Light effects and diel variations of nitrate reductase activity in phytoplankton from the northwest Africa upwelling region

Abstract

Light kinetics and diel cycles of nitrate reductase (NR) activity were studied in the upwelling ecosystem off northwest Africa. The activity of the enzyme showed a strong response to light at low intensities but became saturated at light intensities above 15–30% of the incident light intensity. At higher irradiances, NR activity showed photoinhibition. At sea surface irradiances an average inhibition of 32% in the NR activity was observed.

Diel cycles of NR activity exhibited the following characteristics: a low pre-dawn value (0.03–0.009 μmol NO3−N (μg Chl α)−1), a rapid increase with the onset of daylight, a maximum before noon (tripling the dark value), a secondary maximum in the early afternoon, and an afternoon-evening decrease that coincided with the decrease in sunlight. To simulate these characteristics a continuous polynomial function of light and time was developed