Figure 1.
(a) δ15NNO3+NO2 (log scale) of nitrate solutions (10–300 µmol L−1) following equilibration with 0.1 mL 15N2 gas from lecture bottles procured from three distributors.
Solutions were 40 mL for Sigma-Aldrich and Campro Scientific equilibrations, and 100 mL for Cambridge Isotopes equilibrations. The solid line corresponds to the δ15NNO3 of the control solutions for Sigma-Aldrich and Cambridge Isotopes experiments (δ15NNO3 = 23.5±0.5‰); the dashed line corresponds to controls for Campro Scientific experiments (δ15NNO3 = 14.15±0.1‰). Paired symbols identify replicate experimental treatments. (b) Corresponding apparent δ18ONO3+NO2 of the experimental nitrate solutions. The solid line corresponds to the δ18ONO3 of control solutions for the Sigma-Aldrich and Cambridge Isotope experiments (δ18ONO3 = 18.9±0.3‰); the dashed line corresponds to controls for Campro Scientific experiments (25.4±0.3‰).
Figure 2.
(a) δ15NNO3+NO2 (log scale) of higher sensitivity equilibrations of 10 µmol L−1 nitrate solutions (10 mL) with 2 mL of 15N2 gas from a Cambridge Isotopes or a Sigma-Aldrich bottle.
(b) Corresponding apparent δ18ONO3 (log scale) of higher sensitivity equilibrations of the two stocks. n = the number of experimental replicates.
Table 1.
The quantity of 15N-labeled contaminants detected relative to 15N2 additions.
Figure 3.
(a) δ15NNH4 (log scale) of 5 µmol L−1 ammonium solutions after equilibration with 0.1 mL 15N2 gas from respective Sigma-Aldrich and Cambridge Isotopes lecture bottles vs. control solutions.
Sigma-Aldrich treatments utilized 40 mL ammonium solutions, whereas Cambridge Isotopes treatments utilized 100 mL ammonium solutions. (b) δ15NNH4 of higher sensitivity equilibrations of 5 µmol L−1 ammonium solutions (10 mL) with 2.0 mL 15N2 gas from Cambridge Isotopes lecture bottles vs. control solutions. n = the number of experimental replicates.
Figure 4.
The δ15N of particulate nitrogen (δ15NPN) of D. tertiolecta harvested in stationary phase following growth in media containing sodium nitrate (and no ammonium) and equilibrated with 15N2 gas from Sigma Aldrich or Cambridge Isotopes.
n = the number of experimental replicates.
Table 2.
Inferred N2 fixation rates (nmoles N L−1 d−1) resulting from 15N-labeled contaminants.