Sarah Stewart Johnson
email: sarah.johnson@georgetown.edu

Maëva Millan

Heather Graham

Kathleen C Benison

Amy J Williams

Amy Mcadam

Christine A Knudson

Slavka Andrejkovičová

Cherie Achilles

Lipid Biomarkers in Ephemeral Acid Salt Lake Mudflat/Sandflat Sediments: Implications for Mars

Keywords: astrobiology, Mars, acid salt lakes, lipid biomarkers, biosignatures, lakes

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could be a viable target for biomarkers on Mars. To fully understand the astrobiological potential of these depositional environments, additional investigations of organic preservation in ancient acidic saline sedimentary environments are needed.

Introduction

Although abiotic synthesis of some organic compounds may occur in hydrothermal settings [START_REF] Mccollom | Abiotic synthesis of organic compounds in deepsea hydrothermal environments[END_REF], the vast majority of sedimentary organic matter reflects biological processes that are highly characteristic of the depositional environment. Biomarkers-organic compounds derived from living organisms-are particularly important in reconstructing Earth's microbial history prior to the Cambrian radiation emergence of readily fossilized body features [START_REF] Simoneit | Biomarkers as tracers for life on early Earth and Mars[END_REF]. Biomarkers have been found in rocks and sediments associated with some of the earliest life forms on Earth, and they have been used to characterize both physical environments and primitive biological systems (see examples in [START_REF] Peters | The biomarker guide: Volume 1, Biomarkers and isotopes in the environment and human history[END_REF]. Lipid biomarkers in particular are paramount in the identification of ancient bacterial metabolisms preserved in Archean and Proterozoic rocks [START_REF] Summons | Molecular fossils and microfossils of prokaryotes and protists from Proterozoic sediments[END_REF], as well as in modern microbial communities [START_REF] White | Analysis of microorganisms in terms of quantity and activity in natural environments[END_REF]. Microbial lipids that survive in the rock record typically have highly branched or cyclic hydrocarbons, such as isoprenoids and sterols, which are resistant to the early stages of diagenesis, during which time most degradation occurs. Often the more polar membrane lipids such as fatty acids are rapidly degraded (De Leeuw and Largeau, 1993), although recent research demonstrates that select taphonomic windows exist which allow phospho-and glycolipids to survive initial diagenesis [START_REF] Wilhelm | Xeropreservation of functionalized lipid biomarkers in hyperarid soils in the Atacama Desert[END_REF][START_REF] Wilhelm | Constraints on the Metabolic Activity of Microorganisms in Atacama Surface Soils Inferred from Refractory Biomarkers: Implications for Martian Habitability and Biomarker Detection[END_REF] If similar biomarkers formed and were preserved in the geological record on the surface of Mars, where would we be most likely to find them? The best way to answer this might be to evaluate biomarker preservation in analog mineral environments on Earth. Acid brine lakes and associated mudflats/sandflats on the Yilgarn Craton of Western Australia have been interpreted as an excellent analog for martian sedimentary deposits for three reasons (Benison and Bowen, 2006;[START_REF] Benison | Modern and ancient extremely acid saline deposits: terrestrial analogs for martian environments?[END_REF]. First, there are similar sedimentary textures and sedimentary structures, including laminations, wave ripple cross-bedding, and mudcracks in both terrestrial and martian environments, suggesting processes such as flooding, desiccation, and eolian reworking. Second, there are similar diagenetic features, including iron oxide concretions, displacive crystals, and sulfate veins, likely formed from acid saline groundwaters. Third and most importantly, the sedimentary mineral assemblage on Mars is quite similar to that of acid saline lakes in Western Australia.

The mineral assemblage at acid saline lakes and mudflats/sandflats in southern Western Australis is unusual and includes a siliciclastic component, chlorides, sulfates, iron oxides, phyllosilicates, and opalline silica (Benison et al., 2007;[START_REF] Story | Authigenic phyllosilicates in modern acid saline lake sediments and implications for Mars[END_REF]. Halite, gypsum (and other sulfates such as epsomite and basanite), hematite, kaolinite, and, less commonly, jarosite, alunite, and silica precipitate directly from acid saline lakes as chemical sediments (Benison et al., 2007). Halite and gypsum also grow from acid saline groundwater centimeters below the surface of mudflats and sandflats adjacent to the lakes, making displacive halite and gypsum, as well as halite and gypsum intergranular cements (Bowen et al., 2013). Hematite, jarosite, alunite, and kaolinite also precipitate directly from acid saline groundwaters as early intergranular cements (Benison et al., 2007: Bowen et al., 2012). Hematite concretions form early in Western Australian acid saline lake and mudflat/sandflat sediments and contain hematite, jarosite, gypsum, halite, and kaolinite (Bowen et al., 2008). The siliciclastic component of the sediments includes quartz and feldspar sand and mud grains that the products of weathering of Archean host rocks (Bowen et al., 2013).

