Use of nitrate reductase (NR) activity as an index of rates of nitrate incorporation in marine phytoplankton has been complicated by inadequate assays, and the high degree of regulation of the enzyme under different non-steady-slate conditions. The relationship between NR activity and rates of nitrate incorporation was examined using a modified NR assay (which included bovine serum albumin) under diel periodicity in irradiance, following exhaustion of nitrate, and in the presence of ammonium. Laboratory experiments using the diatom Thalassiosira pseudonana showed that on a 14:10 h light: dark cycle, NR activity had 2 distinct peaks within 1 cycle, one in the middle of the light period, and the other towards the end of the dark period. Each of these peaks has been noted in previous work, both in culture and in natural populations, but this is the first description of both in 1 cycle. Throughout the diel cycle, NR activity closely matched rates of nitrate incorporation calculated from increases in particulate nitrogen in the cultures. As nitrate was exhausted in the cultures, NR activity declined in step with nitrate incorporation rates (estimated from nitrate depletion, increases in particulate nitrogen, or the product of cell growth rate and cell nitrogen content). In cultures where nitrate was substituted by ammonium, no NR activity was detected, but when ammonium was exhausted, background contamination of the medium with nitrate (approximately 1 mu M) was sufficient to cause the appearance of NR activity and the uptake of nitrate. Daily additions of 2 mu M ammonium to cultures growing on nitrate had no effect on the relationship between NR activity and nitrate incorporation rate. The NR assay was used in a preliminary study of natural assemblages of phytoplankton in Monterey Bay, California, USA, during a post-upwelling diatom bloom. Diel periodicity in NR activity was observed that was virtually identical to that found in laboratory cultures. Ammonium inhibition on a time scale similar to that seen in culture was also recorded. These preliminary results are promising for the use of NR activity to estimate rates of nitrate incorporation in field population of phytoplankton.
