Evidence for microbial iron reduction in the methanogenic sediments of the oligotrophic SE Mediterranean continental shelf

Abstract. Dissimilatory iron reduction is probably one of the earliest metabolisms, which still participates in important biogeochemical cycles such as carbon and sulfur. Traditionally, this process is thought to be limited to the shallow part of the sediment column, as one of the energetically favorable anaerobic microbial respiration cascade, usually coupled to the oxidation of organic matter. However, in the last decade iron reduction has been observed in the methanogenic depth in many aquatic sediments, suggesting a link between the iron and the methane cycles. Yet, the mechanistic nature of this link has yet to be established, and has not been studied in oligotrophic shallow marine sediments. In this study we present first geochemical and molecular evidences for microbial iron reduction in the methanogenic depth of the oligotrophic Southern Eastern (SE) Mediterranean continental shelf. Geochemical pore-water profiles indicate iron reduction in two zones, the traditional zone in the upper part of the sediment cores and a deeper second zone located in the enhanced methane concentration layer. Results from a slurry incubation experiment indicate that the iron reduction is microbial. The Geochemical data, Spearman correlation between microbial abundance and iron concentration, as well as the qPCR analysis of the mcrA gene point to several potential microorganisms that could be involved in this iron reduction via three potential pathways: H₂/organic matter oxidation, an active sulfur cycle or iron driven anaerobic oxidation of methane.