The detection of some of these minerals on Mars, particularly jarosite and alunite, serve as strong indicators of past acidic conditions. Jarosite, a hydrated iron sulfate which forms in acid saline waters [START_REF] Klingelhöfer | Jarosite and hematite at Meridiani Planum from Opportunity's Mössbauer spectrometer[END_REF][START_REF] Nordstrom | The effect of sulfate on aluminum concentrations in natural waters: some stability relations in the system Al2O3-SO3-H2O at 298 K[END_REF], was first hypothesized on Mars by [START_REF] Burns | Gossans on Mars: spectral features attributed to jarosite[END_REF], then discovered by the Opportunity Rover in 2004 [START_REF] Klingelhöfer | Jarosite and hematite at Meridiani Planum from Opportunity's Mössbauer spectrometer[END_REF], suggesting that acidic conditions once dominated the landscape at Meridiani Planum. Jarosite has since been detected in multiple other sites on Mars, suggesting it is a relatively common mineral [START_REF] Cull | A new type of jarosite deposit on Mars: Evidence for past glaciation in Valles Marineris?[END_REF][START_REF] Léveillé | Jarosite in Gale Crater, Mars: The Importance of Temporal and Spatial Variability and Implications for Habitiability[END_REF]. Alunite, a hydrated aluminum sulfate in the same family as jarosite, has been found in the Terra Sirenum region of Mars, within a spatially extensive formation within Cross crater where in some areas it is closely associated with kaolinite. Alunite also strongly suggests low pH, sulfuric fluids at or near the martian surface [START_REF] Ehlmann | Discovery of alunite in Cross crater, Terra Sirenum, Mars: Evidence for acidic, sulfurous waters[END_REF].

To analyze the astrobiological potential of mixtures of iron oxides, sulfates, and phyllosilicates in a highly acidic and saline terrestrial Mars analog, this study investigated the types of biomarkers present and the range of biomarker recovery in naturally occurring ephemeral acid salt lakes in the vicinity of Norseman, Western Australia. The lakes are hosted by the deeply weathered Archaean rocks of the Yilgarn Craton, range in diameter from m 2 to km 2 , and are marked by cycles of flooding, evapoconcentration, and desiccation (Benison et al., 2007), as well as winds. Despite the extreme conditions (pHs as low as 1, salinity as high as 32 % total dissolved solids, high fluxes of solar radiation, water stress during desiccation, high metal concentrations, and large shifts in diurnal temperature), these sites are highly habitable, home to a diverse bacterial-, archaeal-, algal-, and fungal-rich microbiome [START_REF] Johnson | Insights from the Metagenome of an Acid Salt Lake: The Role of Biology in an Extreme Depositional Environment[END_REF][START_REF] Mormile | Molecular analysis of the microbial communities of Mars analog lakes in Western Australia[END_REF][START_REF] Zaikova | Microbial communities and their predicted metabolic functions in a desiccating acid salt lake[END_REF]. To assess the lake biomarkers associated with these iron oxide, sulfate, and phyllosilicate-rich lake sediments, a lipid assay was performed using gas chromatography-mass spectrometry (GC-MS), which illuminates the chemical structure of biomolecules from characteristic fragment ions in their mass spectra.

Materials and Methods

Sample acquisition

Sediment samples were collected in June, 2018, from four acid saline lakes within the vicinity of Norseman in southern Western Australia. The ambient air temperatures were high (up to 34.6C). Lakes in the region are ephemeral; of those with standing surface water in June 2018, lake diameters were typically less than a few hundred meters and no more than a meter deep. The pH levels of lake surface waters ranged between 1 and 3.5, as measured in the field with pH indicator strips.

Sediment samples were collected from acid saline mudflats/sandflats approximately 1 m from the active shoreline of flooded Lake Gneiss and Typhoon Lake. Other samples were collected from the lake facies of desiccated Lake Aerodrome and Lake Gilmore. Minerals and water chemistry at these sites changes through time. At time of sampling, Lake Gneiss-the most acidic of the lakes sampled, with a pH of 1 at time of sampling-was underlain by yellowish-brown mud and a thin halite layer of approximately 1 mm in thickness. Nearby Typhoon Lake, with a pH of ~3.5, had a thicker halite layer, approximately 3 mm in thickness. Lake Aerodrome, near an unpaved landing strip on the southern part of Cowan Basin, is composed of a bed of bottomgrowth gypsum crystals up to 5 cm in length. At Lake Gilmore, the salt crust was much more prevalent, approximately 1 cm in thickness.

Sampling in the field and subsequent laboratory procedures were conducted with sterile procedures to avoid contamination. At each sampling site, samples of ~100 g wet weight were collected using precombusted (550°C, 8 hours), organically clean spatulas and organically clean glass jars from just beneath the salt crust to a depth of approximately 5 cm (see Table 1). Samples were kept frozen until lipid extraction. Approximately 10 g of material was subsampled for mineralogical and particulate organic carbon and nitrogen analyses. The other 90 g was reserved for lipid extraction.

