‐sea hydrothermal systems are still poorly constrained, so evolutionary, and ecological relationships remain unclear. One important reason is an insufficient understanding of the formation of diagnostic microbial biosignatures in such settings and their preservation through geological time. This contribution centers around microbial biosignatures in Precambrian deep‐sea hydrothermal sulfide deposits. Intending to provide a valuable resource for scientists from across the natural sciences whose research is concerned with the origins of life, we first introduce different types of biosignatures that can be preserved over geological timescales (rock fabrics and textures, microfossils, mineral precipitates, carbonaceous matter, trace metal, and isotope geochemical signatures). We then review selected reports of biosignatures from Precambrian deep‐sea hydrothermal sulfide deposits and discuss their geobiological significance. Our survey highlights that Precambrian hydrothermal sulfide deposits potentially encode valuable information on environmental conditions, the presence and nature of microbial life, and the complex interactions between fluids, micro‐organisms, and minerals. It further emphasizes that the geobiological interpretation of these records is challenging and requires the concerted application of analytical and experimental methods from various fields, including geology, mineralogy, geochemistry, and microbiology. Well‐orchestrated multidisciplinary studies allow us to understand the formation and preservation of microbial biosignatures in deep‐sea hydrothermal sulfide systems and thus help unravel the fundamental geobiology of ancient settings. This, in turn, is critical for reconstructing life's emergence and early evolution on Earth and the search for life elsewhere in the universe.


| INTRODUC TI ON
Deep-sea hydrothermal systems provide unique insights into life thriving under extreme conditions by any human standards. Driven by energy from Earth's interior, hot fluids circulate in the ocean crust and locally emanate into cool marine environments. Even in the absence of sunlight, these springs can fuel diverse ecosystems, from chemoautotrophs (i.e., micro-organisms that fix inorganic carbon species such as CO 2 ) at the base to heterotrophic organisms such as tubeworms and bivalves at higher trophic levels. Metabolically diverse communities of chemosynthetic micro-organisms utilize redoxactive gases (e.g., H 2 S, CH 4 , H 2 ) and metals (e.g., Fe, Ni, Cu) delivered by hydrothermal fluids (Kelley et al., 2002). Micro-organisms are also ecologically diverse and adapted to highly different temperatures, ranging from psychrophiles and mesophiles that thrive at ambient seawater temperature (ca. 4-45°C) to hyperthermophiles that can tolerate temperatures up to 121°C (Kashefi & Lovley, 2003). The high metabolic and ecologic diversity in these systems is a consequence of steep spatial gradients and temporal variations in various environmental parameters, such as temperature, pH, availability and composition of minerals and organic substrates, and fluid chemistry (e.g., concentrations of metals), resulting in distinct ecological niches (O'Brien et al., 2015;Von Damm, 1995). Therefore, geobiological studies on hydrothermal systems require a combination of geological, geochemical, and microbiological approaches.
Geobiological studies on hydrothermal systems are vital for understanding the emergence and early evolution of life on our planet.
Hydrothermal vents were likely much more widespread on the early Earth due to a much higher heat flow from the mantle (Johnson et al., 2014;Russell et al., 2010). Some of these environments provided ideal conditions for the abiotic synthesis of organic molecules via Fischer-Tropsch-type (FTT) reactions linked to the serpentinization* of ultramafic* rocks. More specifically, these processes involve the hydrothermal reaction between minerals (olivine, pyroxene) and H 2 O, resulting in the formation of H 2 , which then may react with CO 2 from various sources to CH 4 and more complex hydrocarbons (Holm & Charlou, 2001;Konn et al., 2015;McCollom, 2013;McCollom et al., 1999;McCollom & Seewald, 2007;Mißbach et al., 2018;Proskurowski et al., 2008;Rushdi & Simoneit, 2001).
Metal sulfide minerals such as pyrite (FeS 2 ), sphalerite (ZnS), and chalcopyrite (CuFeS 2 ) are essential constituents of hydrothermal deposits, and their redox activity and reactive surfaces may have catalyzed the abiotic synthesis of organic matter under hydrothermal conditions (Huber & Wächtershäuser, 1997;Russell et al., 1994Russell et al., , 2010Wächtershäuser, 1990). Phylogenetic studies suggest that deep-branching (hyper) thermophilic micro-organisms similar to those found around modern hydrothermal vents appear to be the closest living relatives of LUCA* (Weiss et al., 2016). Similarly, the Asgard archaea-a group of micro-organisms proposed as the "missing evolutionary bridge" between prokaryotes and eukaryoteswere discovered in hydrothermal systems (MacLeod et al., 2019;Spang et al., 2015). Notably, hydrothermal systems may also exist in oceans of icy moons such as Enceladus, fueling the idea that life may also have emerged beyond Earth (Deamer & Damer, 2017).
To understand the significance of deep-sea hydrothermal systems in the emergence and early evolution of life, it is critical to examine their geobiology in deep time. One important reason is that models of potential prebiotic chemical evolution in hydrothermal environments must be consistent with conditions and processes in such systems on early Earth. Also, understanding fluid-microbemineral interactions in ancient hydrothermal settings is crucial to identifying metabolic pathways that might have played a vital role in the emergence of the earliest lifeforms. The only direct information on these interactions in our planet's past can be gleaned from the geological record.
The most important ancient equivalents of deep-sea hydrothermal systems are (i) volcanogenic massive sulfide deposits (VMS) and (ii) sedimentary exhalative massive sulfide ores (SEDEX). These deposits form in diverse deep-sea environments (i.e., below the photic zone), ranging from volcanic mid-ocean ridge, ocean-island, and (back-)arc settings to sediment-rich shelves, which accounts for profoundly different facies* (e.g., sulfidic chimney walls vs. sulfidemineralized shales). Geological evidence for such systems on Earth extends back to more than 3.2 billion years ago (Ga) (Hofmann, 2011;Rasmussen, 2000;Vearncombe et al., 1995). Still, little is known about microbial life in ancient hydrothermal sulfide systems, which is due to (i) the low preservation potential of deep-sea deposits covering oceanic crust, (ii) the decreasing abundance of preserved rocks with increasing geological time, (iii) the obliteration of potential biosignatures by destructive processes in the environment and during later stages in history, and (iv) the difficulty in distinguishing biogenic from abiotic features (Georgieva et al., 2021;Javaux, 2019;Lepot, 2020;Westall, 2005). Indeed, compared with other sedimentary settings, hydrothermal environments are characterized by strong chemical disequilibria, which commonly result in the syndepositional alteration of biogenic features and self-assembly of mineral textures* that can resemble biological fingerprints (Fowler & L'Heureux, 1996;Rouillard et al., 2018;Southam & Saunders, 2005).
Therefore, a robust understanding of the formation and preservation of microbial biosignatures in deep-sea hydrothermal environments over geological timescales is vital to studies concerned with life's emergence and early evolution. This paper on microbial biosignatures in Precambrian VMS-and SEDEX-type deposits aims to provide a resource for scientists from across the natural sciences whose research is concerned with life's emergence and early evolution. First, we introduce a range of candidate biosignatures for these environments and discuss their preservation potential under sulfidic hydrothermal conditions and over geological time scales. We then highlight examples of Precambrian deep-sea hydrothermal sulfide deposits for which biosignatures have been reported. Our review stresses the need to understand better the formation and preservation of microbial biosignatures in hydrothermal environments, which is of paramount importance for the search for the earliest life on Earth and, perhaps, beyond.

