Ammonium thresholds for simultaneous uptake of ammonium and nitrate by oyster-pond algae

Abstract

Natural microalgal populations and axenic algal isolates from oyster ponds have been grown either in situ or in controlled conditions, in the presence of ammonium and nitrate as nitrogen sources. Both ions were added at various concentrations, up to 50 μg-at. N·l −1; other nutrients were in excess. In one experiment, urea was also added. Uptake of nitrate was followed by measuring disappearance of nitrate from the medium, and by incorporation of 15NO 3; nitrate reductase (NR) activity and the intracellular nitrate pool were also measured. The uptake of nitrate was prevented by the presence of ammonium above a concentration which varied according to species. The ammonium threshold (in units of μg-at. N·l −1) was ≈ 30 for natural populations and the diatom Navicula ostrearia Bory, ≈ 21 for Nitzschia ovalis (sensu Hustedt), and ≈ 44 for Amphora coffeaeformis (sensu Hendey). Nitrate uptake started at a rate which was ≈ 39% of the eventual maximum rate observed for the natural populations, and from 11 to 17% for cultured strains. The initial low rate was maintained until the ambient ammonium concentration had decreased to ≈ 7.5 μg-at. N·l −1, except for A. coffeaeformis which shifted from slow to fast nitrate uptake at 23.5μg-at. N·l −1. The nitrate uptake system then operated at a slightly higher rate than the one for ammonium (ammonium uptake/nitrate uptake = 0.86). Cultures with an initial ammonium concentration lower than the threshold values did not show an initial low rate or a lag phase for uptake of nitrate. NR activity was detectable even in the presence of ≈ 30 μg-at. NH 4-N·l −1 in the external medium. When urea and nitrate were presented simultaneously, urea was not taken up preferentially, as reported elsewhere; uptake was initially at a reduced rate until the external nitrate concentration decreased to 3.7 μg-at. N·l −1. Then the rate of urea uptake increased to a maximum. It is suggested that because oyster-pond algae have evolved in an environment where concentrations of ammonium, nitrate, and organic nitrogen are continuously high, the threshold of inhibition by ammonium of uptake of these compounds is much higher than for similar pelagic and neritic species, so that they are able to assimilate other sources of nitrogen, such as nitrate and urea, simultaneously with ammonium.