X-Ray Diffraction

Samples were crushed via mortar and pestle and sieved to <63 µm for powder Xray diffraction (XRD) analyses on a Bruker D8 Discover diffractometer. The patterns were acquired from 2-70° 2θ (Cu Kα radiation, λ = 1.54059 Å) and quantitative mineralogy was obtained by Rietveld refinement methods using the MDI (Materials Data Incorporated) Jade analysis software. Due to the poorly-crystalline nature of certain phyllosilicate minerals (e.g., kaolinite, smectite), standard Rietveld refinement methods cannot be used to properly quantify the abundance of such phases. Among the samples analyzed, kaolinite was identified by its characteristic ~7.2Å peak and quantified by combining a whole pattern fitting technique with the Rietveld refinement of the crystalline fraction. The scale factor of a pure kaolinite pattern (Clay Mineral Society KGa-2; relative intensity ratio (RIR) = 2.1) was optimized to yield an abundance relative to the crystalline phases. As discussed below in Section 3, the sample diffraction patterns show lower 001:02l kaolinite intensity ratios than expected for randomized patterns, indicative of acid alteration of the kaolinite (Pentrak et al., 2012). To minimize quantification uncertainties due to kaolinite alteration, the kaolinite abundance was estimated from 02l band scattering, a more stable measurement region compared to the 001 basal peak. Clay minerals were characterized by isolating the < 2 µm size fraction and performing XRD on oriented mounts.

Organic Carbon and Nitrogen

Particulate organic carbon and nitrogen were measured using a CHN Elemental Analyzer at Woods Hole Oceanographic Institution Nutrient Analytical Facility, Massachusetts, according to standard protocols.

Lipid extraction

Sediment samples of ~90 g wet weight (mixtures of sand, mud, salt crystals, and groundwater) were freeze dried for 36 hours. Samples were then crushed to a fine powder using a solvent-washed (3x methanol, 3x methanol:dichloromethane 1:1, 3x dichloromethane) mortar and pestle. Lipids were extracted using a modified Bligh and Dyer method [START_REF] Bligh | A rapid method of total lipid extraction and purification[END_REF] alongside an extraction blank to monitor for any signs of contamination. Extractions took place in pre-washed 50 mL Teflon tubes that were ultra-cleaned by two sequential 30-minute sonications in methanol and dichloromethane. Nineteen mL of a 10:5:4 mixture of methanol: chloroform: dichloromethane-washed H 2 O were added to each dry sample powder (of approximate weight 10 g) within the Teflon tubes. A mixture containing four internal (heneicosane (C 21 ), 2-methyl-octadecanoic acid, 1-nonadecanol (C 19 ) and p-terphenyl-d 14 ) was then added to the total lipid extract was then added to the total lipid extract.

The sample tubes were then vortexed for 5 minutes and sonicated for 20 minutes before centrifugation at 2500 rpm for 10 minutes. The liquid phase was transferred to a precombusted glass separatory funnel and the process repeated three times with the remaining sediment. For the final extraction, 19 mL of a 10:5:4 mixture of methanol: dichloromethane: H 2 O with 1% trichloroacetic acid was used, then 30 mL of DI water was added to the separatory funnel to aid in phase separation. The aqueous phase was reextracted two more times using dichloromethane.

To desulfurize samples, acid-activated and solvent-cleaned copper pellets were added to the 60 mL precombusted glass vial containing the TLE before drying it and allowed to sit overnight. The TLE was reduced to less than 50L under N 2 gas.

Silica gel column separation

Approximately half of the total lipid extract was then applied to a 10-cm silica gel pipet column for each sample. Columns with each sample, alongside an extraction blank to monitor for signs of contamination, were sequentially eluted with 1.4 column-volumes of hexane and 2 column volumes of 8:2 hexane-dichloromethane to obtain the nonpolar fractions F1 and F2; this was followed by 2 column volumes of dichloromethane for fraction F3, 2 column volumes of 8:2 dichloromethane-ethyl acetate for fraction F4, and 3 column volumes of 7:3 dichloromethane-methanol for fraction F5. Each fraction was then gently evaporated under N 2 gas. Because of an issue with clumping of the silica gel, the column fractions were lost for the Lake Gilmore mudflat/sandflat sample (for this reason, only the total lipid extract was analyzed on the GC-MS and only those results are reported in Table 2; for all other samples, results from the better separated fractions are reported in Table 2).

Derivatization for GC-MS

Trimethylsilyl (TMS) reagents were used to derivatize (reducing the polarity of) functional groups in polar fractions F3, F4 and F5 prior to GC-MS analysis. Nonpolar fractions were reconstituted in 20 L of hexanes. 10 L of bis(trimethylsilyl)trifluoroacetamide (BSTFA) in 10 L of pyridine was added then samples were heated at 70 C for 30 minutes for the derivatization to occur. All polar fractions were run on the GC-MS within thirty-six hours of derivatization.

