nai

Executive Summary

Our research activities in Year 2 of CAN-6 were focused on four themes, all of which broadly fall within the team’s efforts in pushing new approaches to habitability, life detection, and signatures of life: 1) Analog settings and the biomolecules of life; 2) Experimental studies of paleoenvironmental and biological proxies; 3) Hadean, Archean, and Proterozoic environments and biosphere; and 4) Building the Astrobiology infrastructure. In Year 2 we significantly expanded our research portfolio from 16 projects in Year 1 to 21 projects in Year 2, as student, post-doc, and staff hiring efforts were completed.

Theme 1: Analog settings and the biomolecules of life

Six projects were pursued under this theme. In Project 1A: Field analog Geology and Astrobiology in support of Mars exploration, Co-I Pascale Ehrenfreund obtained preliminary results from a field research campaign in the area around the Mars Desert Research Station (MDRS) in Utah, a region that has been characterized as geomorphologically and geochemically similar to Mars. FTIR, XRD, and SEM-EDX analysis shows a wide range in compositions, both in terms of mineralogy, volatiles, and organic carbon, and quantifying these to calibrate spectral data taken from Mars will be essential in planning future Mars missions. In Project 1B: Photostability of pigments and amino acids in space environment on the International Space Station, Co-I Pascale Ehrenfreund describes the on-going work with the EXPOSE-R2 space exposure facility on the International Space Station (ISS). The goal of this work is to measure the photostability and photochemical alteration of organic compounds in actual space environments to fully understand the potentially destructive effects of space radiation, particularly from the VUV (vacuum ultraviolet) spectral region. The EXPOSE-R2 facility was launched on July 24th, 2014 to the ISS and was activated in November 2014. EXPOSE-R2 will remain in Low Earth Orbit (LEO) for 12-18 months. In Year 2, Ehrenfreund’s team performed the pre-flight UV/Vis and FTIR measurements needed on pigments, amino acids and mineral samples that are part of EXPOSE-R2.

Co-I Bill Schopf has been studying ways to use the thermal stability of ancient enzymes to infer paleotemperatures. In Project 1C: Studies of early-evolved enzymes in modern organisms may reveal the history of Earth’s ambient temperature over geological time, Schopf reports initial efforts to determine the temperature stability of enzymes associated with biomolecules that have well understood age origins, including cyanobacterial Rubisco (~3000 Ma), metazoan collagen (~700 Ma), vascular plant lignin (~420 Ma), and flowering plant triterpenoids (~100 Ma).

Three projects were aimed at understanding biogeochemical cycling in modern environments that are analogs to Mars. In a study of a modern biogeochemical system, Co-I Eric Roden reports on progress for Project 1D: Iron biogeochemistry in Chocolate Pots hot spring, Yellowstone National Park, where studies of the reactive Fe inventories, Fe speciation, and Fe isotope compositions document a significant role for microbial Fe(III) reduction. Importantly, the Fe mass balance indicates that microbial Fe(III) reduction represents one component of an “internal Fe cycle” in a hot spring environment, suggesting that a wide range of biologically produced Fe components may be produced in circumneutral hot spring environments. In a related study, Project 1E: Microbial communities in Chocolate Pots hot spring, Yellowstone National Park, Co-I Eric Roden discusses new genomic data that document a wide diversity of prokaryotic taxa, including both Bacteria and Archaea, many of which are only distantly related to known taxa. These results show, for the first time, that both Fe(II)-oxidizing and Fe(III)-reducing microbial communities can exist in circumneutral hot spring environments. Turning to cold analog environments, Co-I Eric Boyd reports new results from Project 1F: Chemolithotrophic microbial communities in subglacial sediments, where his group studied chemolithoautotrophs involved in pyrite oxidation in the subglacial environment of Robertson Glacier, Alberta, Canada. These results document that the energetics of FeS₂-based chemosynthetic life in cold-dark environments is sufficient to extensively cycle both Fe and S.

