Denitrification by Anaeromyxobacter dehalogenans, a common soil bacterium lacking the nitrite reductase genes nirS and nirK

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 -mol of NO₃ - or NO₂ - to Fe(III)-grown axenic cultures of A. dehalogenans, 54 (±7) -mol and 113 (±2) -mol 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.