| MICROB IAL B IOS IG NATURE S -CON CEP T AND DEFINITI ON S
The Precambrian comprises the first 4 billion years in Earth's history, and most of this time, life was exclusively microbial (Knoll et al., 2016). Problematically, micro-organisms do not possess hard parts such as bones, shells, or wood. Therefore, reconstructing the earliest evolution of life on Earth cannot rely on such "classic" fossils.
However, rocks can preserve various other types of evidence that indicate the presence of micro-organisms in ancient environments.
These include rock fabrics* and textures, microbial microfossils, as well as minerals and organic matter with specific characteristics that are diagnostic for biologic activity (e.g., morphology, trace element signatures, stable isotope compositions; Figure 1). These features can be understood as microbial biosignatures, that is, signatures preserved in sediments and rocks that potentially testify to the presence of microbial life during their formation deposition.
The identification and interpretation of microbial biosignatures in the geological record are challenging. For one, this is because Earth's oldest rocks may preserve primary features that resemble diagnostic biosignatures but have an abiotic origin (i.e., pseudobiosignatures) (e.g., Brasier et al., 2002;Lowe, 1994;McCollom & Seewald, 2007;McLoughlin et al., 2008;McMahon, 2019;Zawaski et al., 2020). Discussing an abiotic explanation for observed features is relevant in rocks of any age but most critical for studies investigating periods in Earth's history for which the presence of life is not well constrained (i.e., before the Paleoarchean). Furthermore, biosignatures tend to get obscured over geological time scales by various processes. Alteration and destruction of biosignatures commence in the paleoenvironment and continue throughout diagenesis* and perhaps later stages (e.g., metamorphism*, metasomatism*, surface exposure; Manning-Berg et al., 2019;Pinti et al., 2009;Westall, 2005).
Primary signatures preserved in rocks might also be obscured by the formation of secondary minerals or the intrusion of organic matter during much later stages (e.g., Rasmussen et al., 2008;Summons et al., 2021;van Zuilen et al., 2002;Westall & Folk, 2003). For these reasons, a robust knowledge of biosignature formation and preservation (i.e., taphonomy*) is critical to studies concerned with Precambrian geobiology and astrobiology, and the geological context of target records must always be considered.

