Earth’s atmosphere underwent an irreversible, and geologically sudden, change approximately 2.5 billion years ago from oxygen free, to oxygenated, called the Great Oxidation Event (GOE). This change was driven by the evolution of a new form of photosynthesis which produced molecular oxygen as a byproduct. The group of bacteria in which this evolved, Cyanobacteria, are the only organisms to independently harness this form of photosynthesis. While we know that by the time of the GOE, Cyanobacteria were present, we do not know if they were present before the GOE. It has been proposed that Cyanobacteria were restricted to freshwater environments for hundreds of millions of years before the GOE, and only when they were able to inhabit the oceans did the GOE occur. We address this hypothesis by surveying the literature to understand how modern cyanobacteria respond to changes in salinity, as well as running a 1000 generation evolution experiment. We find evidence that just because a cyanobacterial species is found in freshwater does not mean it cannot live in marine salinities, and vice versa. Additionally, we find that prolonged exposure to a different salinity does not result in loss of ability to grow in the ancestral salinity.
Triple-oxygen isotope (δ18O and Δ17O) analysis of sulfate is becoming a common tool to assess several biotic and abiotic sulfur-cycle processes, both today and in the geologic past. Multi-step sulfur redox reactions often involve intermediate sulfoxyanions such as sulfite, sulfoxylate, and thiosulfate, which can rapidly exchange oxygen atoms with surrounding water. Process-based reconstructions therefore require knowledge of equilibrium oxygen-isotope fractionation factors (18α and 17α) between water and each individual sulfoxyanion. Despite this importance, there currently exist only limited experimental 18α data and no 17α estimates due to the difficulty of isolating and analyzing short-lived intermediate species. To address this, we theoretically estimate 18α and 17α for a suite of sulfoxyanions—including several sulfate, sulfite, sulfoxylate, and thiosulfate isomers—using quantum computational chemistry. We determine fractionation factors for sulfoxyanion “water droplets”; using the B3LYP/6-31G+(d,p) method; we additionally determine higher-order method (CCSD/aug-cc-pVTZ and MP2/aug-cc-pVTZ) and anharmonic zero-point energy (ZPE) scaling factors using a suite of gaseous sulfoxy compounds and test their impact on resulting sulfoxyanion fractionation-factor estimates. When including redox state-specific CCSD/aug-cc-pVTZ and anharmonic ZPE scaling factors, our theoretical 18α predictions for protonated isomers closely agree with all existing experimental data, yielding root-mean-square errors of 1.8 ‰ for SO3(OH)-/H2O equilibrium (n = 18 experimental conditions), 2.2 ‰ for SO2(OH)-/H2O (n = 27), and 3.9 ‰ for S2O2(OH)-/H2O (n = 3). This result supports the idea that oxygen exchange occurs via isomers containing oxygen-bound protons. By combining 18α and 17α predictions, we additionally estimate that SO3(OH)-, SO2(OH)-, SO(OH)-, and S2O2(OH) exhibit Δ17O values as much as 0.167 ‰, 0.097 ‰, 0.049 ‰, and 0.153 ‰ more negative than equilibrated water at Earth-surface temperatures (reference line slope = 0.5305). This theoretical framework provides a foundation to interpret experimental and observational triple-oxygen isotope results of several sulfur-cycle processes including pyrite oxidation, microbial metabolisms (e.g., sulfate reduction, thiosulfate disproportionation), and hydrothermal anhydrite precipitation. We highlight this with several examples.
