Nitrogen uptake and assimilation in Enteromorpha intestinalis (L.) Link (Chlorophyta): using 15N to determine preference during simultaneous pulses of nitrate and ammonium

Abstract

We investigated the ability of Enteromorpha intestinalis (L.) Link to take up pulses of different species of nitrogen simultaneously, as this would be an important mechanism to enhance bloom ability in estuaries. Uptake rates and preference for NH 4 + or NO 3 − following 1, 3, 6, 9, 12 or 24 h of exposure to either 15NH 4NO 3 or NH 4 15NO 3 were determined by disappearance of N from the medium. Differences in assimilation rates for NH 4 + or NO 3 − were quantified by the accumulation of NH 4 +, NO 3 −, and atom % 15N in the algal tissue. NH 4 + concentration was reduced more quickly than water NO 3 − concentration. Water column NH 4 + concentration after the longest time interval was reduced from 300 to 50 μM. Water NO 3 − was reduced from 300 to 150 μM. The presence of 15N or 14N had no effect on uptake of either NH 4 + or NO 3 −. 15N was removed from the water at an almost identical rate and magnitude as 14N. Differences in accumulation of 15NH 4 + and 15NO 3 − in the tissue reflected disappearance from the water; 15N from NH 4 + accumulated faster and reached an atom % twice that of 15N from NO 3 −. This outcome suggested that when NH 4 + and NO 3 − were supplied in equal concentrations, more NH 4 + was taken up and assimilated. The ability to take up high concentrations of NH 4 +, and NO 3 − simultaneously is important for bloom-forming species of estuarine macroalgae subject to multiple nutrient species from various sources.