| MICROB IAL B IOS IG NATURE S RELE VANT TO PREC AMB RIAN DEEP-S E A HYDROTHERMAL SULFIDE SYS TEMS
Geobiological studies on Earth's oldest rocks ideally start at the outcrop scale and then progressively zoom in, perhaps down to the micron-or even nanoscale. Accordingly, this survey begins with morphological features ("rock fabrics and textures" and "microbial microfossils": Figure 1a,b), continues with mineralogical and organic components ("mineral precipitates" and "carbonaceous matter": However, this integrative multi-scale strategy is essential to critically F I G U R E 1 Examples of microbial biosignatures comparable to those that might form and preserve in deepsea hydrothermal sulfide deposits; (a) microbial rock fabrics and textures (here: originally sulfidized stromatolite in the ca. 3.48 Ga Dresser Formation, Pilbara Craton); (b) microbial microfossils (here: a Fe(II)-oxidizing bacterium encrusted by Fe-(oxyhydr)oxides that formed through microbially induced precipitation); (c) microbial mineral precipitates (here: recent framboidal pyrite in sediments from the Norsminde Fjord, Denmark); (d) (bio)geochemical signatures (here: μXRF scan showing chemical zoning in the exterior part of a recent black smoker chimney from the Manus Basin, Western Pacific Ocean); (e) carbonaceous matter (here: structural formula of a C 30 hopane, a geologically stable organic molecule that is diagnostic for commonly used bacterial biomarker).
assess the integrity and validity of potential microbial biosignatures in Earth's most ancient rocks and, simultaneously, helps avoid analytical and interpretative pitfalls.
The formation of microbialites in deep-sea hydrothermal sulfide systems results from a complex interplay of abiotic and biological processes. It may be fostered by biologically induced precipitation and/or encrustation of organic templates of biofilms and microbial mats in Fe and S minerals (see the section on Mineral precipitates).
These structures may serve as precursors for the secondary sulfidation of Fe minerals and organic matter driven by reduced sulfur species from volcanic exhalation or microbial sulfur cycling (Baumgartner et al., 2022;Campbell, 2006;Kelley et al., 2002;Little et al., 1998;Russell, 1996). The fabric, texture, and mineralogy of microbialites in deep-sea hydrothermal systems will likely depend on temperature, pH, and fluid chemistry. For instance, these parameters' steep gradients and substantial temporal variations might result in distinct ecological niches occupied by different (stratified) microbial communities and characterized by specific microbe-mineral interactions (O'Brien et al., 2015;Toner et al., 2013). Furthermore, the (trans) formation of various metal sulfides in hydrothermal environments is influenced by metal concentrations (Ehrlich et al., 2021;Park & Faivre, 2022) and gradients in fluid temperature, pH, and redox state (e.g., from hot to cooler: pyrrhotite ± magnetite > chalcopyrite to pyrite > sphalerite ± galena: Hannington, 2014; Figure 1d). These gradients may also control the identity of minerals precipitated in microbial mats and biofilms, potentially resulting in mineralogically and/or geochemically zoned microbialites in ancient hydrothermal deposits.
Although the experimental conditions differed from those prevailing in deep-sea hydrothermal vent environments, this report cautions that features commonly associated with microbialites can also derive from abiotic processes.

