Jp. Montoya et al., A SIMPLE, HIGH-PRECISION, HIGH-SENSITIVITY TRACER ASSAY FOR N-2 FIXATION, Applied and environmental microbiology, 62(3), 1996, pp. 986-993
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.