Light absorption in the photosynthetically active (400 – 700 nm) spectral region is necessary for plant CO2 fixation via photosynthesis. Light absorption in excess of that which can be used for photosynthesis may result in photoinhibition and/or other processes detrimental to normal plant function. Plants have evolved several photoprotective mechanisms to reduce light absorption under stressful conditions. For example, leaf-level reflectance and transmittance increased as a result of chloroplast movement within leaf cells in response to water stress in greenhouse-grown maize and soybean. This has implications for detecting (as a signal and noise) diurnal and stress-related changes in canopy reflectance in field-grown crops. These changes were recently investigated in the field using newly developed instrumentation systems and software. Two hyperspectral spectrometers, an Ocean Optics QE Pro (0.3 nm resolution in the 650 - 813 nm range) and a Flame (2.0 nm resolution in the 340 - 1028 nm range) are coupled through optical shutters to a downward looking fiber (25° field of view) and an upward looking fiber with cosine corrector. The spectrometers can be configured to see sky or surface targets concurrently or separately. This new configuration offers concurrent measures of derived solar induced fluorescence (SIF), and visible and near infrared reflectance on a mobile platform, acquiring spatially averaged responses. Our goal is to use SIF as an indicator of the level of photosynthetic activity in comparison to reflectance-derived indication of photoprotective response. In conducting data acquisition, several technical issues arose. Different spectrometer integration times, due to differing radiometric sensitivities and changing sky conditions, causes differences in measured reflectance between the two spectrometers. Also the approach highlighted the difficulty of obtaining reliable system calibration under varying sky conditions when using near-Lambertian reference panels. While results are promising in detecting SIF along with more conventional remote sensing spectral resolution, further research is needed to refine data acquisition to ensure quality reflectance measurements. We report on technical issues and on our success in tying photoprotection to changes in photosythentic activity.
The rhizosphere is a complex system in which many diverse and heterogeneous small-scale components (e.g, plant roots, fluids, microbes, and mineral surfaces) interact with one another, often in nonlinear ways, giving rise to emergent system behaviors. Ecosystem-scale perturbations, such as nitrogen limitation or drought, drive changes in micro-environments through a cascade of complex interacting processes, leading to a bidirectional feedback across scales between microbial and plant habitats at the microscale and ecosystem function at the macroscale. We are developing a conceptual and numerical framework for multiscale simulation of organic carbon transport, transformation, and disposition in the soil-microbe-plant continuum. The conceptual model comprises a set of directed graphs, with nodes representing system processes and states and edges representing process-state relationships. The graphs are coded in the graphviz syntax enabling dynamic web visualization. Graph nodes are hyperlinked to metadata pages summarizing current understanding of each process or state and its representation in current numerical codes. This conceptual model is available via a git repository and can guide identification of opportunities for coupling (data exchange) between codes operating at different length scales. The numerical implementation of the conceptual model is based on execution of integrated data processing and multiscale modeling scientific workflows. The numerical framework is enabled by a recent development in information technology known as orchestration, a class of solutions to problems of deployment and execution of cloud-oriented software. Orchestration technology is well-suited to automating complex scientific workflows, both in model-coupling efforts and experimental analysis pipelines. Here it is used to flexibly define workflow steps based on precedent events (such as arrival of a new model output in the data repository). It is being applied to integrate several community software packages spanning scales from molecules to ecosystems, linked to experimental data from the Environmental Molecular Sciences Laboratory (a national scientific user facility), to address critical scientific questions related to soil nutrient cycling.
The transport of methane from deep sediments towards the seafloor is widespread in ocean margins and has important biogeochemical implications for the deep ocean . A significant portion (>80%) of methane entering the shallow sediments from below at present is oxidized by microbially-driven anaerobic oxidation of methane (AOM), which mainly involves a microbial consortium of anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria. Isoprenoid Glycerol dialkyl glycerol tetraethers (GDGTs) derived from core lipid membranes of ANMEs are often well preserved in sediment records. Methane Index (MI) is an organic geochemical proxy for methane seepage intensity which weighs in the relative proportion of GDGTs (GDGT-1,-2, and -3) preferentially synthesized by ANMEs with that of non-methane-related biomarker contribution from planktonic and benthic sources (Crenarchaeols) . This study analyzed the GDGT composition of sedimentary core lipids from IODP Site 1230 (Peru Margin) using two silica columns and a high-resolution and accurate mass Orbitrap Fusion Mass Spectrometer. Our results report novel GDGT isomers with concentration peaking at the Sulfate-Methane Transition Zones (SMTZ) with the highest AOM activity around 8 mbsf. Further, these isomers were almost absent above and below the SMTZ. Our observations suggest that these characteristic isomers of GDGT compounds preserved at the SMTZ depth are sourced from ANMEs. Identification of these novel isomers has important implications in refining the MI and additional GDGT based palaeoceanographic proxies like TEX86. 1. Akam et al. (2020), Frontiers in Marine Science 7, 206. 2. Y. G. Zhang et al. (2011), Earth and Planetary Science Letters 307, 525-534.