| Microbial trace fossils
Microbial trace fossils (not to be mistaken with microfossils, see next section) are μm-scale morphological or textural features formed by rock-inhabiting (i.e., endolithic) micro-organisms. These organisms may actively create channels, voids, or cavities within rocks or minerals (Golubic et al., 1981;Ivarsson et al., 2021;Marlow et al., 2015). For instance, oxidative dissolution of Fe sulfides by Fe(II)-oxidizing bacteria can result in distinct cell-sized (i.e., μm-scale) etch-marks or pits on mineral surfaces (Andrews, 1988;Rojas-Chapana & Tributsch, 2004;Thorseth et al., 2001). Such features and associated Fe (oxyhydr)oxides resulting from oxidative dissolution were reported on surfaces of sulfide minerals in modern seafloor hydrothermal deposits (Liu et al., 2020). To the best of our knowledge, there are no reports of microbial trace fossils in Precambrian hydrothermal sulfides. However, etch marks and channels associated with Fe oxides and carbonaceous matter in detrital pyrite in the ca. 3.4 Ga Strelley Pool Formation were interpreted as evidence for microbially induced pyrite oxidation . This suggests that microbial trace fossils may be preserved in ancient hydrothermal sulfides.
A common challenge in studying ancient microbial trace fossils is ensuring their endogeneity* and syngenicity* to the host rock.
Endolithic micro-organisms can inhabit a rock any time after its formation, even billions of years after deposition (Hoshino et al., 2014;McLoughlin et al., 2007;Westall & Folk, 2003). Moreover, it has been shown for other rock types (e.g., pillow basalts, seafloor volcanic glasses, and chert) that microbial trace fossils can be confused with abiotic post-depositional features such as ambient inclusion trails or metamorphic titanite microtubes (e.g., Grosch & McLoughlin, 2014;Knoll & Barghoorn, 1974;Lepot et al., 2011;McCollom & Donaldson, 2019). The degree to which abiotic processes can mimic sulfide bio-alteration features is currently unknown. Moreover, experimental exposure of bio-alteration features to high temperatures is necessary to illuminate their preservation potential in hydrothermal systems.
In Precambrian cherts, microfossils typically range between 10 and 100 μm in size and exhibit spheroidal or filamentous shapes (Duck et al., 2007;Glikson et al., 2008;Golubic & Hofmann, 1976;Javaux & Lepot, 2018;Knoll & Barghoorn, 1977;Rasmussen, 2000;Sugitani et al., 2007). The morphological preservation of such delicate structures may be aided by sulfide-and silica-bearing fluids causing mineral-coating or permineralization in sulfide minerals and/or chert Chert formation typically occurs in the lower temperature zones of deep-sea hydrothermal systems and is often associated with (microbial) Fe oxide formation, resulting in a characteristic jasper facies (Hannington et al., 1998). These rocks could provide promising targets for studying ancient microbial microfossils; indeed, various reported microbial Fe oxide filaments were found in such jasper (Dodd et al., 2017;Little et al., 2004Little et al., , 2021Papineau et al., 2022).
Despite this potential, recognizing microbial microfossils in the rock record remains difficult. One important reason is their small size and simple morphology (Brasier et al., 2002;Buick, 1990).
Furthermore, abiotic processes, such as self-assembly during syn- inhabited niches of hydrothermal environments (Kotopoulou et al., 2022). Moreover, the organic compounds contributing to this auto-assembly may derive from the degradation of primary organic matter that does not testify to a biogenic origin of associated minerals (Brasier et al., 2002;Simoneit, 1993;Simoneit et al., 2004).
Pyritization of such abiotic filaments may yield features resembling previously reported pyritized microfossils in hydrothermal sulfides (Baumgartner et al., 2022;Rasmussen, 2000). The morphological preservation of abiotic biomorphs needs to be tested for hydrothermal conditions, but likely, such features can easily be confused with microbial microfossils in ancient rocks.
Another possible pathway is the modification of the physicochemical microenvironment in microbial communities through their metabolic activity (i.e., "induced biomineralization": Beveridge, 1989;Lowenstam, 1981; or "biologically induced mineralization": Cosmidis & Benzerara, 2022). Organomineralization and induced biomineralization are not mutually exclusive and may co-occur. However, organomineralization is not necessarily linked to metabolic processes or limited to living organic matter and, therefore, can proceed during an organisms' lifetime and/or after its death (i.e., as a taphonomic process). Induced biomineralization, in contrast, requires metabolic activity and, thus, living organisms.
Fe and S minerals are the most promising minerals of potential biogenic origin in hydrothermal sulfide systems because their formation in most sedimentary environments dominantly results from microbial Fe and S cycling Picard et al., 2016).
Important examples are Fe (oxyhydr)oxides such as ferrihydrite (Kappler et al., 2005;Widdel et al., 1993) and magnetite (Chaudhuri et al., 2001;Köhler et al., 2013;Lovley et al., 1987). These minerals commonly exhibit nm-scale particle size and association with organic matter (Han et al., 2021;Miot et al., 2009). The reaction of Fe minerals and dissolved metals (e.g., Fe 2+ , Zn 2+ ) with aqueous sulfide in anoxic to low-oxic settings drives sedimentary sulfide mineral formation (Berner, 1970(Berner, , 1984Labrenz et al., 2000;Popa et al., 2004;Rickard, 1975;Schieber, 2002). Notably, sulfate-reducing bacteria can influence the nucleation, particle size, and morphology of sulfide minerals via templating on cell walls and EPS, as well as through sulfur redox cycling (Donald & Southam, 1999;Ferris et al., 1987; Mansor Identifying the products of induced biomineralization or organomineralization in ancient hydrothermal deposits is challenging. One important reason for this problem is that biological and abiotic precipitates might be texturally and compositionally similar. For instance, magnetite can form during diagenesis or low-grade metamorphism via thermochemical reduction of primary Fe oxyhydroxides such as ferrihydrite and lepidocrocite with sedimentary organic matter (Halama et al., 2016;Köhler et al., 2013;Posth et al., 2013Posth et al., , 2014 Rasmussen, 2000;Schroll & Rantitsch, 2005;Tornos et al., 2014;Wacey et al., 2015;Wilson et al., 2003). While the biogenicity of precipitates has not been unequivocally demonstrated in all of these cases, the combination of petrographic analyses with various mineralogical and geochemical approaches has helped reach a higher degree of confidence.
MTB occur in suboxic to anoxic environments in freshwater and marine settings (Amor et al., 2020). Magnetite-producing MTB are generally more abundant near the oxic-anoxic transition zone, while greigite-producing MTB more widely occur in sulfidic environments (Amor et al., 2020;Reitner et al., 2005). The MTB strain Magnetobacterium bavaricum was found to inhabit recent deep-sea hydrothermal vent chimneys, supporting the potential presence of magnetofossils in ancient deposits from such settings (Suzuki et al., 2004). Indeed, magnetite magnetofossils are widespread in modern deep-sea sediments .
Putative magnetofossils preserved in the ca. 1.9 Ga Gunflint Chert    (Lin et al., 2017). Taken together, magnetofossils seem to be promising candidate biosignatures for geobiological studies on ancient hydrothermal deposits.
The preservation potential of magnetofossils for microbial habitats in hydrothermal sulfide systems needs to be better understood.
Sulfidation reactions may promote the reductive dissolution of magnetite and/or its transformation to Fe sulfide minerals (Bendt et al., 2019;Canfield & Berner, 1987;Poulton et al., 2004;Qian et al., 2010Qian et al., , 2013. More experimental work is required to identify whether the transformation products of such reactions preserve biogenic characteristics. Moreover, high-temperature metamorphic reactions may produce magnetite crystals in the size range of MTB magnetite. Indeed, putative magnetofossils in the Martian meteorite ALH84001 are now widely considered abiotic products of high-temperature reactions (e.g., Bell, 2007;Brearley, 2003;Treiman, 2003; but see McKay et al., 1996;Thomas-Keprta et al., 2000). Therefore, the unambiguous identification of magnetofossils in ancient hydrothermal deposits requires distinct criteria that collectively differentiate them from abiotic precipitates (for a detailed review, see Kopp & Kirschvink, 2008).

