IRON-MEDIATED EFFECTS ON NITRATE REDUCTASE IN MARINE-PHYTOPLANKTON

The potential activity of nitrate reductase was determined in uni-alga l cultures in the laboratory and in natural marine phytoplankton assem blages. In the laboratory bioassays, distinct differences in nitrate r eductase activity were observed in iron replete versus depleted cultur es for Emiliania huxleyi, Isochrysis galbana and Tetraselmis sp. Cells from iron-depleted cultures had 15 to 50 percent lower enzyme activit y than those from iron-replete cultures. Upon addition of iron, nitrat e reductase activity was enhanced in depleted cells up to levels compa rable to those of the replete cells. Bioassays in the northern North S ea conducted in 1993, under low iron conditions, demonstrated similar results. Upon addition of 2.5 nM iron, a distinct enhancement, to a ma ximum of three times, of nitrate reductase activity was observed withi n 32 h after addition. Therefore, iron can stimulate nitrate reductase activity. In spite of the clean techniques used, some nitrate reducta se activity was always observed. Iron deficiency was shown to impair n itrate reductase activity, but it is unlikely that nitrate reduction w ould cease completely.