Response of Marine Denitrifying Bacterial Populations to Nitrous Oxide: a Natural Sink?

The influence of nitrous oxide upon denitrifying populations was investigated in moderately saline shallow Texas coastal bays. Homogenized sediment samples were incubated under a N2O atmosphere for one week, after which nosZ DNA marker for nitrous oxide reductase enzyme was extracted and analyzed by quantitative polymerase chain reaction (qPCR) and semi-quantitative polymerase chain reaction/denaturing gradient gel electrophoresis (PCR/DGGE). qPCR analysis indicated a significant (over an order of magnitude) increase in nosZ copy number in response to N2O addition, with PCR/DGGE indicating a significant population shift towards a small number of select organisms. Intriguingly, nirK and nirS markers for nitrite reductase enzymes in denitrifying bacteria did not show a corresponding increase, suggesting that observed nosZ peak was not associated with typical denitrifying populations. It is possible that N2O consumption in these sediments might not be driven by normally abundant denitrifiers, but rather by a niche-specific guild of bacteria converting N2O to N2 and naturally present in sediments at low numbers. These organisms are capable of responding rapidly to increased N2O supply. Our results suggest that net biological emission of N2O from the sediments is regulated by bacteria at both production and consumption stage, and disturbance of either could result in enhanced N2O emission.