Animal migrations mark the largest daily movement of biomass on Earth today, but who performed the first diurnal migration? Extant benthic microbial mats inhabiting Lake Huro’s low-oxygen, high-sulfur submerged sinkholes that resemble life on early Earth, may offer some answers. Herein, mats are dominated by motile filaments of purple-pigmented cyanobacteria capable of oxygenic and anoxygenic photosynthesis, and pigment-free chemosynthetic sulfur-oxidizing bacteria. We captured time-lapse images of diurnal vertical migration between phototactic cyanobacteria and chemotactic sulfur-oxidizing bacteria – dramatically turning the mat surface purple at dawn and white at dusk. Alternating waves of vertically migrating photosynthetic and chemosynthetic filaments rapidly tracked diurnally fluctuating light; observations corroborated with intact mats under simulated day-night conditions. Both types of filaments increased in surface coverage non-linearly, albeit at different rates. During their respective surface takeovers, maximum nightly rate of movement for white chemosynthetic filaments occurred an hour before that of purple cyanobacteria during the day. However, though slow to start at dawn, the cyanobacteria’s maximum rate of movement was double that of the chemosynthetic bacteria, leading to greater total coverage over the span of the day. Such synchronized diurnal “tango” might have been the largest daily mass movement of life during the long Archean and Proterozoic eras, when the biosphere was mostly benthic, and played a critical role in optimizing photosynthesis, chemosynthesis, carbon burial, and oxygenation. Further studies of extant microbial “mat worlds” will add to the expanding knowledge of Earth’s biodiversity and physiologies, and may aid our ongoing search for life in extraterrestrial waters.
To our knowledge, this is the first study to report the different biodegradation sequences of saturated hydrocarbon compounds by two bacteria— XJ16 and XJ19—using semiquantitative analyses of the gas chromatography–mass spectrophotometry (GC–MS) data of biodegraded oils over 90-day simulation, whcih demonstrating the effects of bacterial species on the biodegradation sequences. The general biodegradation sequence of compounds for XJ16 was similar to that reported previously: -alkanes (most easily to biodegrade) > -alkylcyclohexanes > dicyclic sesquiterpenes > steranes > hopanes. However, the general biodegradation sequence of compounds for XJ19 was different: dicyclic sesquiterpenes (most easily to biodegrade) > steranes > hopanes > -alkylcyclohexanes > -alkanes. The total biodegradation ratios of -alkanes, -alkylcyclohexanes and dicyclic sesquiterpenes by XJ16 were 69.5%, 52.9%, and 48.3% higher than those by XJ19, respectively. The -alkane/-alkylcyclohexane biodegradation sequence for XJ16 and XJ19 were different, but the dicyclic sesquiterpene biodegradation sequences for these two bacteria were the same. However, the total biodegradation ratios of the steranes and hopanes by XJ19 were 12.64% and 18.56% higher than those by XJ16, respectively. For both strains, the biodegradation sequences of some biomarkers were as follows: Cdiastrane > Cdiastrane, C-5α(H)-homopregnane > C-5α(H)-pregnane, βαC20S > βαC20R, αβC20S > αβC20R, αααC20R > αααC20R > αααC20R, Tm > Ts and CM > CH. Moreover, preferential biodegradation of the lower-molecular-weight homologues (C > C > C > C) was observed, with R epimer over the S epimer.
