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Executive Summary

Biosignatures from Earth and Beyond

The Penn State Astrobiology Research Center pushed ahead with notable advances in several very exciting areas. Overall, we continue to have a focus on developing new biosignatures, searching for biosignatures in relevant microbial environments, searching for biosignatures in ancient rocks, and the discovery and exploration of exoplanets. Our efforts focus on creating innovative approaches for the analyses of cells and other organic material, finding ways in which metal abundances and isotope systems reflect life, and developing creative approaches for using environmental DNA to study present and past life. Also, the Earth’s Archean and Proterozoic eons offer the best opportunity for investigating a microbial world, such as might be found elsewhere in the cosmos. The ancient record on Earth provides an opportunity to see what geochemical signatures are produced by microbial life and how these signatures are preserved over geologic time. We have been investigating microbial life in some of Earth’s most mission-relevant ecosystems with a current focus on both biosignatures of life in ancient stratified ocean analogs and biosignatures of life in extremely energy-limited environments. Finally, our center has had great recent successes with the discovery and characterizations of exoplanets.

Developing New Biosignatures

One of our tasks right now is the use of DNA as a biosignature. We continue to analyze DNA recovered from multiple horizons of deeply-buried sediment from both the Costa Rica Margin and Hydrothermal Mounds near Okinawa. We have numerous horizons with extracted and sequenced DNA from Okinawa hydrothermal mound, Costa Rican Margin, and the Equatorial Pacific. DNA analysis (genetics and genomics) are proceeding. In general, the analysis have revealed that (1) Archaea make up a substantial proportion of the subsurface biosphere, (2) Chlorofexi make up a significant portion of the bacterial populations, and (3) some notable microbial groups increase as the sediment temperatures increase.

Another significant task is investigating the microorganisms in methane seep environments. Microbial aggregate abundance within the carbonate interior exceeds that of seep sediments, and molecular diversity surveys reveal methanotrophic communities within protolithic nodules and well-lithified carbonate pavements. Aggregations of microbial cells within the carbonate matrix actively oxidize methane as indicated by stable isotope FISH–nanoSIMS experiments and ¹⁴CH₄ radiotracer rate measurements. Carbonate-hosted methanotrophy extends the known ecological niche of these important methane consumers and represents a previously unrecognized methane sink that warrants consideration in global methane budgets. Also, applying compound-specific carbon isotopic analysis to seep sediments, we have found over 100 permit ¹³C range for molecules derived from methane-oxidizing archaea. This is a significant results because it indicates that geochemical signatures of methane seep microbes can preserve signatures including both lipid biomarkers and isotope heterogeneity. Also related to biosignature preservation, our team has been investigating tetragonal microstructures present in the carbonates that resemble seep-endemic Methanosarcinales cell clusters. Despite morphological similarities to the seep-endemic microbes that likely mediated the authigenesis of Eel River Basin carbonates and sulfides, detailed petrographic, SEM, and magnetic microscopic imaging, remnant rock magnetism, laser Raman, and energy dispersive X-ray spectroscopy, suggest that these microstructures are not microfossils, but rather mineral structures that result from the diagenetic alteration of euhedral Fe-sulfide framboids. Electron microscopy shows that during diagenesis, reaction rims composed of Fe oxide form around framboid microcrystalites.

Recently, our work has extended beyond preservation of methane seep microbiota on Earth to the preservation of such a community on Mars. Seven distinct fluids representative of putative martian groundwater were used to calculate Gibbs energy values in the presence of dissolved methane under a range of atmospheric CO₂ partial pressures. In all scenarios, AOM is exergonic, ranging from – 31 to – 135 kJ/mol CH₄. A reaction transport model was constructed to examine how environmentally relevant parameters such as advection velocity, reactant concentrations, and biomass production rate affect the spatial and temporal dependences of AOM reaction rates. Two geologically supported models for ancient martian AOM are presented: a sulfate-rich groundwater with methane produced from serpentinization by-products, and acid-sulfate fluids with methane from basalt alteration. The simulations presented in this study indicate that AOM could have been a feasible metabolism on ancient Mars, and fossil or isotopic evidence of this metabolic pathway may persist beneath the surface and in surface exposures of eroded ancient terrains.

Another portion of our research involves rock weathering to soil. This year, we have made very good progress on our investigations of Fe mobility and isotopes in soil profiles on diabase and basalt on Earth as a way of understanding weathering on Mars.

