US EPA

1528599 

Journal Article 

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

Buchwald, C; Santoro, AE; Mcilvin, MR; Casciotti, KL 

2012 

1 

Limnology and Oceanography
ISSN: 0024-3590
EISSN: 1939-5590 

57 

5 

1361-1375 

10.4319/lo.2012.57.5.1361 

WOS:000308640600009 

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.