GC-MS conditions

An injection volume of 1 L from each fraction was analyzed on a ThermoFisher Trace 1310 gas chromatograph coupled to a ISQ-LT quadrupole mass-spectrometer (GC-MS). The GC is equipped with a programmed-temperature vaporization (PTV) inlet, a 30 m MXT-5 capillary column with 0.25 mm i.d. and 0.25 m film thickness and helium (purity ≥ 99.999%) was used as carrier gas. The PTV injector temperature was set at 330°C to prevent any condensation of the compounds analyzed. Injections were performed in split mode with a 5 mL/min split flow and a 1.2 mL/min constant carrier flow rate. The five fractions were analyzed using the same GC temperature program: initial temperature of 60°C immediately followed by a 10°C/min ramp to 150°C, immediately followed by a second ramp temperature at 4°C/min to 350°C held for 10 minutes. This temperature program allowed all the compounds targeted to be analyzed while being well separated.

The MS transfer line and ion source temperatures were both set at 300°C. The MS was set to scan the ions produced from the electron impact ionization source (70eV) in the range m/z 30 to 600. A solvent delay of 6 minutes was applied to preserve the MS filament and prevent saturation of the GC column by the solvents associated with the fractions and the BSTFA derivatization reagent.

Peak identifications

Chromatograms and the mass spectra for different compounds were viewed using the Xcalibur qual browser software from ThermoFisher. Identifications were based on spectral reduction by AMDIS software followed by NIST Spectral Library software matches when available. Otherwise, spectra were identified by comparison with a laboratory-acquired spectral library and retention times of known molecules.

Mineralogy Results

Mineralogical composition derived from XRD analyses of the samples is shown in Figure 2. Quartz, halite, and kaolinite were present in all samples. Quartz was dominant mineral in Aerodrome (93.5 wt.%), Typhon (85.8 wt.%) and Gilmore samples (39.7 wt.%). Halite content varies between 2.4 (Aerodrome) to 16 wt.% (Gilmore and Gneiss).

Aerodrome and Typhon, both quartz rich samples, contained very low amounts of kaolinite (0.8-1 wt.%) compared to Gilmore and Gneiss (7.7-10.6 wt.%). Kaolinite exhibited a striking decrease in d 001 peak intensity at 12.2 °2Ѳ compared to that expected from kaolinite (Fig. 2). This preferential decrease in d 001 peak intensity compared to d 02l peak intensity (20 °2Ѳ) is a characteristic of acid attack [START_REF] Pentrák | Stability of kaolin sand from the Vyšný Petrovec deposit (south Slovakia) in an acid environment[END_REF]) XRD analyses of oriented mounts of the < 2 um size fraction of each sample (not reported) confirmed the presence of kaolinite in each sample and also revealed the presence of a trace 10 Å phyllosilicate that could be either illite or a mica mineral.

Plagioclase is identified in most samples (1.7-6.6 wt.%) as well as alunite, which is most abundant (49.5 wt.%) in the sample from the most acidic lake Gneiss (pH=1, Table 1). Gilmore is the only sample containing gypsum with significant abundance of 29 wt.%. Hematite and goethite were identified as minor phases in Aerodrome and Gneiss with 0.6 and 1.8 wt.%, respectively.

Mineral identifications have been made previously for some of these Western Australian lakes and adjacent mudflats/sandflats at different time and spatial scales (Benison et al.., 2007;Bowen et al., 2012;[START_REF] Story | Authigenic phyllosilicates in modern acid saline lake sediments and implications for Mars[END_REF]. To best understand the context of the new mineral data presented here for the four sampling sites, one should consider the wider mineralogy of Western Australian acid saline lakes mineralogy. The mineral suite is heterogeneous over meter-scales and changes in mineral relative abundances in response to flooding, evapoconcentration, desiccation, and host rock composition (Benison et al., 2007).

Lipid Results

Typical GC-MS chromatograms showing the most abundant lipids recovered from the Bligh and Dyer extracted nonpolar fractions (F1 and F2) and polar (F3, F4, and F5) fractions. Representative chromatograms are presented in Figures 3, 4, and5. Identifications are noted in Table 2. The following internal standards were used: heneicosane (C 21 ), 2-methyl-octadecanoic acid, 1-nonadecanol (C 19 ) and p-terphenyl-d 14 .

n-Alkanes were identified in fractions F1 and F2 following Bligh-Dyer extractions and column separation. The n-alkanes range in carbon number from 17 to 35 while the most dominant homologues are the C 25 -C 31 n-alkanes; no evidence of cyanobacteria in short chain n-alkanes was seen. There is a strong odd over even carbon number predominance (See Figure 3).

Fatty acids in the polar fractions range in carbon number from 12 to 32 and nalcohols from 14 to 32. Even numbered carbon compounds dominate the distributions of fatty acids and n-alcohols (See Figure 4). Sterols, ursolic acid derivatives, and betulin were also found in high levels in the polar fractions (See Figure 5). Based on retention times and fragmentation patterns, we also uncovered likely pentacyclic triterpenoids in some of the sampled sites, possibly bacterial hopanes or oleanane-type compounds.

