A simple, high-precision, high-sensitivity tracer assay for N-2 fixation

We describe a simple, precise, and sensitive experimental protocol for direct measurement of N-2 fixation using the conversion of N-15(2) to organic N. Our protocol greatly reduces the limit of detection for N-2 fixation by taking advantage of the high sensitivity of a modern, multiple-collector isotope ratio mass spectrometer, This instrument allowed measurement of N-2 fixation by natural assemblages of plankton in incubations lasting several hours in the presence of relatively low-level (ca, 10 atom%) tracer additions of N-15(2) of the ambient pool of N-2. The sensitivity and precision of this tracer method are comparable to or better than those associated with the C2H2 reduction assay, Data obtained in a series of experiments in the Gotland Basin of the Baltic Sea showed excellent agreement between N-15(2) tracer and C2H2 reduction measurements, with the largest discrepancies between the methods occurring at very low fixation rates, The ratio of C2H2 reduced to N-2 fixed was 4.68 +/- 0.11 (mean a standard error, n = 39), In these experiments, the rate of C2H2 reduction was relatively insensitive to assay volume, Our results,: the first for planktonic diazotroph populations of the Baltic, confirm the validity of the C2H2 reduction method as a quantitative measure of N-2 fixation in this system, Our N-15(2) protocols are comparable to standard C2H2 reduction procedures, which should promote use of direct N-15(2) fixation measurements in other systems.