Excessive dissolved inorganic nitrogen (DIN) added to the urban river systems by point-source inputs, such as untreated wastewater and wastewater treatment plant (WWTP) effluent, constitutes a water-quality problem of growing concern in China. However, very little is known about their impacts on DIN retention capacity and pathways in receiving waters. In this study, a spatially-intensive water quality monitoring campaign was conducted to support the application of the river water quality model WASP7.5 to the PS-impacted Nanfei River, China. The DIN retention capacities and pathway of a reference upstream Reach A, a wastewater-impacted Reach B and an effluent-dominated Reach C were quantified using the model results after a Bayesian approach for parameter estimation and uncertainty analysis. The results showed that the untreated wastewater discharge elevated the assimilatory uptake rate but lowered its efficiency in Reach B; while the WWTP effluent discharge elevated both denitrification rate and efficiency and made Reach C a denitrification hotspot with increased nitrate concentration and hypoxic environment. The effects of the point-source inputs on the DIN retention pathways (assimilatory uptake vs. denitrification) were regulated by their impacts on river metabolism. Despite different pathways, the total DIN retention ratios of Reaches A, B and C under low-flow conditions were 30.3% km-1, 14.3% km-1 and 6.5% km-1, respectively, which indicated the instream DIN retention capacities were significantly impaired by the point-source inputs. This result suggests that the DIN discharged from point-source inputs to urban rivers will be transported downstream with the potential to create long-term ecological implications not only locally but also regionally.
Microbial decomposition of carbon and biogenic methane in coal is one of the most important issues in CBM exploration. Microbial C-N-S functional genes in different hydraulic zones of high rank coal reservoirs was studied, demonstrating high sensitivity of this ecosystem to hydrodynamic conditions. The results shows that hydrodynamic strength of the 3# coal reservoir in Shizhuangnan block gradually weakened from east to west, forming the transition feature from runoff area to stagnant area. Compared with runoff area, the stagnant area has higher reservoir pressure, gas content and ion concentrations. The relative abundance of genes associated to C, N and S cycling was increased from the runoff area to stagnant area, including cellulose degrading genes, methane metabolism genes, N cycling genes and S cycling genes. This indicates that the stagnant zone had more active microbial C-N-S cycle. The machine learning model shows that these significantly different genes could be used as effective index to distinguish runoff area and stagnant area. Carbon and hydrogen isotopes indicate that methane in the study area was thermally generated. The methanogens compete with anaerobic heterotrophic bacteria to metabolize limited substrates, resulting in low abundance of methanogens. Meanwhile, the existence of methane oxidizing bacteria suggests biogenic methane was consumed by methanotrophic bacteria, which is the main reason why biogenic methane in the study area was not effectively preserved. In addition, weakened hydrodynamic conditions increased genes involved in nutrient cycling contributed to the increase of CO2 and consumption of sulfate and nitrate from runoff area to stagnant area.
One of the largest tropical tidal ranges in the world occurs in King Sound, a semi-enclosed embayment in the tropical Kimberley region of Western Australia. Incubations of phytoplankton within King Sound displayed reduced photosynthetic efficiency, elevated maximum photosynthetic rates, and no measurable photo-inhibition. A response typical of high light adapted phytoplankton despite decreased water clarity and low ambient nutrient concentrations in the estuary. This is in contrast with the adjacent shelf where phytoplankton, associated with a deep chlorophyll maximum, display high photosynthetic efficiency, and strong light inhibition typical of low light adaptation. Remote sensing and numerical modelling suggest that spatial and temporal variations in tidal mixing drive changes in light variability and in photo-acclimation. In King Sound phytoplankton experience the largest variations in light over short timescales where diatoms dominate since they can rapidly acclimate to water column light conditions by adjusting pigment within the cell. The photo-physiological response of the phytoplankton in the Sound, suggests that acclimation to alternate weak and strong mixing exposes them to cyclical changes in light intensity delaying the onset of photo-inhibition, allowing higher maximum photosynthetic rates to be attained. These findings highlight the importance of a multifaceted approach to understanding the links between physics and photo-acclimation strategies employed by phytoplankton to more accurately determine rates of depth-integrated productivity in complex coastal areas.
