Dissimilatory nitrate reduction to ammonium driven by iron tolerant Lutibacter in coastal sediments

The processes of denitrification and dissimilatory NO₃^- reduction to ammonium (DNRA) occupy a competing pivotal point in the nitrogen cycle. Denitrification leads to a loss of fixed nitrogen, while DNRA recycles NO₃^- as bioavailable ammonium. Iron (II) is known to enhance DNRA, by acting as an electron donor, however, the organisms responsible for iron driven DNRA remain poorly understood. Here we undertook incubations of sediment with NO₃^- and Fe²⁺ additions followed by process measurements and metagenomic analysis to measure DNRA and microbial community structure. Addition of Fe²⁺ stimulated DNRA, and greatly increased the relative abundance of flavobacteria (Lutibacter) in slurries over a period of 12-24 days compared to controls. We isolated a strain of Lutibacter from these slurries which mediated organotrophic and mixotrophic DNRA (while oxidising sulphide). No genes capable of Fe²⁺ oxidation were detected in agreement with experiments that showed this strain had no ability to oxidise Fe²⁺. The addition of Fe²⁺ to pure cultures had no significant effect on DNRA, suggesting these bacteria are not stimulated nor inhibited by the presence of Fe²⁺. As such, in contrast to previous studies, we find Fe²⁺ addition to sediments enhances DNRA by favouring Fe tolerant Lutibacter and is uncoupled Fe²⁺ oxidation in coastal sediments. This has implications for how pollutants such as higher Fe²⁺ concentrations and disturbance can lead to bacterial community shifts that enhance nitrogen retention within ecosystems. Isolates of these robust bacteria have the potential to help recover NO₃^- as NH₄⁺ in novel water treatment systems.