Activity of Autotrophic Fe(II)-Oxidizing Denitrifiers in Freshwater Lake Sediments

In recent years, the coupling of Fe(II) oxidation and denitrification has attracted more attention. However, the activity of autotrophic Fe(II)-oxidizing denitrifiers in freshwater environments and their overall contribution to total nitrogen removal remain unknown. In this study, sediment microcosms incubated under different treatment conditions revealed that when extra Fe(II) was added, biological Fe(II)-oxidizing denitrification contributed at least 26% of the total denitrification in some organic carbon-limited sediments. In addition, the autotrophic CO2 fixation rates (31.8–84.2 nmol of C g–1 day–1) were detected via 13C labeling experiments following depletion of bioavailable organic C, which demonstrated the activity of autotrophic Fe(II)-oxidizing denitrifiers. Furthermore, high-throughput sequencing analysis of the nirS and nirK transcripts during long-term incubations without the addition of organic C revealed that addition of Fe(II) changed the community composition of nirS-type instead of nirK-type denitrifiers, which suggests a dominant role of nirS-type autotrophic Fe(II)-oxidizing denitrifiers. Paracoccus and Thiobacillus were significantly enriched during the long-term incubation, indicating that they might be the key autotrophic Fe(II)-oxidizing denitrifying genera in freshwater lake sediments. This study provides the first evidence of autotrophic Fe(II)-oxidizing denitrifier activity in freshwater environments at a molecular level and highlights the important role of autotrophic Fe(II)-oxidizing denitrifiers in freshwater sediments.