Experimental studies of the interactions between biomolecules and minerals under conditions simulating harsh planetary environments provide key insights into possible prebiotic processes and the search for life. Despite protection from cosmic rays, UV, and oxidative degradation, buried biosignatures may undergo diagenetic processes that decrease the concentration of organic matter. Additionally, other degradation mechanisms occur as a result of elevated temperatures, pressures, mineral-organic interactions, and fluid/brine processes. In this study, we aim to provide a fuller understanding of preservation potential by considering several variables, including pressure, temperature, the mineral matrix environment, and fluid chemistry (salinity, pH, composition). This research expands previous anhydrous work to investigate the influence of lower pressure regimes, especially in a combined fluid/brine environment with various mineral matrices. To test the preservation potential of various biomolecules, we subjected samples to temperature, pressure, fluid, and mineral matrix conditions representative of different environmental stressors. The starting materials included: 1) isolated organic compounds added to various mineral standards, 2) An endolithic and microbe-rich natural calcite deposited from a CO2-rich hot spring, 3) cyanobacteria necromass. Experiments were conducted in three different devices 1) a piston-cylinder press reaching up to 15 kbar and 550 °C, 2) high-volume batch reaction vessels generating up to 15 MPa pressure and 80 °C, and 3) ambient pressure, high temperature furnaces. Samples were analyzed by GC-MS and LC-MS, while ICP-MS, XRD, and Raman were used for additional characterization. The influence of pressure can be clearly identified. Similarly, fluid transport, complex thermal degradation, and oxidation mechanisms are identified.
In both natural and built environments, microbes on occasions manifest in spherical aggregates instead of solid-affixed biofilms. These microbial aggregates are conventionally referred to as granules. Cryoconites are mineral rich granules that appear on glacier surfaces and are linked with expanding surface darkening, thus decreasing albedo, and enhanced melt. The oxygenic photogranules (OPGs) are organic rich granules that grow in wastewater with photosynthetic aeration and present potential for net autotrophic wastewater treatment in a compact system. Despite obvious differences inherent in the two, cryoconite and OPG pose striking resemblance. In both, the order Oscillatoriales in Cyanobacteria envelope inner materials and develop dense spheroidal aggregates. We explore the mechanism of photogranulation on account of high similarity between cryoconites and OPGs. We contend that there is no universal external cause for photogranulation. However, cryoconites and OPGs, as well as their intra variations, which are all are under different stress fields, are the outcome of universal physiological processes of the Oscillatoriales interfacing goldilocks interactions of stresses, which select for their manifestation as granules. Finding the rules of photogranulation may enhance engineering of glacier and wastewater systems to manipulate their ecosystem impacts.
