New Approaches to Characterizing Fe(III) Bioreduction by Hyperthermophiles: Combining Physiological Potential with Mineral Spectroscopies

Abstract:

Fe(III) is a widely available electron acceptor in many mildly-reducing deep-sea hydrothermal vents and terrestrial hot springs. Dissimilatory iron reduction, or the extracellular reduction of Fe(III) to Fe(II), is an integral biogeochemical process at these sites. Most of what is known about microbial Fe(III) reduction, however, has been established for mesophiles rather than the hyperthermophiles that are ubiquitous in hot environments. Our study examines the rates and constraints of Fe(III) bioreduction by hyperthermophilic archaea in order to address the types of Fe(III) (oxyhydr)oxides that are favored for growth of hyperthermophiles, the rates of growth and Fe(II) production for these organisms, and the mineralogy of Fe(III) bioreduction and possible variations with electron acceptor.

We synthesized a range of nanophase Fe(III) (oxyhydr)oxides (2-line-ferrihydrite, 6-line-ferrihydrite, lepidocrocite, hematite, goethite, maghemite and akaganéite) and examined cell growth and Fe(II) production rates of Pyrodictium sp. Su06 and Pyrobaculum islandicum on the different Fe(III) (oxyhydr)oxides at 90°C and 95°C, respectively. Two different aggregate sizes of 2-line ferrihydrite and one of 6-line ferrihydrite were used to understand the effect of mineral aggregate size and shape on bioreduction. Direct cell counts and ferrozine assays were used to monitor cell growth and Fe(II) production respectively. Transformed mineral products were characterized using Mössbauer and attenuated total reflectance (ATR) spectroscopies. Preliminary results suggest that Pyrodictium sp. Su06 can only utilize 2-line ferrihydrite, producing up to 35 mM Fe(II) for smaller aggregates, and 10 mM Fe(II) for larger aggregates. P. islandicum on the other hand, can reduce 2-line-ferrihydrite, goethite, and lepidocrocite, producing up to 8 mM Fe(II) with ferrihydrite, 7 mM with lepidocrocite, and 2 mM with goethite as an electron acceptor. Initial results from Mössbauer and ATR spectra combined suggest that nanophase magnetite is formed through the bioreduction of ferrihydrite. Our work provides new insights into characterizing hyperthermophile microbe-mineral interactions and the potential biosignatures that result.