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, mu ltiple-collector isotope ratio mass spectrometer, This instrument allo wed measurement of N-2 fixation by natural assemblages of plankton in incubations lasting several hours in the presence of relatively low-le vel (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 comparabl e to or better than those associated with the C2H2 reduction assay, Da ta obtained in a series of experiments in the Gotland Basin of the Bal tic Sea showed excellent agreement between N-15(2) tracer and C2H2 red uction measurements, with the largest discrepancies between the method s 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 e xperiments, the rate of C2H2 reduction was relatively insensitive to a ssay volume, Our results,: the first for planktonic diazotroph populat ions 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 sy stems.