The utility of isotopic distributions in the environment is premised on knowledge of the magnitudes of isotope discrimination by individual biogeochemical reactions, which is most often gained through lab studies of cultured organisms. Our current focus is on the isotope effects of nitrate-consuming processes, in particular, nitrate assimilation by photosynthetic organisms (central to studies of nutrient supply and uptake in the surface ocean) and denitrification by heterotrophic bacteria (central to studies of the global ocean’s input/output budget of fixed N). While the core goal of this work is to help with environmental and paleoceanographic application of the N isotopes, the sensitivities of isotope effects also offer insight into cell physiology and enzyme mechanism.
Increase in nitrate δ15N and δ18O during nitrate assimilation by three species of diatoms and E. huxleyi grown under various environmental conditions. a) Change in nitrate δ15N relative to initial nitrate δ15N plotted as a function of 1n(f) (f being the fraction of initial nitrate that remains in the medium). The slopes of individual experiments approximate the N isotope effect on NO3-(15ε). Dashed lines show slopes for 15ε of 5 and 25‰. b) The δ15N vs. δ18O change in NO3-. The slope of the linear regression and its standard error (1.00 ± 0.01) are inclusive of all the experiments combined. Dashed lines show slopes of 1.1 and 0.9, for comparison. From Granger et al. (2004)
References
We report the first measurements of coupled nitrogen (N) and oxygen (O) isotopic variations of nitrate (NO3-) during its assimilation by laboratory cultures of marine phytoplankton and derive the N and O kinetic isotope effects for nitrate assimilation by three species of diatoms (Thalassiosira weissflogii, Thalassiosira oceanica, and…
In order to (i) establish the biological systematics necessary to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate (15N/ 14N and 18O/16O) in the environment and (ii) investigate the potential for isotopes to elucidate the mechanism of a key N cycle enzyme, we measured the nitrate N and O isotope effects ( 15ε and 18ε) for nitrate…
The link between similarity in amino acid sequence,for ammonia monooxygenase (AMO) and isotopic discrimination for ammonia oxidation (εAMO) was investigated in β-subdivision ammonia-oxidizing bacteria. The isotope effects for ammonia oxidation in pure cultures of the nitrifying strains. Nitrosomonas marina, Nitrosomonas C-113a, Nitrosospira…
The 15N/14N of nitrate in the external medium and intracellular pool of the cultured marine diatom Thalassiosira weissflogii (Grun.) Fryxell et Hasle was measured during nitrate assimilation under low light, a 12:12-h light:dark cycle, low temperature, or low iron conditions. The 15N/14N of the nitrate in the medium and the particulate matter…
In order to provide biological systematics from which to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate (15N/14N, 18O/16O, respectively) in the environment, we previously investigated the isotopic fractionation of nitrate during its assimilation by mono-cultures of eukaryotic algae (Granger et al., 2004). In this study, we…
We report the first measurements of coupled nitrogen (N) and oxygen (O) isotope fractionation of nitrate by laboratory cultures of denitrifying bacteria. Two seawater strains (Pseudomonas stutzeri, Ochrobactrum sp.) and three freshwater strains (Paracoccus denitrificans, Pseudomonas chlororaphis, Rhodobacter sphaeroides) were examined. Among…
Using batch cultures, the 15N/14N (hereafter δ15N) of diatom-bound organic matter was measured and compared to the δ15N of total diatom biomass during the progressive consumption of a nitrate pool in four polar diatom species (Fragilariopsis cylindrus, Fragilariopsis kerguelensis, Pseudo-nitzschia seriata, and Thalassiosira nordenskioeldii) and…
Experiments with two well-studied denitrifiers and one recently isolated marine suboxic zone denitrifier show that the cellular-level denitrification N isotope effect ( 15ε) is typically lower than the canonical value of ∼25‰ under many conditions prevalent in the ocean. Across all three strains, 15ε is 10-15‰ at cellular nitrate reduction…