Henry J. Sun

Application of Fe isotopes to tracing the geochemical and biological cycling of Fe

Over 100 high-precision Fe isotope analyses of rocks and minerals are now available, which constrain the range in d 56 Fe values (per mil deviations in 56 Fe/ 54 Fe ratios) in nature from � 2.50xto +1.5x. Re-assessment of the range of d 56 Fe values for igneous rocks, using new ultra-high-precision analytical methods discussed here, indicate that igneous Fe is isotopically homogeneous to F0.05x, which represents an unparalleled baseline with which to interpret Fe isotope variations in nature. All of the isotopic variability in nature lies in fluids, rocks, and minerals that formed at low temperature. Equilibrium (‘‘abiotic’’) isotopic fractionations at low temperatures may explain the range in d 56 Fe values; experimental measurements indicate that there is a large isotopic fractionation between aqueous Fe(III) and Fe(II) (DFe(III)–Fe(II)=2.75x). However, many of the natural samples that have been analyzed have an unquestionable biologic component to their genesis, and the range in d 56 Fe values are also consistent with the experimentally measured isotopic fractionations produced by Fereducing bacteria. In this work, we touch on a number of aspects of Fe isotope geochemistry that bear on its application to geochemical problems in general, and biological cycling of metals in particular. We report on new state-of-the-art Fe isotope analytical procedures, which allow precisions of F0.05x( 56 Fe/ 54 Fe) on samples <300 ng in size. In addition, we discuss the implications of experimental work on Fe isotope fractionations during metabolic processing of Fe by bacteria and the need to take a ‘‘mechanistic’’ approach to understanding the pathways in which Fe isotopes may be uniquely fractionated by biology. Additionally, we discuss experimental methods, such as the use of enriched isotope tracers that are necessary to evaluate if experimental isotope exchange reactions are transient kinetic fractionations, equilibrium isotopic exchange reactions, or a combination of both, which can be caused by the complexities of multiple isotope exchange reactions taking place in an experimental system. D 2002 Elsevier Science B.V. All rights reserved.

Isotopic fractionation between Fe(III) and Fe(II) in aqueous solutions

Abstract Large equilibrium isotope fractionation occurs between Fe(III) and Fe(II) in very dilute (≤22 mM Cl − ) aqueous solutions, reflecting significant differences in bonding environments. Separation of Fe(III) and Fe(II) is attained by rapid and complete precipitation of Fe(III) through carbonate addition, followed by separation of supernatant and ferric precipitate; experiments reported here produce an equilibrium Δ Fe(III)–Fe(II) =+2.75±0.15‰ for 56 Fe/ 54 Fe at room temperature (22±2°C). The timescales required for attainment of isotopic equilibrium have been determined by parallel isotope tracer experiments using 57 Fe-enriched iron, which are best fitted by a second-order rate law, with K =0.18±0.03 s −1 . Based on this rate constant, ∼15–20% isotopic exchange is estimated to have occurred during Fe(III)–Fe(II) separation, which contributes Fe(III)–Fe(II) . Under the experimental conditions used in this study, >97% Fe(II) exists as [Fe II (H 2 O) 6 ] 2+ , and >82% Fe(III) exists as [Fe III (H 2 O) 6 ] 3+ and [Fe III (H 2 O) 6− n (OH) n ] 3− n ; assuming these are the dominant species, the measured Fe isotope fractionation is approximately half that predicted by Schauble et al. [Geochim. Cosmochim. Acta 65 (2001) 2487–2497] at 20–25°C. Although this discrepancy may be due in part to the experimentally unknown isotopic effects of chloride interacting with Fe-hexaquo or Fe-hydroxide complexes, or directly bonded to Fe, there still appears to be at this stage a >1‰ difference between prediction and experiment.

