Possible evidence for dissimilatory bacterial magnetite dominating the magnetic properties of recent lake sediments

Rock magnetic measurements, including low temperature susceptibility and Curie Temperature experiments, show that the most recent sediments from the Bay of Vidy, close to Lausanne on the north shore of Lake Geneva, Switzerland, are rich in superparamagnetic magnetite. The 137Cs dated increase in magnetite concentration coincides with the beginning of sewage de-phosphatization by the addition of iron chloride. Extracts of fine-grained magnetite, imaged by transmission electron microscopy, form amorphous masses visually comparable to those detected as the product of extracellular magnetite production by bacteria. This is the first indication that extracellular, dissimilatory magnetite may survive and make a dominant contribution to the magnetic properties of sediments. Final confirmation of the bacterial origin of the magnetite will depend on rRNA sequencing

Biogenic Magnetite Formation through Anaerobic Biooxidation of Fe(II)

The presence of isotopically light carbonates in association with fine-grained magnetite is considered to be primarily due to the reduction of Fe(III) by Fe(III)-reducing bacteria in the environment. Here, we report on magnetite formation by biooxidation of Fe(II) coupled to denitrification. This metabolism offers an alternative environmental source of biogenic magnetite

Geobacteraceae are important members of mercury-methylating microbial communities of sediments impacted by waste water releases

Microbial mercury (Hg) methylation in sediments can result in bioaccumulation of the neurotoxin methylmercury (MMHg) in aquatic food webs. Recently, the discovery of the gene hgcA, required for Hg methylation, revealed that the diversity of Hg methylators is much broader than previously thought. However, little is known about the identity of Hg-methylating microbial organisms and the environmental factors controlling their activity and distribution in lakes. Here, we combined high-throughput sequencing of 16S rRNA and hgcA genes with the chemical characterization of sediments impacted by a waste water treatment plant that releases significant amounts of organic matter and iron. Our results highlight that the ferruginous geochemical conditions prevailing at 1–2 cm depth are conducive to MMHg formation and that the Hgmethylating guild is composed of iron and sulfur-transforming bacteria, syntrophs, and methanogens. Deltaproteobacteria, notably Geobacteraceae, dominated the hgcA carrying communities, while sulfate reducers constituted only a minor component, despite being considered the main Hg methylators in many anoxic aquatic environments. Because iron is widely applied in waste water treatment, the importance of Geobacteraceae for Hg methylation and the complexity of Hgmethylating communities reported here are likely to occur worldwide in sediments impacted by waste water treatment plant discharges and in iron-rich sediments in general