With a warming climate, solar activity including Sunspot Number (SSN) and large-scale climate phenomena including EI Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Arctic Oscillation (AO) have induced changes in climate extremes and changes in the hydrological cycle in arid-semiarid regions of the world, thus a detailed investigation of climate variability can play a key role in water resources management, drought monitoring, ecological restoration and sustainable development. In this study, we used wavelet coherence (WTC) based on continuous wavelet transform (CWT) to assess the impacts of SSN, ENSO, PDO and AO on multiple interacting hydrological processes and identify the teleconnection patterns and lead-lag relationships between the four principal modes and changes in extreme temperature and precipitation events, meteorological drought, and streamflow variability in Xinjiang, an arid-semiarid region of China. The results indicated that solar activity and climatic oscillations were viewed as the primary drivers for periodic variation of extreme temperature events and the evolution of drought in Xinjiang. For instance, the ENSO positively affected warm extremes with intermittent coherence in the 2–6-year band during 1984–2000 and had negative correlations with cold extremes in the 2–6-year band at the interannual scale. Compared with warm extremes, variability in cold extremes was much more sensitive to the activity pattern of AO. It was clear that the coherence of temperature variables in Xinjiang with PDO was weaker than that with ENSO and AO, and there was a nonsignificant covariance between PDO and extreme temperature events. In addition, the WTC spectra showed that teleconnection factors including solar activity and three large-scale climate phenomena had significant impacts on annual and monthly drought evolution, and AO had the strongest influence on annual standardized precipitation evapotranspiration index (SPEI) values. In general, compared with SSN, ENSO, PDO and AO all showed clear leading effects on precipitation extremes variability and annual streamflow variability for a specific time and frequency, and solar activity’s influences might be transferred by ENSO to precipitation extremes or streamflow variability at the 2–7-year band. Overall, the warming and wetting trend in Xinjiang may be a local manifestation of global multivariate climate change. Thus, our findings will have important implications for designing best practice strategies for water resource management and ecological restoration in similar arid-semiarid basins around the world.
There has been a significant increase in the amount and accuracy of mineral data (from resources like Mindat, MED or the GEMI) and the improvements in technological resources make it possible to explore and answer large, outstanding scientific questions, such as, understanding the mineral assemblages on Earth and how they compare to assemblages and localities on other planets. In the last couple of years, affinity analysis methods have been used to:1) Predict unreported minerals at an existing locality, 2) Predict localities for a set of known minerals. We’ve chosen to call this application “Mineral Association Analysis”. Affinity analysis is an unsupervised machine learning method that uses mined association rules to find interesting patterns in the data. Most of the metrics used to evaluate market basket analysis methods focus on either the ability of the model to ingest large amounts of data, or using a metric based comparison of various algorithms used for association rule mining, or on evaluating the rules mined to more efficiently generate association rules. However, when patterns generated in an unsupervised method are used to predict the occurrences of entities such as minerals, there needs to be a way to evaluate the predictions made by the model. It’s in such an area that there has been very little work. In this abstract, we explore the development of a new method to evaluate the results of association rule mining algorithms specifically when used when the association rules generated are utilized in a predictive setting.  Prabhu et. al (2019). In AGU Fall Meeting Abstracts (EP23D-2286).  Morrison et al. Nat. Geo. (2021) In Prep.  Agrawal et al. (1993) SIGMOD’93.  Sharma et al. (2012) IJERT 1(06).  Üstündağ and Bal (2014) Proc. in Comp.
Soil carbon cycling and ecosystem functioning can strongly depend on how microbial communities regulate their metabolism and adapt to changing environmental conditions to improve their fitness. Investing in extracellular enzymes is an important strategy for the acquisition of resources, but the principle behind the trade-offs between enzyme production and growth is not entirely clear. Here we show that the enzyme production rate per unit biomass may be regulated in order to maximize the biomass specific growth rate. Based on this optimality hypothesis, we derive mathematical expressions for the biomass specific enzyme production rate and the microbial carbon use efficiency, and verify them with experimental observations. As a result of this analysis, we also find that the optimal enzyme production rate decays hyperbolically with the soil organic carbon content. We then show that integrating the optimal extracellular enzyme production into microbial models may change considerably soil carbon projections under global warming, underscoring the need to improve parameterization of microbial processes.