Discussion

On Earth, organics are often sequestered in tandem with phyllosilicate minerals, such as smectites and associated non-crystalline iron and aluminum oxides/hydroxides [START_REF] Wattel-Koekkoek | Mean residence time of soil organic matter associated with kaolinite and smectite[END_REF]. Micrometer size phyllosilicate minerals dominate the organic matter stabilization literature, in large part because these minerals have relatively large surfaces areas and typically carry a substantial charge, allowing them to chemically stabilize organic matter [START_REF] Keil | Mineral matrices and organic matter[END_REF]. These observations suggest that terrains dominated phyllosilicates are the best targets for lipid biomarker recovery, thus, sites with analogous phyllosilicate deposits on Mars, such as Jezero Crater, have been targeted for future exploration [START_REF] Ehlmann | Geochemical consequences of widespread clay mineral formation in Mars' ancient crust[END_REF]. But would phyllosilicates found in association with other mineral groups, such as the sulfate and iron oxides bearing phyllosilicates of the Upper Murray unit at Gale crater [START_REF] Rampe | Mineral Trends in Early Hesperian Lacustrine Mudstone at Gale Crater, Mars[END_REF], also be worth targeting for sample return?

Oxidized minerals have long been considered weak targets for the preservation of organic molecules [START_REF] Klein | Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins[END_REF]. Thermodynamics suggest that organics will quickly react with SO 4 2-and Fe 3+ , leaving behind simpler compounds or CO 2 as well as more reducing forms of sulfur or Fe 2+ [START_REF] Stumm | Aquatic chemistry: chemical equilibria and rates in natural waters[END_REF][START_REF] Sumner | Poor preservation potential of organics in Meridiani Planum hematite-bearing sedimentary rocks[END_REF]. The kinetics of organic carbon oxidation in the presence of iron are thought to be extremely rapidmonths to years under most conditions-largely due to the generation of free radicals and H 2 O 2 on the surface of iron-bearing minerals, and the calculations of Stumm and Morgan (1996) suggest that Fe(III)-bearing minerals are not stable at the low electron potentials necessary for organic carbon stability [START_REF] Stumm | Aquatic chemistry: chemical equilibria and rates in natural waters[END_REF][START_REF] Sumner | Poor preservation potential of organics in Meridiani Planum hematite-bearing sedimentary rocks[END_REF]. Sulfate reduction in the presence of organics, while slower in general, requires the transfer of more electrons and is thought to occur at a faster rate at low pH conditions [START_REF] Ohmoto | Kinetics of reactions between aqueous sulfates and sulfides in hydrothermal systems[END_REF].

Mineral stability fields, however, depend critically on ion pairing in solution. This in turn depends on brine compositions and pH. Although significant progress has been made in this area-on how Mg, Ca, and Fe-sulfates might have formed from the evaporation of acidic fluids derived from weathering olivine-bearing basalt [START_REF] Tosca | Geochemical modeling of evaporation processes on Mars: Insight from the sedimentary record at Meridiani Planum[END_REF], on how iron-bearing phyllosilicates might have formed from weathering and hydrothermal alteration of Martian basalt [START_REF] Catalano | Thermodynamic and mass balance constraints on iron-bearing phyllosilicate formation and alteration pathways on early Mars[END_REF], and on how clay-bearing sequences might have formed through downward percolation and neutralization of acidic H 2 SO 4 -HCl solutions [START_REF] Zolotov | Chemical models for martian weathering profiles: Insights into formation of layered phyllosilicate and sulfate deposits[END_REF])-these compositions are not fully constrained for the early Martian surface. Further, investigations on Earth of late Archaean and early Paleoproterozoic banded iron formations, such as the Gunflint Iron Formation (1.878 +/-0.001 Ga), have shown at least some evidence of co-preservation of organics in the presence of iron oxides [START_REF] Fralick | The age of the Gunflint Formation, Ontario, Canada: single zircon U Pb age determinations from reworked volcanic ash[END_REF]. Fatty acid methyl esters have been found in natural terrestrial Fe(III)-dominated rock samples in Iron Mountain, California-analogs of martian rocks that host iron-oxides, such as those in the lower mound of Mt. Sharp [START_REF] Williams | Lipid Detection in Fe (III)-Dominated Samples to Prepare for the Tetramethylammonium Hydroxide (TMAH) Wet Chemistry Experiment on the SAM Instrument Suite[END_REF]. Lipid biomarkers, including labile fatty acids associated with Synechococcus-Chloroflexi mats, have also been shown to survive early stages of diagenesis in iron hot springs like the Chocolate Pots in Yellowstone National Park [START_REF] Parenteau | Production and early preservation of lipid biomarkers in iron hot springs[END_REF]. The entombment of microbial cells by iron oxides in hot springs and sulfur and iron-rich streams may improve lipid preservation by sequestering the organics from enzymatic degradation via heterotrophs [START_REF] Parenteau | Microbial biosignatures in iron-mineralized phototrophic mats at Chocolate Pots Hot Springs, Yellowstone National Park, United States[END_REF][START_REF] Williams | Filamentous Hydrous Ferric Oxide Biosignatures in a Pipeline Carrying Acid Mine Drainage at Iron Mountain Mine, California[END_REF][START_REF] Tan | The Fate of Lipid Biosignatures in a Mars-Analogue Sulfur Stream[END_REF].

