Diversity of Iron Oxidizing and Reducing Bacteria in Flow Reactors in the Äspö Hard Rock Laboratory

<div><p>Processes of iron mineralization are of great significance to the understanding of Early-Earth geochemistry. Of specific interest are processes at circumneutral pH, where chemical oxidation of Fe can outcompete biological oxidation. To better understand microbially-induced mineral formation and the composition of the involved microbial communities, we set up a series of flow-reactors in the Äspö Hard Rock Laboratory, a 3.6 km tunnel that runs under the Baltic Sea. Various aquifers of Fe<sup>2+</sup>-rich brackish to saline waters penetrate the tunnel through a series of fractions. The reactors were set up with different combinations of light and aeration conditions, and were connected to three aquifers of differing chemical composition and age. Using a combination of 454 pyrosequencing and CAtalyzed Reporter Deposition Fluorescent In Situ Hybridization we analyzed the bacterial community from these reactors in two consecutive seasons half a year apart. A general decrease in diversity was observed towards the deep part of the tunnel. Multivariate modeling of the community composition and environmental parameters shows that the overall diversity of the microbial community is controlled by salinity as well as carbon and nitrogen sources. However, the composition of iron oxidizing bacteria is driven by pH, O<sub>2</sub> and the availability of Fe<sup>2+</sup>. The latter is mostly supplied by Fe<sup>3+</sup> reduction in the reactors. Thus the reactors form a self-sustained ecosystem. Several genera of known aerobic and anaerobic iron oxidizing bacteria were found. <i>Mariprofundus</i> sp. was found to be dominant in many of the samples. This is the first detection of this marine species in groundwater. The microbial community in the reactors is unique in each site, while that in the exposed tunnel is more homogenous. Therefore we suggest that the flow reactors are a good model system to study the nonaccessible microbial communities that are likely present in cracks and crevices of the surrounding bedrock.</p></div>