Model projections predict tropical forests will experience longer periods of drought and more intense precipitation cycles under a changing climate. Such transitions have implications for structure-function relationships within microbial communities. We examine how chronic drying might reshape prokaryotic and fungal communities across four lowland forests in Panama with a wide variation in mean annual precipitation and soil fertility. Four sites were established across a 1000 mm span in mean annual precipitation (2335 to 3300 mm). We expected microbial communities at sites with lower MAP to be less sensitive to chronic drying than sites with higher MAP; while fungal communities to be more resistant to disturbance than prokaryotes. At each location, partial throughfall exclusion structures were established over 10 x 10 m plots to reduce direct precipitation input. After the first nine months of throughfall exclusion, prokaryotic communities showed no change in composition. However, 18 months of throughfall exclusion resulted in markedly divergent prokaryotic community responses, reflecting MAP and soil fertility. We observed the emergence of a “drought microbiome” within infertile sites, whereby the community structure of the experimental drying plots at the lower MAP sites diverged from their respective control sites and converged towards overlapping assemblages. Furthermore, taxa increasing in relative abundance under throughfall exclusion at the highest MAP became more similar to taxa characteristic of the control plots at the lowest MAP site, suggesting a shift toward communities with life-history traits selected 1
The stable hydrogen isotope composition of persistent biomolecules is used as a paleoenvironmental proxy. While much previous work has focused on plant leaf wax-derived n-alkanes, the potential of bacterial and archaeal lipid biomarkers as carriers of H isotope signatures remains underexplored. Here we investigated H isotope distributions in the membrane lipids of the ammonia-oxidizing chemoautotroph Nitrosopumilus maritimus strain SCM1. Hydrogen isotope ratios were measured on the biphytane chains of tetraether membrane lipids extracted from steady-state continuous cultures cultivated at slow, medium, and fast growth rates. In contrast to recent work on bacterial fatty acids, where the direction and magnitude of isotopic fractionation varies widely (ca. 600 ‰ range) in response to the choice of substrate and pathways of energy metabolism, archaeal biphytane data in the present work are relatively invariant. The weighted average 2H/1H fractionation values relative to growth water (2εL/W) only ranged from 272 to 260 ‰, despite a three-fold difference in doubling times (30.8 hr to 92.5 hr), yielding an average growth-rate effect of 0.2 ‰ hr-1. These 2εL/W values are more depleted than all heterotrophic archaeal and bacterial lipid H isotope measurements in the literature, and on par with those from other autotrophic archaea, as well as isoproenoid-based lipids in photoautotrophic algae. N. maritimus values of 2εL/W also varied systematically with the number of internal rings (cyclopentyl + cyclohexyl), increasing for each additional ring by 6.4 ± 2.7 ‰. Using an isotope flux-balance model in tandem with a comprehensive analysis of the sources of H in archaeal lipid biosynthesis, we use this observation to estimate the kinetic isotope effects (KIEs) of H incorporation from water; from reducing cofactors such as ferredoxin, and for the transhydrogenation reaction(s) that convert the electron-donor derived NADH into NADPH for anabolic reactions. Consistent with prior studies on bacteria, our results indicate the KIEs of reducing cofactors and transhydrogenation processes in archaea are highly fractionating, while those involving exchange of water protons are less so. When combined with the observation of minimal growth-rate sensitivity, our results suggest biphytanes of autotrophic 3HP/4HB Thaumarchaeota may be offset from source waters by a nearly constant 2εL/W value. Together with the ring effect, this implies that all biphytanes originating from a common source should have a predictable ordering of their isotope ratios with respect to biphytane ring number, allowing precise reconstruction of the original δ2H value of the growth water. Collectively, these patterns indicate archaeal biphytanes have potential as paleo-hydrological proxies, either as a complement or an alternative to leaf wax n-alkanes.
Documentation of primitive terrestrial life signatures and the methods used to characterize them will be relevant to identify signatures of biological origin at the surface of Mars. In this respect, fossils of chemolithotrophic microorganisms found in ancient volcanic rocks on Earth are used as analogues for the kinds of microorganisms that could be found on Mars. Indeed, chemolithotrophs are the ancient forms of life thought to inhabit the first habitats on Earth and potentially on Mars when the two planets had similar conditions in their early history, i.e. an atmosphere, geological activity and enough liquid water in specific habitable localities for a prolonged period of time.