| Carbonaceous matter
All known life is based on reduced carbon, and organisms are the primary source of organic matter in sediments and rocks on Earth (Peters et al., 2005a(Peters et al., , 2005bvan Zuilen, 2019). Particularly interesting are organic molecules with specific biological sources (e.g., lipids, pigments) and their hydrocarbon derivates that are stable over geological timescales and retain source diagnostic structural characteristics. These compounds are commonly termed "molecular fossils" or "biomarkers" (Eglinton et al., 1964;Peters et al., 2005aPeters et al., , 2005b; Figure 1e). Organic matter in modern and ancient deposits can be chemically and compositionally complex.
It is operationally divided into proportions that are extractable and non-extractable with organic solvents (i.e., bitumen and kerogen, respectively) (Durand, 1980; Figure 3). Bitumen comprises mixtures of organic compounds that were directly preserved as free molecules or released through the thermal degradation of macromolecular fractions such as kerogen (Vandenbroucke & Largeau, 2007; Figure 3). The kerogen is particularly important since it usually comprises the bulk of the total organic matter in sediments and sedimentary rocks (typically >90% w/w: e.g., Peters et al., 2005aPeters et al., , 2005b. Kerogen formation is complex but essentially involves the degradation, polymerization, and condensation of biomolecules (Durand, 1980;Farrimond et al., 2003;Vandenbroucke & Largeau, 2007). Given that a post-depositional emplacement can be excluded, most of the bitumen preserved in ancient samples is derived from the thermal degradation of the corresponding kerogen during burial.
During burial, the degradation of organic matter is mainly driven by increasing temperatures, resulting in a progressive loss F I G U R E 3 Temperature and pressure conditions relevant to the preservation of biosignatures in hydrothermal deposits. The increase in temperature (30°C/km) and pressure (0.27 kbar/km) as a function of depth are given for continental crust since the geothermal gradient is highly variable in the oceanic crust (ca. 40-80°C/km). Note that the heat flow can drastically increase in proximity to hydrothermal vents at oceanic spreading centers (red arrow). Boundaries between metamorphic facies are not sharp, and the relationship between temperature and pressure conditions during metamorphism varies strongly between different plate tectonic settings. Note that diagenetic processes commence at temperatures lower than the upper limit of microbial life and can thus affect, and be affected by, living microbial communities. Sforna et al., 2018). However, the involved processes are complex, and the abiotic synthesis of organic matter is by no means an inevitable consequence of serpentinization. Furthermore, it has not been demonstrated that FTT-derived compounds would evolve into kerogens , a crucial prerequisite for preserving over geological time scales. These fundamental uncertainties may result from the facts that FTT products show no distinct characteristics that would allow their discrimination from biotic compounds (McCollom & Seewald, 2006;Mißbach et al., 2018) and that the presence of potential abiotic organics in Earth's history could have been masked by organic matter from biological sources. Hence, the quantitative significance of organic matter deriving from FTT synthesis under hydrothermal conditions remains unknown for any time in Earth's history.
Thus, graphite in ancient metamorphic rocks may originate from the mixing of fluids containing carbon from either abiotic or biogenic sources and may be emplaced both syngenetically or from exogenous sources during younger metamorphic events (Heijlen et al., 2006;Lepland et al., 2011;Papineau et al., 2011;Papineau, De Gregorio, Cody, et al., 2010;. These potential sources of carbonaceous further complicate the identification of primarily biogenic materials in the rock record.