Growth on Geological Time Scales in the Antarctic Cryptoendolithic Microbial Community

In the process of biogenous weathering of Beacon sandstone in the McMurdo Dry Valleys (Ross Desert), Antarctica, periods of microbial growth, on the time scale of 103-104 years, alternate with sudden exfoliation events. The present study addressed the question of whether microbial growth is continuous between exfoliation events or whether each exfoliation is followed by a period of comparatively rapid growth and then an extended period of steady state. The color intensity (Munsell lightness value) of the rock surface is an indicator of relative age of the crust within the exfoliation cycle, permitting measurement of changes in microbial biomass on a geological time scale. Results indicate that microbial growth is continuous and that exfoliation occurs when the microbial biomass reaches the carrying capacity of the cryptoendolithic habitat.

Fluorescence Microscopy as a Tool for In Situ Life Detection

The identification of extant and, in some cases, extinct bacterial life is most convincingly and efficiently performed with modern high-resolution microscopy. Epifluorescence microscopy of microbial autofluorescence or in conjunction with fluorescent dyes is among the most useful of these techniques. We explored fluorescent labeling and imaging of bacteria in rock and soil in the context of in situ life detection for planetary exploration. The goals were two-fold: to target non-Earth-centric biosignatures with the greatest possible sensitivity and to develop labeling procedures amenable to robotic implementation with technologies that are currently space qualified. A wide panel of commercially available dyes that target specific biosignature molecules was screened, and those with desirable properties (i.e., minimal binding to minerals, strong autofluorescence contrast, no need for wash steps) were identified. We also explored the potential of semiconductor quantum dots (QDs) as bacterial and space probes. A specific instrument for space implementation is suggested and discussed.

Communities Adjust their Temperature Optima by Shifting Producer-to-Consumer Ratio, Shown in Lichens as Models: I. Hypothesis

An apparent paradox exists in the ecology of Antarctic lichens: their net photosynthetic temperature optimum (around 0°C) lies far below the temperature optima of their constituent algae and fungi (around 20°C). To address this paradox, we consider lichens as microbial communities and propose the “community adaptation” hypothesis, which posits that in each thermal regime there is an equilibrium between photosynthetic primary producers (photobionts), and heterotrophic consumers (mycobiont and parasymbiont fungi). This equilibrium, expressed as the producer/consumer ratio (Rp/c), maximizes the fitness of the community. As respiration increases with temperature, more rapidly than does photosynthesis, Rp/c will shift accordingly in warm habitats, resulting in a high-growth temperature optimum for the community (the lichen). This lends lichens an adaptive flexibility that enables them to function optimally at any thermal regime within the tolerance limits of the constituent organisms. The variable equilibrium of producers and consumers may have a similar role in thermal adaptation of more complex communities and ecosystems.

Communities adjust their temperature optima by shifting producer-to-consumer ratio, shown in lichens as models: II. Experimental verification.

The community adaptation hypothesis [7] predicts that lichens, simple communities of microorganisms, can adapt to a wide range of thermal regimes by regulating the ratio of primary producers (algae) and consumers (fungi): Rp/c. To test this hypothesis, we determined Rp/c values by image analysis of cross sections of herbarium specimens of the lichen Cladina rangiferina, which is widely distributed between the Arctic and the tropics. We found that Rp/c for C. rangiferina increases with summer temperature by more than one order of magnitude, consistent with the hypothesis. To assess the ecological significance of community adaptation (Rp/c regulation), other adaptive mechanisms (e.g., photobiont substitution, genetic adaptation, and photosynthetic acclimation in North American Cladina spp.) were studied. Laboratory investigations with algae and fungi isolated in culture from live specimens suggested that the role of these mechanisms is relatively minor and cannot account for the high degree of lichen adaptability.

