NITRATE REDUCTASE-ACTIVITY AS A MEASURE OF NITROGEN INCORPORATION IN RHIZOSOLENIA-FORMOSA (PERAGALLO,H.) - INTERNAL NITRATE AND DIEL EFFECTS

The nitrate reductase (NR) activity of cultures of Rhizosolenia formos a H. Peragallo was measured to determine whether an improved MZ 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 rat es and cell nitrogen quotas (r(2) = 0.90), but tended to overestimate calculated nitrogen demand as cultures became increasingly nitrate-def icient. These two estimates were more similar when diel,fluctuations i n NR activity were incorporated into NR rates. Diel fluctuations were evident in cultures in both nutrient-replete (8 mu M) and low-external -nitrate (1 mu M) conditions. Average night NR activity was 32% of its maximum in nutrient-replete cultures, and 20% of its maximum in low-e xternal-nitrate cultures. A dark vs. light nitrate-uptake experiment r evealed that this species is capable of accumulating much larger inter nal nitrate pools in the dark (13.3 mM) than in the light (3.2 mM). In ternal nitrate concentrations were followed in a time-course experimen t to examine the effect they had on NR activity. In general, NR activi ty was positively correlated to both internal- and external-nitrate co ncentrations. In our experiments, external nitrate concentrations appe ared to limit the maximum NR activity of this species rather than inte rnal nitrate concentrations, although NR activity was induced in cells containing internal nitrate pools regardless of the external nitrate concentration. (C) 1998 Elsevier Science BN. All rights reserved.