Microbial Communities Associated with Biogenic Iron Oxide Mineralization in Circumneutral pH Environments

Lithotrophic growth on iron is a metabolism that has been found in a variety of neutral pH environments and is likely important in sustaining life in microaerophilic solutions, especially those low in organics. The composition of the microbial communities, especially the organisms that are responsible for iron oxidation, and carbon and nitrogen fixation, are not known, yet the ability to recognize these contributions is vital to our understanding of iron cycling in natural environments. Our approach has been to study the microbial community structure, mineralogy, and geochemistry of ~20 cm thick, 100's meters long, fluffy iron oxide-encrusted biological mats growing in the Piquette Mine tunnel, and to compare the results to those from geochemically similar environments. In situ measurements (Hydrolab) and geochemical characterization of bulk water samples and peepers (dialysis sampling vials) indicate that the environment is microaerobic, with micromolar levels of iron, high carbonate and sulfate, and typical groundwater nitrate and nitrite concentrations. 16S rDNA clone libraries show that the microbial mat and water contain communities with considerable diversity within the Bacterial domain, a large proportion of Nitrospira and Betaproteobacteria, and no Archaea. Because clone library data are not necessarily indicative of actual abundance, fluorescence in-situ hybridization (FISH) was performed on water, mat, and sediment samples from the Piquette mine and two circumneutral iron- and carbonate-rich springs in the Oregon Cascade Range. Domain- and phylum-level probes were chosen based on the clone library results (Nitrospira, Beta- and Gammaproteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, and Planctomyces). FISH data reveal spatial associations between specific microbial groups and mineralized structures. The organisms responsible for making the mineralized sheaths that compose the bulk of the iron oxide mat are Betaproteobacteria (probably Leptothrix spp.). However, only a small proportion of the cells in the mat reside within the sheaths. Most are located on or around the sheaths, which provide a physical framework for the community. Preliminary results from FISH experiments on the iron-rich spring samples show some similarities, including an abundance of Betaproteobacteria. Enrichment and isolation experiments are being performed to identify the iron-oxidizing organisms. Iron-oxidizers have been enriched from all sites. In some cultures it has been difficult to isolate the iron-oxidizing organisms from a non-iron-oxidizing heterotroph, possibly indicating co-dependence. Knowledge of the microbial community structure and the metabolic activities of key members will enable us to better understand the processes and chemical conditions which generate iron oxide deposits found in the geologic record on Earth and possibly extraterrestrial habitats.