Evidence for photochemical production of reactive oxygen species in desert soils

The combination of intense solar radiation and soil desiccation creates a short circuit in the biogeochemical carbon cycle, where soils release significant amounts of CO2 and reactive nitrogen oxides by abiotic oxidation. Here we show that desert soils accumulate metal superoxides and peroxides at higher levels than non-desert soils. We also show the photogeneration of equimolar superoxide and hydroxyl radical in desiccated and aqueous soils, respectively, by a photo-induced electron transfer mechanism supported by their mineralogical composition. Reactivity of desert soils is further supported by the generation of hydroxyl radical via aqueous extracts in the dark. Our findings extend to desert soils the photogeneration of reactive oxygen species by certain mineral oxides and also explain previous studies on desert soil organic oxidant chemistry and microbiology. Similar processes driven by ultraviolet radiation may be operating in the surface soils on Mars.

Stereo-Specific Glucose Consumption May Be Used to Distinguish Between Chemical and Biological Reactivity on Mars: A Preliminary Test on Earth

Two alternative hypotheses explain the degradation of organics in the Viking Labeled Release experiment on Mars. Either martian soil contains live indigenous microorganisms or it is sterile but chemically reactive. These two possibilities could be distinguished by the use of pure preparations of glucose isomers. In the laboratory, selected eukaryotes, bacteria, and archaea consumed only D-glucose, not L-glucose, while permanganate oxidized both isomers. On Mars, selective consumption of either D- or L-glucose would constitute evidence for biological activity.

Planetary Imaging in Powers of Ten: A Multiscale, Multipurpose Astrobiological Imager

Contextual, multiscale astrobiological imaging is necessary to discover, map, and image patchy microbial colonization in extreme environments on planetary surfaces. The large difference in scale--several orders of magnitude--between search environment and microorganisms or microbial communities represents a challenge, which to date no single imaging instrument is able to overcome. In support of future planetary reconnaissance missions, we introduce an adapter-based imager, built from an off-the-shelf consumer digital camera, that offers scalable imaging ranging from macroscopic (meters per pixel) to microscopic (micrometers per pixel) imaging, that is, spanning at least 6 orders of magnitude. Magnification in digital cameras is governed by (1) the native resolution of the CCD/CMOS chip of the camera, (2) the distance between camera and object to be imaged (focal length), and (3) the built-in optical and digital zoom. Both telezoom and macro mode alone are usually insufficient for microscopic imaging. Therefore, the focal distance has to be shortened, and the native CCD resolution of the camera has to be increased to attain a microscopic imaging capability. Our adapter-based imager bridges the gap between macroscopic and microscopic imaging, thereby enabling for the first time contextual astrobiological imaging with the same instrument. Real-world applications for astrobiology and planetary geology are discussed, and proof-of-concept imagery taken with our prototype is presented.

Martian Superoxide and Peroxide O2 Release (OR) Assay: A New Technology for Terrestrial and Planetary Applications

This study presents an assay for the detection and quantification of soil metal superoxides and peroxides in regolith and soil. The O2 release (OR) assay is based on the enzymatic conversion of the hydrolysis products of metal oxides to O2 and their quantification by an O2 electrode based on the stoichiometry of the involved reactions. The intermediate product O₂˙⁻ from the hydrolysis of metal superoxides is converted by cytochrome c to O2 and by superoxide dismutase (SOD) to ½ mol O2 and ½ mol H2O2, which is then converted by catalase (CAT) to ½ mol O2. The product H2O2 from the hydrolysis of metal peroxides and hydroperoxides is converted to ½ mol O2 by CAT. The assay method was validated in a sealed sample chamber by using a liquid-phase Clark-type O2 electrode with known concentrations of O₂˙⁻ and H2O2, and commercial metal superoxide and peroxide mixed with Mars analog Mojave and Atacama Desert soils. Carbonates and perchlorates, both present on Mars, do not interfere with the assay. The assay lower limit of detection, when using luminescence quenching/optical sensing O2-electrodes, is 1 nmol O2 cm(-3) or better. The activity of the assay enzymes SOD and cytochrome c was unaffected up to 6 Gy exposure by γ radiation, while CAT retained 100% and 40% of its activity at 3 and 6 Gy, respectively, which demonstrates the suitability of these enzymes for planetary missions, for example, on Mars or Europa.