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Project Progress

Iron biogeochemical cycling in circumneutral pH systems is an increasingly important astrobiological target in light of recent discoveries on Mars by Curiosity (Grotzinger et al., 2013). Previous research into microbial ferric iron Fe(III) reduction in Yellowstone National Park geothermal springs has focused on high temperature, low pH environments where microbial communities are able to utilize soluble Fe(III) as an electron acceptor for respiration (e.g. Kozubal et al., 2012). Much less attention has been paid to Fe(III)-reducing microbial communities in lower temperature, circumneutral pH environments, where solid phase Fe(III) oxides are the dominant forms of ferric iron. This study explored the potential for microbial reduction of Fe(III) oxides in the warm (ca. 40-60 C), circumneutral pH (ca. 6.0-6.5) Chocolate Pots hot springs (CP) in Yellowstone National Park. The work was motivated by the idea that internal Fe redox cycling driven by microbial Fe(III) reduction could influence Fe isotope fractionation by liberating dissolved, mobile Fe(II) from a relatively heavy pool of deposited Fe(III) oxides, thus reducing the overall degree of fractionation within fluids in the hot spring (Wu et al., 2013). Such internal Fe redox cycling has important implications for interpretation of Fe isotope fractionation patterns as a signature of microbial redox metabolism.

Endogenous microbial communities at two sites within the CP hot spring deposits (“Vent”, near the vent source; and “Mid”, roughly mid-way down the flow path; see Fig. 1) were able to reduce native CP Fe(III) oxide-silica coprecipitates, as documented in most probable number (MPN) enumerations and enrichment culture experiments. The MPN results indicated the presence of substantial numbers (2.4 × 10⁸ and 1.5 × 10⁵ cells per mL for the Mid and Vent sites, respectively) of Fe(III)-reducing organisms that could utilize a mixture of acetate and lactate as the carbon and energy source.

The enrichment culture studies showed that the observed Fe(III) reduction was enzymatic rather sulfide-driven, as neither exclusion of sulfate, or the presence of molybdate (an inhibitor of microbial sulfate reduction) had deleterious effect on Fe(III) oxide reduction (data not shown). Other experiments showed that Fe(III)-reducing organisms in the enrichments could utilize either acetate or lactate alone, or a combination of H₂ and small amounts of acetate, to sustain Fe(III) reduction activity.

Microbial communities in the enrichments were analyzed by high-throughput pyrosequencing of 16S rRNA gene amplicons from the 10^th transfer of the enrichment cultures. The sequencing revealed an abundance of the well-known Fe(III)-reducing bacterial species, Geobacter metallireducens, as well several other novel organisms with the potential to contribute to Fe(III) reduction (Fig. 2).

A shotgun metagenomic (paired-end Illumina sequencing) analysis of the enrichment cultures is in progress to explore the identity and function of G. metallireducens as well as other less well-characterized organisms in the cultures. Of particular interest are the likely presence of thermotolerance genes in the G. metallireducens metagenome, as well as outer membrane cytochrome genes that may be indicative of other Fe(III)-reducing organisms and provide evidence for pathways of electron flow in these cultures. Additional research is currently underway to track Fe isotope fractionation coupled to DIR in the enrichment cultures, and in core samples obtained in an August 2013 field campaign. The Fe geochemistry (bulk and Fe isotopic) determined in these studies will be related to the genomic capacity of reconstructed microbial genomes from metagenomic sequence information using recently described methods.

References cited

Grotzinger, J. P., D. Y. Sumner, L. C. Kah, K. Stack, S. Gupta, L. Edgar et al. 2013. A habitable fluvio-lacustrine environment at Yellowknife Bay, Gale Crater, Mars. Science.

Kozubal, M. A., W. P. Inskeep, and R. E. Macur. 2012. Geomicrobiology of iron oxide mats from acidic geothermal springs in Yellowstone National Park: Microbial community structure, isolation of novel species and mechanisms of iron oxidation. Front. Microbiol. 3:109.

Wu, L., R. P. Poulson-Brucker, B. L. Beard, E. E. Roden, and C. M. Johnson. 2013. Iron isotope characteristics of hot springs at Chocolate Pots, Yellowstone National Park. Astrobiology 13:1091-1101.