Iron may inhibit autolytic enzymes, and it has been recently shown that large amounts of organic carbon in marine sediments are directly bound to reactive iron phases, generating a "rusty sink" which can favor the long-term preservation of organics [START_REF] Lalonde | Preservation of organic matter in sediments promoted by iron[END_REF][START_REF] Parenteau | Production and early preservation of lipid biomarkers in iron hot springs[END_REF]. It may be that Fe oxides are also home to preferential sorption sites at the mouth of micropores (<2 nm) and smaller mesopores (2 to10 nm); the surface structure of goethite crystals, for instance, can have defects resulting in this surface roughness [START_REF] Kaiser | Mineral surfaces and soil organic matter[END_REF][START_REF] Keil | Mineral matrices and organic matter[END_REF].

While there have been few systematic investigations of the preservation potential of sulfate minerals on Earth, organics-including amino acids and their amine degradation products-have been detected in terrestrial sulfate mineral matrices, including gypsum from the Anza-Borrego Desert, California, gypsum from the Haughton impact crater, Canada, and gypsum, anhydrite, and jarosite samples from Panoche Valley, California (Aubrey et al., 2006), as well as acid saline lake gypsum from northern Chile [START_REF] Benison | Could microorganisms be preserved in Mars gypsum? Insights from terrestrial examples[END_REF]. Microorganisms have been observed as trapped in acid halite and gypsum from Western Australia [START_REF] Conner | Acidophilic halophilic microorganisms in fluid inclusions in halite from Lake Magic, Western Australia[END_REF][START_REF] Conner | Acidophilic halophilic microorganisms in fluid inclusions in halite from Lake Magic, Western Australia[END_REF], as well as unusual clumps of organic bodies and sulfate crystals in evaporite minerals grown in acid saline lakes (Benison et al. 2008). Magnesium sulfates have also been shown to protect organics trapped inside the crystal lattice or present in fluid inclusions from oxidation induced by the decomposition of perchlorates, powerful oxidants present on the Martian surface [START_REF] François | Magnesium sulfate as a key mineral for the detection of organic molecules on Mars using pyrolysis[END_REF].

Our study is among the first to investigate the biomarker potential of mixtures of iron oxides, sulfates, and phyllosilicates in a highly acidic terrestrial Mars analog. We found a strong odd over even carbon number predominance in long-chain n-alkanes (carbon preference index, or CPI, ranged from 3.6 to 5.7), indicating a terrigenous origin, probably in the primary form of cuticular waxes [START_REF] Eglinton | Leaf epicuticular waxes[END_REF]. These have also been shown to be carried in eolian dust [START_REF] Gagosian | Long-range transport of terrestrially derived lipids in aerosols from the South Pacific[END_REF][START_REF] Simoneit | Biogenic lipids in particulates from the lower atmosphere over the eastern Atlantic[END_REF]. The dominance of even numbered carbon compounds in the distributions of fatty acids and odd numbered compounds in the n-alcohols also suggest a strong terrigenous plant origin [START_REF] Eglinton | Leaf epicuticular waxes[END_REF]). An algal origin of the long-chain fatty acids does, however, remain a possibility, though other signs of algae, such as unsaturated fatty acids, would be expected [START_REF] Volkman | Microalgal biomarkers: a review of recent research developments[END_REF]. The predominance of C 29 sterols over C 27 and C 28 sterols again suggests a land plant source [START_REF] Holland | Treatise on Geochemistry[END_REF], and ursolic acid derivatives and betulin are indicative of higher plant leaf waxes [START_REF] Jäger | Solubility studies of oleanolic acid and betulinic acid in aqueous solutions and plant extracts of Viscum album L[END_REF].

The overall assemblage of saturated alkanes, alcohols, fatty acids and sterols we recovered strongly point to the overprinting of plant debris, likely originating from the low-diversity eucalypt forests that exist on the Yilgarn Craton [START_REF] Yates | The distribution and status of eucalypt woodlands in Western Australia. Temperate eucalypt woodlands in Australia: biology, conservation, management and restoration[END_REF], though low abundances of microbial lipids were also detected. We surmise that amount of plant biomass simply overshadowed the traces of microbial biomass in the sediment, as diverse microbial communities have been observed within lake water and mud/sandflat sediment as part of genomic studies [START_REF] Zaikova | Microbial communities and their predicted metabolic functions in a desiccating acid salt lake[END_REF][START_REF] Johnson | Insights from the Metagenome of an Acid Salt Lake: The Role of Biology in an Extreme Depositional Environment[END_REF][START_REF] Mormile | Molecular analysis of the microbial communities of Mars analog lakes in Western Australia[END_REF]. It is possible that there's a preservational bias for plant material in this system (perhaps sequestered in such a way that's more conducive to preservation) or simply for larger molecules (plant terpenoids, potential hopanes), yet easily hydrolyzed shorter chain fatty acids (e.g. C 16 and C 18 , likely associated with microbial communities) were also observed. While microbes may be present as part of an active subsurface microbial community, the plant lipids (very long chain alkanes, plant terpenoids, etc.) are from dead organisms-washed into the lake basins at some point in the past. This suggests that structural biomarkers not only associated with extant biology are preserved through at least the early stages of diagenesis in the biosedimentary system represented by these lakes (Table 2).

