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 N-2. 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 mu mol of NO3- or NO2- to Fe(III)-grown axenic cultures of A. dehalogenans, 54 (+/- 7) mu mol and 113 (+/- 2) mu mol N2O-N, respectively, were produced and subsequently consumed. The conversion of NO3- to N-2 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 N-2 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. IMPORTANCE Current attempts to gauge N loss from soils rely on the quantitative measurement of nirK and nirS genes and/or transcripts. In the presence of iron, the common soil bacterium Anaeromyxobacter dehalogenans is capable of denitrification and the production of N-2 without the key denitrification genes nirK and nirS. Such chemodenitrifiers denitrify through combined biotic and abiotic reactions and have potentially large contributions to N loss to the atmosphere and fill a heretofore unrecognized ecological niche in soil ecosystems. The findings emphasize that the comprehensive understanding of N flux and the accurate assessment of denitrification potential can be achieved only when integrated studies of interlinked biogeochemical cycles are performed.