Nitrate reductase activity as a measure of nitrogen incorporation in Rhizosolenia formosa (H. Peragallo): Internal nitrate and diel effects

Abstract

The nitrate reductase (NR) activity of cultures of Rhizosolenia formosa H. Peragallo was measured to determine whether an improved NR assay used to measure NR activity in individual Rhizosolenia mats provided a good estimate of nitrate incorporation rates. NR rates compared well with calculated estimates of daily nitrogen demand based on growth rates and cell nitrogen quotas ( r 2=0.90), but tended to overestimate calculated nitrogen demand as cultures became increasingly nitrate-deficient. These two estimates were more similar when diel fluctuations in NR activity were incorporated into NR rates. Diel fluctuations were evident in cultures in both nutrient-replete (8 μM) and low-external-nitrate (1 μM) conditions. Average night NR activity was 32% of its maximum in nutrient-replete cultures, and 20% of its maximum in low-external-nitrate cultures. A dark vs. light nitrate-uptake experiment revealed that this species is capable of accumulating much larger internal nitrate pools in the dark (13.3 mM) than in the light (3.2 mM). Internal nitrate concentrations were followed in a time-course experiment to examine the effect they had on NR activity. In general, NR activity was positively correlated to both internal- and external-nitrate concentrations. In our experiments, external nitrate concentrations appeared to limit the maximum NR activity of this species rather than internal nitrate concentrations, although NR activity was induced in cells containing internal nitrate pools regardless of the external nitrate concentration.