A HIGH-SENSITIVITY NITRATE REDUCTASE ASSAY AND ITS APPLICATION TO VERTICALLY MIGRATING RHIZOSOLENIA MATS

A high sensitivity assay for nitrate reductase (NR) was developed that permitted determination on samples as small as 0.2 mu g chlorophyll a . We combined a revised NR assay chemistry with chemiluminescent deter mination of nitrite to improve sensitivity approximately 50-fold over colorimetric techniques. Results from colorimetric and chemiluminescen t techniques were linearly related (r(2) = 0.96, p = 0.0001). The NR a ssay reagents did not interfere with chemiluminescent nitrite determin ations although the reagents did contain detectable amounts of nitrite . The assay chemistry and incubation temperature required optimization for our species. We used the assay to measure NR rates in macroscopic Rhizosolenia mats from the North Pacific Ocean. Rates were adequate t o support calculated nitrogen demands of mats. Floating mats had signi ficantly higher NR activity and internal nitrate pools than sinking ma ts, providing more evidence to support the hypothesis that these pelag ic diatoms can migrate below the nutricline to exploit deep nitrate po ols and then return to the surface for photosynthesis.