Abstract
Nitrate reductase (NR) activity in the surf-zone diatom Chaetoceros armatum T. West was studied in its natural habitat at Copalis Beach, Olympic Peninsula, Washington (USA) over a period of 18 months. Variations of enzyme activity were found, depending on the season and the time of day. General seasonal trends in NR activity were correlated with nitrate concentrations in the surf-zone. The results also indicated that several factors may limit the activity of the enzyme. This is one of the reasons why NR activity could not be correlated with environmental nitrate concentration on the basis of individual measurements at a given point in time. Enzyme inhibition due to the presence of ammonium occurred on several occasions throughout the study period, and could be detected by the NR assay. This phenomenon for the most part masked any pattern of diel periodicity in NR activity that might have been expected to exist in cells in the natural environment. Cultures of C. armatum and Asterionella socialis Lewin and Norris were also studied in the laboratory. A diel fluctuation in NR activity corresponding to the pattern shown in natural phytoplankton by other authors was exhibited in C. armatum under a light-dark regime of 8 h light – 16 h dark and a light intensity of 400 lux.
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