Diversity of iron oxidizing and reducing bacteria as samples in flow reactors and in-situ locations at the Aspo Hard Rock Laboratory in Sweden.

Processes of iron mineralization are of great significance to the understanding Early-Earth geochemistry. Of specific interest are processes at circumneutral or slightly alkaline pH, where chemical oxidation of Fe can outcompete biological oxidation. The Äspö Hard Rock Laboratory (Sweden) consists of a 3.6 km tunnel that runs under the Baltic Sea to a depth of 460 m below sea level. Through a series of fractions, various aquifers of Fe2+-rich brackish to saline waters, penetrate the system and harbor a variety of mineralizing microbial mats. To better understand microbially-induced mineral formation and the nature of the involved microbial communities, flow-reactors were set up with different combinations of light and aeration conditions and were connected to 3 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 in these reactors in two consecutive years. A general decrease in diversity was observed towards the deep part of the tunnel. Modeling shows that the overall diversity of the microbial community is controlled by salinity as well as carbon and nitrogen sources. However, the composition of IOB is driven by pH, O2 and the availability of Fe2+. The latter is mostly supplied by Fe(III) reduction in the reactors. Thus the reactors form a self sustained ecosystem. Several genera of known aerobic and anaerobic iron oxidizing bacteria were found. Mariprofundus sp. was found to be dominant in many of the sites. This is the first description of this marine species from groundwater. The microbial community in the reactors is unique per 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 non-accessible microbial communities found in cracks and crevices of the surrounding bedrock.