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Project Progress

Metals such as Fe, Mn, Ni, Cu and Mo are extremely low in abundance in natural waters, but these metals are used in bacterial enzymes, coenzymes, and cofactors. While it is well known that microbes secrete siderophores to extract Fe from their environment, it is not understood how these siderophores attack minerals to extract the FeIII, nor is it understood how bacteria extract other micronutrients. Strategies for targeted metal uptake by organisms evolved as bioavailability of trace elements changed. If uptake processes left evidence in the rock record, trace metal signatures may document biological activity. For example, studies investigating Fe dissolution and isotopic fractionation by microorganisms have shown promise for detecting biosignatures. N₂-fixing microorganisms (diazotrophs) which require Mo as well as Fe may produce “molybdophores”. Recent evidence indicates that when the N2-fixing soil bacterium Azotobacter vinelandii is grown in the laboratory in the presence or absence of fixed nitrogen, Fe in solution (Δ⁵⁶Fe/⁵⁴Fe) becomes isotopically light, and Mo in solution (Δ⁹⁷Mo/ ⁹⁵Mo) becomes isotopically heavy, compared to controls.. Mechanisms of fractionation as these metals are assimilated into the organisms are the subject of ongoing investigations. We have recently published a review paper on how assimilatory and dissimilatory processes of microorganisms affect metals in the environment (Liermann et al. 2007), and a paper on fractionation of Fe and Mo during bacterial assimilation (Wasylenki et al. 2007). In addition, we have published papers discussing results of studies investigating the interactions of a methanogen with nickel-containing silicates (Hausrath et al. 2007), and a cyanobacterium with phosphate-containing apatite (Schaperdoth et al. 2007).

Although the importance of iron in subsurface environments is widely recognized, it is difficult to study because of its complex biogeochemical pathways resulting in distinct pattern of isotopically light and heavy Fe pools within a soil profile. Fe isotopes are fractionated both by kinetic and equilibrium isotope fractionation mechanisms and both during biotic and abiotic reactions. We are currently investigating the kinetics of dechelation of iron from siderophores and other iron chelators during translocation in soil environments. Recent abiotic kinetic experiments show distinct variations in the kinetics of iron dechelation from ferric acetohydroxamic acid (Fe-aha), ferric EDTA (Fe-EDTA), and ferrioxamine B (Fe-DFMB). The initial rate constants (k’) vary four order of magnitude with k’(Fe-aha)>k’(Fe-EDTA)>k’(Fe-DFMB). In contrast, isotopic results from these experiments didn’t show any kinetic effect on iron isotope fractionation during abiotic dechelation from the different ligands. A manuscript discussing these investigations is in preparation. We will extend our investigations in future experiments by studying biodegradation of different organic ligands and the subsequent release of iron.

Libby Hausrath, a graduate student who recently defended her PhD thesis, has investigated basalt and olivine weathering in a Mars analog environment, the Sverrefjell volcano in Svalbard. Weathered basalt samples were collected for analysis of biotic and abiotic weathering, and the potential identification of biomarkers. The main source of biotic weathering in the arctic is considered to be lichens, which are known to release organic acids and “lichen acids”, which have been shown to enhance weathering and complex metals from a mineral substrate. Data generated by microscopic and spectroscopic methods indicate that physical weathering plays a larger role than chemical weathering in this environment. Understanding the duration of time that Martian rocks were exposed to liquid water is of great interest because it influences the interpretation of the climate history and the potential for life on that planet. The presence or absence of primary minerals may provide constraints for the presence, duration and characteristics of liquid surface water on Mars. Weathering rates are very sensitive to the pH of the reacting fluid. If pH values of terrestrial and Martian weathering solutions were similar, then mineral persistence ages on Mars are likely to be > those on Earth.

We have compiled field terrestrial persistence ages for 8 common rock-forming phases (plagioclase, volcanic glass, quartz, feldspar, micas, pyroxene, amphibole, and olivine) collected from dated chronosequences representing a wide climatic spectrum ranging from -10C to 30C mean annual temperature and 400 mm to 4500 mm mean annual precipitation. The extent to which these minerals persist may help constrain the rates at which primary phases weather under field conditions on Earth, and likely represent minimum mineral persistence times on Mars if pH values were similar.
Relative mineral dissolution rates at different pH values can be predicted from laboratory dissolution experiments. We have compared relative mineral weathering rates observed in the field with laboratory predicted trends. Relative mineral weathering rates observed for basalt in Svalbard (Norway), Pennsylvania,and Costa Rica are explainable by pH. These results suggest that the pH-dependence of laboratory rates can be used to interpret relative mineral persistence on Mars to yield information about the pH of the reacting fluid. We also interpret both terrestrial and Martian weathering profiles using reactive transport modeling, which can yield insights into the duration of weathering. The interpretation of weathering profiles on Mars is a promising approach to study that planet’s aqueous history. Data from these studies have been presented at several conferences, and a manuscript is in review (Hausrath et al., submitted to Geology).