Our results build on other recent findings that suggest organics could be found in mineralogically diverse sediments on Mars, including those with jarosite group minerals (alunite, jarosite), smectites, and other phyllosilicates where multiple redox states give rise to habitable environments and microbial activity may be concentrated at interfaces, in the past or even today in the planet's subsurface. For example, a recent study of mudstones and sandstones in the Painted Desert of Arizona also revealed far higher concentrations of amino acids, in addition to carboxylic and dicarboxylic acids, alcohols, aliphatics and other complex molecules, in the presence of clay minerals and jarosite compared to the presence of clay minerals alone (Noe [START_REF] Dobrea | Preservation of Organics at the Painted Desert: Lessons for MSL and Beyond[END_REF].

The processes affecting the preservation of organics in later stages of diagenesis, however, requires additional investigation, and we have only investigated the very earliest stages of diagenesis as part of this study. There are very few examples of these types of depositional systems on Earth, but the ancient acid ephemeral saline lakes of the Permian Opeche Shale of the Williston Basin, North Dakota, provide one such opportunity [START_REF] Benison | Recognizing acid lakes and groundwaters in the rock record[END_REF][START_REF] Benison | Extremely acid Permian lakes and ground waters in North America[END_REF]. Here the oxidation of sulfide minerals along with other acidification processes, which may have been mediated by a combination of climate, rock-water interactions, and microbes, gave rise to extremely acidic waters (pH<1) in tectonically stable, closed-drainage basins (Benison and Bowen, 2015). These ancient sites precipitated evaporite minerals in an arid climate characterized by abundant, carbonate-poor red siliciclastics, much like lithified strata that have been observed on Mars at Gale crater. The discovery of lipid biomarkers in places like the Permian Opeche Shale would further bolster the search for remnant organic matter on Mars.

Cosmic rays, UV irradiation, peroxides, and a multitude of non-aqueous chemical oxidations over very long timescales are critical challenges to the recovery of Martian biomarkers (Summons et al., 2007), and it is likely that deep subsurface samples from ancient acid saline playa lakes would be better targets for biomarker preservation on Mars than the shallow subsurface samples we analyzed here.

While hyperaridity is a major challenge to microbial survival on Earth, the hyperaridity of the Martian surface, which is 100-1000 times drier than the driest regions on Earth, may facilitate the preservation of lipids generated during environmentally favorable periods [START_REF] Wilhelm | Xeropreservation of functionalized lipid biomarkers in hyperarid soils in the Atacama Desert[END_REF]. There is extensive evidence that aqueous conditions were widespread on Mars in its ancient past, so if microbial life did arise on Mars prior to the desertification of the planet, biomarkers may have still be preserved in the geologic record.

Conclusion

Our investigation of the types of biomarkers recoverable within the naturally occurring acid salt lakes in the vicinity of Norseman, Western Australia-a terrestrial analog with striking mineralogical and sedimentary similarities to Mars-has revealed the vestiges of several plant biomolecules alongside trace microbial lipids. The resilience of lipids from dead organic material in the saline sediments represented by these lakes lends support to the idea that iron oxides and sulfates, in tandem with phyllosilicates, may be a viable target for preserved biomarkers on Mars, and motivates investigations of organic preservation in acidic saline pans that have been preserved in our planet's deep geologic record.

Acknowledgements

We would like to thank Angus Turner for assistance in our initial survey of the area and Roger Summons for insightful advice on the manuscript.

  Figure 1: Sampling sites at ephemeral acid saline lakes in southern Western Australia. A. Map of Australia showing approximate locations of sampling sites. B. -D. Satellite images of lakes, courtesy of Google Earth and Landsat/Copernicus. Each satellite image is keyed to location on map of Australia to left. E. Photo of Gneiss Lake at flooding stage and surrounding acid saline mud/sandflat with yellow and brown sediments. F.Photo of Lake Aerodrome at desiccation stage. Red car on right for scale. G. Lake Typhoon at flooding stage. Arrow points to sampling site on acid saline mud/sandflat, 1 m from lake shore. H. Bedding plane view of sampling site at Lake Gneiss mud/sandflat. I. Bedding plane view of sampling site at desiccated Lake Gilmore. Spoon for scale. Lake names are taken fromBenison et al. (2007) andBowen and Benison (2009).