| Trace elements
Trace elements partitioning from fluids into minerals is a function of their abundance in the fluid and their compatibility in the mineral.
Together, this can be mathematically expressed as the partition coefficient (K)*. Micro-organisms can affect K when mineral formation occurs in equilibrium with their cytoplasm (intracellularly), as is the case for many trace elements in magnetite produced by MTB (e.g., Ni, Zn, Cu, Pb: Amor et al., 2015). Also, mediation of mineral formation by organic templates (e.g., cell wall surface, EPS; extracellularly) can affect K in magnetite formed by dissimilatory Fe-reducing bacteria (e.g., Ni, Zn in magnetite: Han et al., 2021), or in sulfide minerals in microbial mats (e.g., As, Zn, Pb, Ni: Huerta-Diaz et al., 2012;Labrenz et al., 2000;Valdivieso-Ojeda et al., 2014). This suggests that trace element signatures in minerals could be used as fingerprints of microbial activity.
Metals and metalloids also form the active centers of many essential enzymes (e.g., Ni, Mo, Zn: Fraústo da Silva & Williams, 2001), resulting in the enrichment of these elements in organic matter in sediments and rocks (Cameron et al., 2012;Cavalazzi et al., 2021;Hickman-Lewis et al., 2020;Liermann et al., 2007;Reitner et al., 2015). Moreover, the affinity of many trace elements to organic matter can result in their enrichment in living and dead biomass during diagenesis (Huerta-Diaz et al., 2012;Petrash et al., 2016;Sforna et al., 2016). These processes are by no means restricted to "normal" sedimentary environments and may also occur in hydrothermal environments. Indeed, sulfidic stromatolites from the ca. 3.48 Ga Dresser Formation (Pilbara, Western Australia) with enrichments of transition metals and metalloids in early diagenetic pyrite were interpreted to reflect their binding to organic matter .
Nevertheless, using trace elements as biosignatures is highly challenging. This is because K is also influenced by other parameters that are usually not well-constrained for paleoenvironments (e.g., mineral precipitation rates and temperature). Also, trace element concentrations in seawater have changed through geological time (Saito et al., 2003;Williams & Fraústo Da Silva, 2003) and are unknown for local paleoenvironments. These problems are amplified in hydrothermal systems, where element concentrations show steep spatial gradients and strong temporal variations (Kelley et al., 2002;Von Damm, 1995). The high capacity of sulfide minerals to abiotically sequester a broad range of metal(loid) s may result in enrichments of trace elements that are commonly associated with biological influence (e.g., Ni) (Berner et al., 2013;Dellwig et al., 2002;Gregory et al., 2015;Raiswell & Plant, 1980;Reitner et al., 2015). Also, trace metals may bind to organic matter of abiotic origin, which may originate from FTT synthesis in specific hydrothermal systems (Holm & Charlou, 2001;Konn et al., 2015;McCollom, 2013;McCollom et al., 1999;McCollom & Seewald, 2007;Ménez et al., 2018;Mißbach et al., 2018;Proskurowski et al., 2008;Rushdi & Simoneit, 2001;Sforna et al., 2018). Finally, mineral-fluid exchange during diagenesis or metamorphism might cause secondary modification of primary trace element signatures (Houghton et al., 2004;Monecke et al., 2002;Petrash et al., 2016;Schad et al., 2021). Future research must address these issues by improving paleoenvironmental proxies and conducting experimental studies to understand the long-term preservation of biogenic trace element fingerprints in minerals under hydrothermal conditions.