Theme 2: Experimental studies of paleoenvironmental and biological proxies

Seven projects were pursued under this theme. Co-I Huifang Xu led two projects aimed at understanding the origin of dolomite, which, despite its great abundance in the geologic record, is very difficult to produce under laboratory conditions. In Project 2A: The catalysis effect of extracellular polymeric substances excreted by fermentative bacteria on Ca-Mg carbonate precipitation, Xu’s team showed that polysaccharides have a catalytic effect on Ca-Mg carbonate crystallization, and found evidence that anaerobic fermenting bacteria may have a role in sedimentary dolomite formation. In addition, Xu’s group reports the results of modeling efforts aimed at understanding dehydration of Mg²⁺, an important step in dolomite formation, where, in Project 2B: Modeling the role of adsorbed hydrogen sulfide in weakening Mg-water complex on dolomite (104) surface, they found that sulfide sorption changes the electrostatic properties of dolomite surfaces such that Mg²⁺ dehydration is enhanced.

Three projects were aimed at understanding the isotopic properties of carbonates for use in paleothermometry or tracers of weathering and microbial cycling. In Project 2C: Developing the ¹³C-¹⁸O clumped isotope thermometer, PI Clark Johnson led an effort to calibrate this thermometer, which has the advantage over “traditional” isotopic thermometers in that the temperature dependence of ¹³C-¹⁸O clumping is independent of the fluid composition. The new approach taken in calibrating this thermometer has been use of seed crystals to minimize kinetic isotope effects; the results demonstrate that such effects can be minimized in experimental calibrations. In Project 2D: Magnesium isotopes in carbonates as a tracer of marine conditions in the early Earth, Co-I Brian Beard’s team studied stable Mg isotope fractionations in dolomite, and demonstrated a clear correlation with temperature. With this new calibration, stable Mg isotopes in carbonates hold promise in understanding weathering and hydrothermal fluxes in the ancient oceans. Co-I Max Coleman led an effort that takes a new view of carbonate-associated sulfate, which traditionally has been used to infer seawater sulfate compositions. In Project 2E: Carbonate-associated sulfate (CAS) as a tracer of ancient microbial ecosystems, however, Coleman showed unusual S and O isotope compositions of CAS can identify sediment-based sulfate reduction in isolation from the ambient oceans.

Co-PI Brian Beard led an experimental study of Si isotope fractionation between aqueous Si and Fe-Si gels, under conditions appropriate for the Archean oceans. As described in Project 2F: Silicon isotopes as a tracer of the coupled Fe-Si cycle in the Archean, Beard’s group demonstrated that Si isotope exchange is fairly rapid, even at low temperatures, and that the effect of Fe on Si isotopes is significant, both in terms of the kinetics of isotopic exchange and the magnitude of the fractionation factor. These results will be critical in understanding the processes involved in producing Archean jaspers and iron formations.

In Project 2G: Iron isotope fractionations among oxide minerals under acidic conditions, PI Clark Johnson describes the results of experiments on the stable Fe isotope fractionation and exchange kinetics for Fe(III) hydroxide (goethite) under low-pH conditions. Such conditions have not been previously studied for Fe isotopes, but are important for understanding hydrothermal environments. Johnson’s team found that as pH decreases, isotopic exchange also decreases, indicating a control by the extent of sorbed Fe(II).

