Certain microbes, when faced with a shortage of oxygen (O2) in their environment, are ableto utilize nitrate (NO3−) as an alternate electron acceptor for respiration. The most commonand well-studied of the dissimilatory nitrate respiration pathways is denitrification, whichconverts nitrate (NO3−) and nitrite ions (NO2−) to gaseous products including nitricoxide (NO), nitrous oxide (N2O) and dinitrogen (N2). Each of these reductions iscatalysed by a different enzyme, and not all microbes possess the full suite necessary tocompletely convert NO3− to N2. Denitrification enzymes require a metal co-factor; forexample, the reduction of N2O to N2 is catalysed by a Cu-containing nitrous oxidereductase (N2OR). Microbes with a complete denitrifying pathway not only have theadditional pressure to reduce the accumulated denitrification intermediates (NO2−,NO, and N2O) to inert N2, but also to effectively scavenge trace-metals from theirenvironment. In a series of lab incubations, we examined the transcriptome of a modeldenitrifying organism (Paracoccus denitrificans) under aerobic and anaerobic conditions.Then we investigated the effects of metal depletion on P. denitrificans and metaladditions on wetland sediments. We found that the transition from aerobic to anaerobicconditions is tightly regulated requiring only 200 genes. The majority of those genes arecontrolled by regulatory factors that respond to carbon, O2, NO3−, and NO2−environmental signals. Under low Cu availability N2OR remains non-functional and N2Oaccumulates, although N2OR is transcribed and translated. This disrupted denitrificationpathway is comparable to organisms lacking N2OR. When considering the anaerobicenvironment of wetland sediments, we found that Mo, Fe, and Cu addition at μM levelsnotably enhanced denitrification and significantly reduced the accumulation of N2O.These biochemical process rate observations were concurrent with changes in themicrobial group abundances for nitrite – and nitrous oxide – reducers, suggesting anecological advantage for complete denitrifiers when electron acceptors are limited inthe environment. Emerging from our studies, firstly, a denitrification regulatorymodel is proposed and secondly, trace-metal availability should be considered as anadditional controlling factor when studying denitrification and N2O emissions. [ABSTRACT FROM AUTHOR]