Abstract
The versatile soil bacterium Anaeromyxobacter dehalogenans lacks the hallmark denitrification genes nirS and nirK (encoding NO2−→NO reductases) and couples growth to NO3− reduction to NH4+ (respiratory ammonification) and to N2O reduction to N2. A. dehalogenans also grows by reducing Fe(III) to Fe(II), which chemically reacts with NO2− to form N2O (i.e., chemodenitrification). Following the addition of 100 μmol of NO3− or NO2− to Fe(III)-grown axenic cultures of A. dehalogenans, 54 (±7) μmol and 113 (±2) μmol N2O-N, respectively, were produced and subsequently consumed. The conversion of NO3− to N2 in the presence of Fe(II) through linked biotic-abiotic reactions represents an unrecognized ecophysiology of A. dehalogenans. The new findings demonstrate that the assessment of gene content alone is insufficient to predict microbial denitrification potential and N loss (i.e., the formation of gaseous N products). A survey of complete bacterial genomes in the NCBI Reference Sequence database coupled with available physiological information revealed that organisms lacking nirS or nirK but with Fe(III) reduction potential and genes for NO3− and N2O reduction are not rare, indicating that NO3− reduction to N2 through linked biotic-abiotic reactions is not limited to A. dehalogenans. Considering the ubiquity of iron in soils and sediments and the broad distribution of dissimilatory Fe(III) and NO3− reducers, denitrification independent of NO-forming NO2− reductases (through combined biotic-abiotic reactions) may have substantial contributions to N loss and N2O flux.