Theme 3: Hadean, Archean, and Proterozoic environments and biosphere

Six projects were pursued under this theme. The lack of preservation of significant crust on Earth prior to ~3.5-3.7 Ga makes understanding the early impact history of the Earth difficult, but Co-I Aaron Cavosie describes, in Project 3A: Searching for ancient impact events through detrital shocked zircons, a new approach of looking for shocked (impact) zircons in detrital mineral populations. Cavosie completed an initial survey of detrital zircons the Yilgarn craton (Australia), North China craton (China), Wyoming craton (USA), and the Superior craton (Canada), but, so far, zircons with shocked microstructures were not found. His search continues. Turning to the well-preserved 3.4 Ga rocks in the Pilbara craton (Australia), Co-I John Valley discusses research aimed at determining the origin of Archean cherts in Project 3B: Genesis of high-δ¹⁸O Archean chert, Pilbara craton, Australia. It has been commonly assumed that the most “primary” cherts, that is those that most closely reflect equilibrium with ancient seawater, as those that have the highest δ¹⁸O values, yet careful field and petrographic studies coupled to in situ O isotope analysis paints a more complex picture, indicating that very careful study is required before inferring ancient seawater conditions. The question of the extent of continental weathering early in the Archean has recently gained attention due to issues of nutrient limitation and its effects on the early biosphere, which include questions on the availability of phosphorus, an element sourced to continental weathering. In a new project not included in the original proposal, PI Clark Johnson reports, in Project 3C: The role of early continental weathering in providing a habitable planet, on new results using the ⁸⁷Rb-⁸⁷Sr isotope system that suggest much more extensive continental weathering in the early Archean than previously thought. Although a large effort has been invested in research on the evolution of photosynthesis in the Archean, there has been relatively little attention given to biological iron cycling in the early Archean, despite the fact that Fe-based metabolisms are deeply rooted in the ¹⁶S/¹⁸S tree of life. In Project 3D: A microbial iron shuttle in early Earth marine basins, PI Clark Johnson discusses new results that document a vigorous microbially-driven iron cycle in the ~3 Ga Pongola Basin (South Africa), and this work demonstrates that Fe-based metabolisms did indeed produce a large “footprint” in marine systems in the Archean. Turning to the Proterozoic, Co-I Bill Schopf reports on two projects aimed at understanding the biosphere after the Great Oxidation Event, leading up to the Cambrian explosion. In Project 3E: Sulfur-cycling fossil bacteria from the 1.8 Ga Duck Creek Formation provide promising evidence of evolution’s null hypothesis, Schopf addresses an important issue for evolution: are there species where essentially no evolution has occurred when there is no environmental drivers for change. Based on studies of the 1.8 Ga Duck Creek Formation (Australia), Schopf looked at sulfur-cycling microbes and compared these to modern species, noting their remarkable similarity. Schopf argues that this provides support for Darwin’s “null hypothesis”. Moving to the latest Proterozoic, Schopf reports on a second research effort, Project 3F — Apatitic latest Precambrian and Early Cambrian fossils provide direct evidence of concentrations of environmental oxygen, where he is developing an innovative way to estimate oxygen contents based on trace-element substitution in apatite, a mineral that is common in permineralization of fossils. If this method proves successful, Schopf will apply this to understanding the rise of oxygen before the Cambrian explosion.

Theme 4: Building the Astrobiology infrastructure

Commensurate with the charge that NAI teams contribute to building the astrobiology infrastructure, we report on two projects that was pursued in Year 2 aimed at increasing the astrobiological capabilities of instrumentation. In _Project 4A: New in situ techniques (CLSM and Raman) solve the problem presented by the disaggregation of acid-macerated organic-walled microfossils_, Co-I Bill Schopf reports on new methods for non-destructive in situ analysis of organic carbon, including microfossils, using confocal laser scanning microscopy (CLSM), and Raman spectroscopy, and these methods will likely be essential in analysis of samples returned from Mars that will be highly limited in quantity. In a second project aimed at maximizing analysis capability for samples returned from planetary missions, Co-I John Valley reports on new efforts to determine four sulfur isotopes by in situ methods. Mass-independent sulfur isotope compositions have been prominent in discussions on the evolution of oxygen in the atmosphere on Earth, but analysis of the rare isotopes ³³S and ³⁶S can be challenging using in situ methods. In _Project 4B: New SIMS procedures for in situ analysis of mass-independent fractionation of S isotopes_, Valley discusses new approaches for determining accurate and precise ³²S-³³S-³⁴S-³⁶S analysis.