Table 1 :

 1 Acid saline lake systems give clues about past environments and the search for life on Mars. Icarus, 183, 225-229. Benison KC, Bowen BB, Oboh-Ikuenobe FE, Jagniecki EA, LaClair DA, Story SL, Mormile MR, and Hong B-Y. (2007) Sedimentology of acid saline lakes in Environmental characteristics for the sediment samples for the four mud/sandflats sampled in this study. Previously observed pH data from Supplemental Information in Bowen and[START_REF] Mormile | Molecular analysis of the microbial communities of Mars analog lakes in Western Australia[END_REF]. It is interesting to note that the higher levels of particulate organic matter and nitrogen are associated with the most alunite-rich samples.

	Aubrey A, Cleaves HJ, Chalmers JH, Skelley AM, Mathies RA, Grunthaner FJ,
	Ehrenfreund P, and Bada JL. (2006) Sulfate minerals and organic compounds on
	Mars. Geology, 34, 357-360.
	Benison KC. (2013) Acid saline fluid inclusions: examples from modern and Permian
	extreme lake systems. Geofluids. Nov;13(4):579-93.
	Benison KC, and Bowen BB (2015) The evolution of end-member continental waters:
	The origin of acidity in southern Western Australia. GSA Today, 25, 4-10.
	Benison KC, Jagniecki EA, Edwards TB, Mormile MR, Storrie-Lombardi MC (2008).
	"Hairy blobs:" microbial suspects preserved in modern and ancient extremely acid
	lake evaporites. Astrobiology. 8(4):807-21.
	Benison KC, and Bowen BB. (2006)

Table 2 .

 2 Detections shown as percentages of total alkanes, fatty acids, alcohols, and 640 sterols in each sample. Darker shading indicates higher abundances. 641 9

. Figures Figure 1:


Pierre Monnin

Joël Legrand

Adrien Coulet

PGxCorpus and PGxLOD: two shared resources for knowledge management in pharmacogenomics

Corresponding author: adrien.coulet@inria.fr Pharmacogenomics (PGx) studies the impact of genetic factors on drug response phenotypes. Atomic knowledge units in PGx have the form of ternary relationships linking one or more drugs, one or more genetic factors, and one or more phenotypes. Such relationships state that a patient having the specified genetic factors and being treated with the specified drugs is likely to experience the given phenotypes. PGx knowledge is of particular interest for the development of precision medicine which aims at tailoring drug treatments to each patient to reduce adverse effects and maximize drug efficacy. However, PGx knowledge is scattered across many sources (e.g., reference databases, the biomedical literature) and suffers from very heterogeneous levels of validation, i.e., some PGx relationships are extensively studied and have been translated into clinical practice, but most are only observed on small-size cohorts or not reproduced yet and necessitate further investigation. Consequently, there is a strong interest in extracting and integrating knowledge units from these different sources into a unique place to provide a consolidated view of the state-of-the-art knowledge of this domain and drive to the validation, or moderation, of insufficiently validated knowledge units. To this aim, we created and share with the community two resources: PGxCorpus and PGxLOD.

PGxCorpus is a manually annotated corpus, designed for the automatic extraction of PGx relationships from text [START_REF] Legrand | PGxCorpus, a manually annotated corpus for pharmacogenomics[END_REF]. In specialized domains such as PGx, state-of-the-art approaches rely on supervised models trained or fine-tuned on previously annotated texts. PGxCorpus has been built by 11 annotators and consists of 945 sentences from PubMed abstracts annotated with (i) PGx entities of interest, i.e., genetic factors (e.g., genes, variants, haplotypes), drugs, and phenotypes, and (ii) relationships between these entities, associated with a type (e.g., causes, decreases, transports) and an attribute (positive, hypothetical, or negative). It includes 2,875 relationships, each seen at least four times and in total by four different annotators. PGxCorpus is available at http://pgxcorpus.loria.fr.

PGxLOD is a knowledge graph that gathers 50,435 PGx relationships extracted from expert databases such as PharmGKB and from the literature [START_REF] Monnin | PGxO and PGxLOD: a reconciliation of pharmacogenomic knowledge of various provenances, enabling further comparison[END_REF]. It implements Semantic Web and FAIR best practices. Relationships of the literature are extracted with a model trained on PGxCorpus. Besides PGx relationships, PGxLOD includes knowledge about genetic factors, drugs, and phenotypes (i.e., PGx key entities) imported from ClinVar, DisGeNET, DrugBank, SIDER, etc. to compose a graph of 88 million triples. We have paid particular attention at connecting PGx relationships that come from independent data sources but may be similar or equivalent with the development of both rule-based and machine learning matching approaches. PGxLOD is available at https://pgxlod.loria.fr.

These resources open perspectives with applications such as predicting pharmacogenes or mining molecular explanations of adverse drug reactions [START_REF] Bresso | Investigating ADR mechanisms with explainable AI: a feasibility study with knowledge graph mining[END_REF]. Additional analyses of PGxLOD offer the potential to guide PGx research by identifying knowledge that requires additional validation. Besides biomedical applications, PGxCorpus and PGxLOD offer challenging experimental settings to both NLP (discontinued entities, ternary relationships) and graph mining tasks (heterogeneity and arity of PGx relationships).