Sedimentary Dynamics of a Modern Iron Oxidizing Microbial Ecosystem

We have investigated an iron-oxidizing microbial mat community in the outflow of an artesian spring located in Potawatomi State Park, Sturgeon Bay, WI. This system supports abundant iron oxidizing bacteria, which exist in diverse hydrologic conditions and across an iron concentration gradient. Over three years of monitoring have shown this to be a constant microbial community. This system, therefore, provides an excellent natural laboratory to investigate the dynamics of a lithotrophic ecosystem.

The microbial ecosystem is dominated by Leptothrix spp but also contains Gallionella spp. The system has an extremely low external organic input, confirming lithotrophic primary production. Characterization of the system in the field includes analyses of pH, nitrogen speciation, as well as oxygen and iron profiles with depth and distance from the spring. XRD analysis documents changes in mineralogy during early diagenesis in both field and laboratory incubation samples. Morphological changes are observed microscopically on an SEM. We also investigated rare earth element (REE) sorption and early digenetic modification in these microbial ecosystems.

Lithotrophic microbial ecosystems are proposed to have been important in the Precambrian based on parameters such as high dissolved iron and low dissolved oxygen levels. Our investigation provides a lens through which sediments derived from iron-bacteria can possibly be viewed. Our study indicates that morphologically distinct fossils are unlikely to be preserved through diagenesis. We also show that iron bacterial ecosystems can produce iron oxides that may be preserved in the geological record—despite high organic matter production that could lead to microbial iron reduction. Our study also demonstrates the utility of REE analysis as a proxy in ferruginous sediments. Despite high rates of organic matter remineralization and iron phase recrystallization, there is no significant change in normalized REE patterns.