Oxygen isotopic composition of nitrate and nitrite produced by nitrifying cocultures and natural marine assemblages

The delta O-18 value of nitrate produced during nitrification (delta O-18(NO3,nit)) was measured in experiments designed to mimic oceanic conditions, involving cocultures of ammonia-oxidizing bacteria or ammonia-oxidizing archaea and nitrite-oxidizing bacteria, as well as natural marine assemblages. The estimates of delta O-18(NO3,nit) ranged from -1.5 parts per thousand +/- 0.1 parts per thousand to +1.3 parts per thousand +/- 1.4 parts per thousand at delta O-18 values of water (H2O) and dissolved oxygen (O-2) of 0 parts per thousand and 24.2 parts per thousand vs. Vienna Standard Mean Ocean Water, respectively. Additions of O-18-enriched H2O allowed us to evaluate the effects of oxygen (O) isotope fractionation and exchange on delta O-18(NO3,nit). Kinetic isotope effects for the incorporation of O atoms were the most important factors for setting overall delta O-18(NO3,nit) values relative to the substrates (O-2 and H2O). These isotope effects ranged from +10 parts per thousand to +22 parts per thousand for ammonia oxidation (O-2 plus H2O incorporation) and from +1 parts per thousand to +27 parts per thousand for incorporation of H2O during nitrite oxidation. delta O-18(NO3,nit) values were also affected by the amount and duration of nitrite accumulation, which permitted abiotic O atom exchange between nitrite and H2O. Coculture incubations where ammonia oxidation and nitrite oxidation were tightly coupled showed low levels of nitrite accumulation and exchange (3% +/- 4%). These experiments had delta O-18(NO3,nit) values of -1.5 parts per thousand to +0.7 parts per thousand. Field experiments had greater accumulation of nitrite and a higher amount of exchange (22% to 100%), yielding an average delta O-18(NO3,nit) value of +1.9 parts per thousand +/- 3.0 parts per thousand. Low levels of biologically catalyzed exchange in coculture experiments may be representative of nitrification in much of the ocean where nitrite accumulation is low. Abiotic oxygen isotope exchange may be important where nitrite does accumulate, such as oceanic primary and secondary nitrite maxima.