In looking at potential biosignatures, we have also needed to further understand prebiotic chemistry. If molecules are readily formed and preserved in the absence of life, then they are not reliable biosignatures. In extending known prebiotic chemistry, we found pyridine carboxylic acids in CM2 carbonaceous chondrites. These compounds could be potential precursor molecules for ancient coenzymes. We also have found that lipid vesicles, which form can be formed fairly robustly in prebiotic experiments, protect activated acetic acid. Methyl thioacetate, or activated acetic acid, has been proposed to be central to the origin of life and an important energy currency molecule in early cellular evolution. Our recent work suggests that the hydrophobic regions of prebiotic vesicles and early cell membranes could have offered a refuge for this energetic molecule, increasing its lifetime in close proximity to the reactions for which it would be needed. This model of early energy storage evokes an additional critical function for the earliest cell membranes.

Also, part of our team, J. W. Schopf and co-workers have described a chert-permineralized fossilized sulfuretum from the ~1.8 Ga Duck Creek group of Western Australia. These microfossils demonstrate very striking resemblance to modern microbes in the same type of ecosystem. The Schopf group has also shown that ~775 Ma pyritized microbes are anaerobic sulfur-circlers.

Biosignatures in Relevant Microbial Environments

Our work this year on biosignatures in relevant microbial environments has focused on two specific areas: biosignatures of life in extremely energy-limited environments and biosignatures of life in ancient stratified ocean analogs.

In our work on extremely energy-limited environments, we have found geochemical niches of various Fe-oxidizing acidophiles in an acidic coal mine drainage environment. The distribution of microorganisms at the two acid mine sites could be best explained by a combination of iron(II) concentration and pH. Populations of the Gallionella-like organism were restricted to locations with pH >3 and iron(II) concentration >4 mM, while Acidithiobacillus spp. were restricted to pH <3 and iron(II) concentration <4 mM. We have also used metagenomic sequencing to gain insight into sulfur precipitation in a terrestrial subsurface lithoautotrophic ecosystem. The Frasassi and Acquasanta Terme cave systems in Italy host isolated lithoautotrophic ecosystems characterized by sulfur-oxidizing biofilms with up to 50% sulfur by mass. We retrieved four nearly-complete genomes of Sulfurovum-like organisms as well as a Sulfuricurvum spp. Analyses of the data indicate that the Epsilonproteobacteria are autotrophic and therefore provide organic carbon to the isolated subsurface ecosystem. Multiple homologs of sulfide-quinone oxidoreductase (Sqr), together with incomplete or absent Sox pathways, suggest that cave Sulfurovum-like Epsilonproteobacteria oxidize sulfide incompletely to elemental sulfur, consistent with previous evidence that they are most successful in niches with high dissolved sulfide to oxygen ratios. In contrast, we recovered homologs of the complete complement of Sox proteins affiliated Gamma-proteobacteria and with less abundant Sulfuricurvum spp. and Arcobacter spp., suggesting that these populations are capable of the complete oxidation of sulfide to sulfate. Similarly, we made good progress on showing there is a biogeographical distribution of Acidithiobacillus populations in extremely acidic subaerial cave biofilms.

In our work on life in ancient stratified ocean analogs, we have explored a Proterozoic-analog microbial ecosystem discovered at Little Salt Spring (FL) in 2011. It has unique potential to help us understand biosignatures of earth evolution, specifically the long delay in the continued oxidation of the surface earth after the initial rise at the Great Oxidation Event (GOE, ~2.4 Ga). The sinkhole is weakly stratified and has low concentrations of oxygen, sulfide and sulfate throughout the water column, similar to the geochemistry hypothesized for Proterozoic oceans. The sinkhole hosts a mixed oxygenic/anoxygenic community of microbial phototrophs. This site thus provides an opportunity to investigate biogeochemical and microbial processes in a Proterozoic ocean analog, including the ecological and geochemical controls on oxygen production and organic matter preservation. In addition, the microbial ecosystem at the site produces large quantitites of hopanoids, an important class of organic biomarkers that can be preserved in rocks for billions of years. A second major field campaign in early Nov. 2014 was spearheaded by Macalady’s group in collaboration with two scientists from the Max Planck Institute for Marine Microbiology in Bremen, Germany, and two European scientific and technical divers from the Hydra Institute who are experts in the underwater deployment of microsensors. Microsensor measurements were carried out both for in situ microbial mats and mat samples maintained in the field lab at the site. These data are currently being used as input to a redox and oxygen budget for the mat ecosystem.

Biosignatures in Ancient Rocks

The Kump group is curently analyzing FAR-DEEP cores that span the putative “oxygen overshoot” associated with the termination of the Great Oxidation Event, 2.0 billion years ago. The volcanic rocks in question are highly oxidized. Our hypothesis is that oxygen-enriched groundwaters altered these rocks during a time interval when atmospheric oxygen concentrations approached modern levels, falling subsequently to lower values characteristic of the ensuing billion years. Kump has also proposed a new explanation for the “second rise of atmospheric oxygen” in the Neoproterozoic (ca. 850 Ma).