| Stable isotopes
Metabolic processes are commonly associated with massdependent stable isotope fractionation*, leading to different isotope ratios in the products compared with the reactants (Hoefs, 2021).
For instance, photo-and chemoautotrophic organisms prefer the lighter over the heavier stable C isotope ( 12 C and 13 C, respectively) for carbon fixation. Consequently, biological organic matter is isotopically depleted relative to the inorganic carbon pool, as expressed in negative δ 13 C Org values (Eigenbrode & Freeman, 2006;Hayes, 2001;Hoefs, 2021;Schidlowski, 2001). Since heterotrophic organisms usually conserve the isotopic composition of their substrates with only minor variations, modern, and ancient biological organic matter typically exhibits δ 13 C signatures between ca. −20‰ and −30‰. Thus, 13 C-depleted carbonaceous matter preserved in rocks and minerals may be a valuable fingerprint of life.
However, compounds from these experiments also yielded highly variable offsets between δ 13 C org and δ 13 C inorg (30‰ to −36‰: McCollom & Seewald, 2006). Therefore, a consistent offset between δ 13 C org and δ 13 C inorg of at least 20‰ to 30‰ across different facies within one system might serve as a biosignature in hydrothermal deposits (cf. Schidlowski, 2001).
This may point to a purely abiotic sulfide source or microbial sulfur cycling at low sulfate concentrations, as expected for Archean environments (Shen et al., 2001). Indeed, the combined analysis of δ 34 S, Δ 33 S*, and Δ 36 S (i.e., quadruple sulfur isotopes) on Paleoarchean pyrites and barites suggests that microbial sulfur cycling was established as early as ca. 3.5 Ga (Baumgartner, Caruso, et al., 2020;Philippot et al., 2007;Shen et al., 2001;Shen et al., 2009;Ueno et al., 2008;; but see Liu et al., 2021;Watanabe et al., 2009). Therefore, applying quadruple sulfur isotopes provides a powerful tool to elucidate microbial sulfur cycling in ancient hydrothermal systems, where traditional approaches can prove challenging.

| Sulphur Springs Group
The ca. 3.2 Ga Sulphur Springs Group (Pilbara, Western Australia) contains the Earth's oldest recognized VMS deposits and formed in a setting comparable to modern-day volcanic back-arc basins in a water depth of ca. 1000 m (Brauhart et al., 1998;Huston et al., 2019;Vearncombe et al., 1995). Based on its geological setting, mineralogy, and locally occurring mineral textures (dendritic, botryoidal, and colloform sulfide), the upper part of the Sulphur Springs deposit has been interpreted as analog to modern black smoker systems (Vearncombe et al., 1995). The ore mineralization consists mainly of pyrite, sphalerite, chalcopyrite, and galena (Brauhart et al., 1998;Huston et al., 2019), which is consistent with recent hydrothermal sulfide systems and modern VMS deposits (Hannington, 2014).
Spherical sulfide minerals in the Sulphur Springs Group were tentatively interpreted as mineralized bacteria (Vearncombe et al., 1995). Notably, colloform chert in the Sulphur Springs deposit contains pyritic filaments interpreted as microfossils of thermophilic, S-cycling prokaryotes living in the subseafloor of a hydrothermal system (Rasmussen, 2000). Their occurrence in paragenetically early chert suggests that the filaments predate the main phase of VMS mineralization and are more likely associated with low-temperature hydrothermal activity (<110°C), consistent with the presence of microbial life (Rasmussen, 2000). These pyrite filaments exhibit a nano-porous texture and are associated with nitrogen-enriched organic matter, which is consistent with a biological origin, but could also be explained by abiotic crystal growth and the localized adsorption of organic matter (Wacey et al., 2014). Hydrothermally generated oil encapsulated in fluid inclusions and pyrobitumen associated with sulfide minerals in the Sulphur Springs deposits display δ 13 C org values between −29.1‰ and −36.9‰, in line with a biological origin (Rasmussen & Buick, 2000). Notably, filamentous 13 C-depleted carbonaceous structures (δ 13 C org = −26.8‰ to −34.0‰) resembling microbial remains have also been observed in black shales directly overlying the VMS deposit (Duck et al., 2007). In summary, the Sulphur Springs deposit represents a prime target in the search for biosignatures of early life in deep-sea hydrothermal environments.
However, the biological origin of the reported filamentous microfossils remains to be further scrutinized.
The depositional environment of Barney Creek Formation was variably interpreted as a restricted deep marine setting (Bull, 1998;Jackson et al., 2000) or a saline lacustrine system (Crick, 1992;French et al., 2020). Ore mineralization in the deposit consists of pyrite, sphalerite, and galena precipitated from an oxidized hydrothermal brine rich in sulfate (Large et al., 1998;Logan, 1979 Logan, 1977). Moreover, microdigitate and columnar stromatolites with pyritic and siliceous mineralogy, as well as crinkly laminae composed of pyrite, were reported from the deposit (McGoldrick, 1999). The δ 34 S signatures of −13‰ to +15‰ in early diagenetic sulfides suggest that microbial sulfur metabolism was involved in mineral formation (Eldridge et al., 1993). Organic matter in the HYC deposit has experienced significant hydrothermal alteration but still encodes information of ore genetic and geobiological significance (Chen et al., 2003;Greenwood et al., 2013;Holman, Greenwood, et al., 2014;Logan et al., 2001;Williford et al., 2011). Preserved organic biosignatures include δ 13 C Org characteristics and biomarkers, among others indicating the presence of sulfate-reducing and sulfideoxidizing bacteria in the environment Logan et al., 2001). In summary, the HYC deposit provides rare clues on microbial sulfur cycling in Precambrian hydrothermal sulfide systems.

