Steady-state nitrogen isotope effects of N-2 and N2O production in Paracoccus denitrificans

Nitrogen stable-isotope compositions (delta(15)N) can help track denitrific ation and N2O production in the environment, as can knowledge of the isotop ic discrimination, or isotope effect, inherent to denitrification. However, the isotope effects associated with denitrification as a function of disso lved-oxygen concentration and their influence on the isotopic composition o f N2O are not known. We developed a simple steady-state reactor to allow th e measurement of denitrification isotope effects in Paracoccus denitrifican s. With [dO(2),] between 0 and 1.2 mu M, the N stable-isotope effects of NO 3- and N2O reduction were constant at 28.6 parts per thousand +/- 1.9 parts per thousand and 12.9 parts per thousand +/- 2.6 parts per thousand, respe ctively (mean +/- standard error, n = 5). This estimate of the isotope effe ct of N2O reduction is the first in an axenic denitrifying culture and plac es the delta(15)N Of denitrification-produced N2O midway between those of t he nitrogenous oxide substrates and the product N, in steady-state systems. Application of both isotope effects to N2O cycling studies is discussed.