Intensive cryptic microbial Fe cycling in a low Fe environment

Iron redox reactions play an important role in carbon remineralization, supporting large microbial populations in terrestrial and aquatic sediments where iron is abundant. Water columns with low iron content are globally widespread, but microbial iron cycling in these systems has largely been ignored. We found evidence for unexpectedly high turnover rates of iron in the low (1-2 µmol/l) iron Lake Cadagno. Light appeared to intensify iron cycling, leading to iron oxidation rates of 1.4-13.8 µmol•l-1•d-1 during the day, which is similar to ferruginous lakes with well-studied microbial iron cycles. If considered as the sole contributor to microbially-mediated iron oxidation, photoferrotrophy may account for up to 10% of total primary production in the chemocline. We could identify the organisms responsible for the intense iron cycling in 16S rDNA libraries and by the successful cultivation of some of these bacteria. Based on incubation experiments, we propose a model in which iron is oxidized by photoferrotrophs and microaerophiles, and iron oxides are immediately reduced by heterotrophic iron reducers, resulting in a cryptic iron cycle. Microbial iron cycling in water column redoxyclines with low iron content, especially those within the photic zone, may be much more important than previously believed.