| Gaobanhe massive sulfide deposit
The ca. 1.43 Ga Gaobanhe massive sulfide deposit (North China) is a SEDEX deposit formed in a submerged graben* system on the North China Craton* (Kusky & Li, 2003;Li & Kusky, 2007). Hydrothermal fluid exhalation caused synsedimentary ore mineralization with an upward zonation from pyrite at the bottom to Zn-Pb-sulfide at the top (Kusky & Li, 2003). Rhenium-osmium isotope data and traceelement patterns in the Gaobanhe sulfide phases suggest local hydrothermal overprint during the Mesozoic break-up of the North China Craton (Gao et al., 2020). Nevertheless, the massive sulfide deposits contain the oldest reported morphologically preserved black smoker chimneys reported to date, offering an opportunity to study the association of ancient microbial life with these structures (Li & Kusky, 2007).
The chimney structures contain dome-shaped build-ups of concentric botryoidal and columnar sulfides interlayered with organic matter that are interpreted as microbialites (Li & Kusky, 2007).
Putative pyritic microfossils within the sulfide chimneys include filamentous, coccoidal, and rod-shaped structures locally associated with framboidal pyrite (Li & Kusky, 2007). Perhaps the deposit also preserves biomarkers (Xia et al., 2008), but the provided information does not allow for adequately assessing the quality and validity of the data. Nonetheless, the Gaobanhe deposit is a promising target for studying microbial biosignatures but needs further investigation in greater detail.

| CON CLUDING REMARK S
Deep-sea hydrothermal sulfide systems might have been crucial for the emergence of life, making ancient deposits from such settings highly relevant to deep-time geobiology and astrobiology.

G LOSSA RY
Allochthonous: Rocks, sediment, mineral particles, or organic matter which did not form at the place of deposition but away from their current location.
Craton: Precambrian cores of modern continents; characterized by high crustal thickness and structural rigidity that prevented their subduction and enabled their long-term preservation. δ−/Δ-notation: Deviation of an isotopic ratio in a sample from a corresponding isotopic ratio in a reference material in ‰, e.g., for stable sulfur isotopes (Marin-Carbonne et al., 2018): For more information on stable isotope systematics and commonly used reference materials, readers are referred to Hoefs (2021).
Diagenesis: Transformation of sediments into sedimentary rocks through progressive lithification. Diagenetic processes commence immediately after the deposition of the primary sediments and proceed through burial. Diagenesis is poorly defined concerning temperatures, but 150°C is commonly taken as an upper limit.

Endogeneity:
Occurring within the analyzed rock or mineral.
Fabric: Components, structural elements, and their geometry within a rock. Commonly used terms to describe fabrics of microbial sediments and microbialites include domal, columnar, layered, laminated, clotted, colloform, botryoidal, fenestral, microdigitate, and peloidal. For examples with images, readers are referred to Grey and Awramik (2020). Mass-dependent (isotope) fractionation: Describes the relative change in the abundance of single isotopes of an element proportional to their mass. The degree of fractionation follows a linear function with a slope governed by the mass differences of the investigated isotope ratios (m 3 -m 1 vs. m 2 -m 1 ), as commonly demonstrated in a three-isotope plot (e.g., 56 Fe/ 54 Fe vs. 57 Fe/ 54 Fe). The magnitude of fractionation increases with the relative mass difference between isotopes.

Mass-independent (isotope) fractionation: A deviation from
the linear function of the mass-dependent fractionation that is expressed through the relative difference Δ from this line (e.g., Δ 57 Fe).
Metamorphism: Pressure-and/or heat-induced structural and mineralogical transformation of rocks. This transformation commonly includes the deformation of the precursor rock (protolith), coarsening of mineral crystals, and the formation of new minerals from the breakdown of existing, no longer stable minerals.
Metamorphism does not include chemical changes to the protolith (i.e., it is "isochemical"). Serpentinization: Hydrothermal alteration of olivine and pyroxene minerals in rocks, yielding serpentine, magnetite, and brucite as well as highly alkaline fluids (pH 9-11) rich in H 2 .

Syngenicity:
Having formed at the same time as the host rock.

Taphonomy:
The study of how organisms and biogenic materials are altered and/or preserved in the fossil record.
Texture: Morphological features of individual mineral particles within sediment or rock, such as particle size, shape, and organization.
Ultramafic: Rocks that primarily consist of Fe-and Mg-rich minerals (e.g., olivine, pyroxene). Important examples are peridotite (the rock that constitutes Earth's upper mantle) and komatiite (a volcanic rock that was a widespread constituent of the Archean crust).

R E FE R E N C E S
Addadi, L., & Weiner, S. (1985). Interactions between acidic proteins and crystals: Stereochemical requirements in biomineralization.
Proceedings of the National Academy of Sciences of the United States of America, 82, 4110-4114.