A high sensitivity nitrate reductase assay and its application to vertically migrating Rhizosolenia mats

Abstract

A high sensitivity assay for nitrate reductase (NR) was developed that permitted determination on samples as small as 0.2 ug chlorophyll a. We combined a revised NR assay chemistry with chemiluminescent determination of nitrite to improve sensitivity approximately 50-fold over colorimetric techniques. Results from colorimetric and chemiluminescent techniques were linearly related (r2 = 0.96, p = 0.0001). The NR assay reagents did not interfere with chemiluminescent nitrite determinations 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 to support calculated nitrogen demands of mats. Floating mats had significantly higher NR activity and internal nitrate pools than sinking mats, providing more evidence to support the hypothesis that these pelagic diatoms can migrate below the nutricline to exploit deep nitrate pools and then return to the surface for photosynthesis. © Inter-Research 1997.