The Kasting group is looking at hydrodynamic escape of hydrogen from H₂- or H₂O-rich primitive atmospheres. They are developing a two-component model to describe this process. Old single-component models evidently do not obey the diffusion limit, so are are trying to remedy that.

Biosignatures in Extraterrestrial Environments

We have worked to improve the velocity precision of the HET spectrograph to the level where it can do Kepler followup and detect low mass planets orbiting the nearest stars. We have identified the dominant sources of error in the iodine technique at HET and Keck, and are working to mitigate them. Ming Zhao and Sharon Wang have obtained echelle spectra at high resolution (R~450,000) of in-use iodine cells, including the one at HET. These spectra, made at multiple temperatures will finally allow us to determine the sources of variation in iodine spectra taken at Fourier Transform Spectrographs, and remove modeling the iodine cells from the Doppler error budget at HET and Keck. We have discovered that the dominant source of noise at Keck Observatory is the presence of weak telluric features in the optical spectra. These features are usually masked out of all Doppler analysis, but apparently the weakest lines remain at a level sufficient to be the “tallest tent pole” in the error budget. Sharon Wang is implementing new code to remove them completely and improve the precision there, improving our sensitivity to low-mass planets.

We have published a hypothesis regarding the origin of the Lunar Farside Highlands, with implications for the giant impact hypothesis and lunar formation. The lunar farside highlands problem refers to the curious and unexplained fact that the farside lunar crust is thicker, on average, than the nearside crust. Here we recognize the crucial influence of Earthshine, and propose that it naturally explains this hemispheric dichotomy. Since the accreting Moon rapidly achieved synchronous rotation, a surface and atmospheric thermal gradient was imposed by the proximity of the hot, post-giant impact Earth. This gradient guided condensation of atmospheric and accreting material, preferentially depositing crust-forming refractories on the cooler farside, resulting in a primordial bulk chemical inhomogeneity that seeded the crustal asymmetry. Our model provides a causal solution to the lunar highlands problem: the thermal gradient created by Earthshine produced the chemical gradient responsible for the crust thickness dichotomy that defines the lunar highlands.

The Penn State Habitable Planet Finder is currently being designed and built including progress on very high levels of temperature and pressure control. Work by Mahadevan and Robertson has shown that stellar activity can masquerade as planet RV signatures (GLiese581d). The M dwarf star Gliese 581 was believed to host four planets, including one (GJ 581d) near the habitable zone that could possibly support liquid water on its surface if it is a rocky planet. Analyzing stellar activity using the Hα line, we measure a stellar rotation period of 130 ± 2 days and a correlation for Hα modulation with radial velocity. Correcting for activity greatly diminishes the signal of GJ 581d (to 1.5 standard deviations) while significantly boosting the signals of the other known super-Earth planets. GJ 581d does not exist, but is an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g.

Sigurdsson and workers have also been investigating lithopanspermia. Material from the surface of a planet can be ejected into space by a large impact and could carry primitive life-forms with it. We have performed n-body simulations of such ejecta to determine where in the Solar System rock from Earth and Mars may end up. We found that, in addition to frequent transfer of material among the terrestrial planets, transfer of material from Earth and Mars to the moons of Jupiter and Saturn is also possible, but rare. We expect that such transfers were most likely to occur during the Late Heavy Bombardment or during the ensuing 1–2 billion years. At this time, the icy moons were warmer and likely had little or no ice shell to prevent meteorites from reaching their liquid interiors. We also note significant rates of re-impact in the first million years after ejection. This could re-seed life on a planet after partial or complete sterilization by a large impact, which would aid the survival of early life during the Late Heavy Bombardment.

Ford’s group is analyzing astronomical observations to characterize the architectures of individual planetary systems and to develop a statistical understanding of the distribution of planet and planetary systems. Graduate student Benjamin Nelson is characterizing the architecture of individual planetary systems with Doppler observations, and performing exploratory statistical analyses of Doppler observations as part of a larger effort to improve the precission of Doppler observations. Graduate student Shabram is focusing on the orbital eccentricity distribution of Kepler’s planet candidates. Graduate student Robert Morehead is investigating Kepler’s multiple transiting planet candidate systems to compute false alarm probabilities, confirm planets and characterize the distribution of mutual inclinations in planetary systems with multiple planets. Research associate Daniel Jontof-Hutter is characterizing the masses and orbits of planetary systems with transit timing variations detected by Kepler.