Nitrogen uptake in light versus darkness of the seagrass Zostera noltei: integration with carbon metabolism

We conducted a study that shows that light and dark conditions do not affect the uptake rates of ammonium and nitrate by the seagrass Zostera noltei. This is an important advantage over some seaweed species in which these rates are severely reduced at night. In the light, the ammonium uptake rates were initially higher (15 and 20molg(-1)h(-1)) and stabilized at a rate of 5molg(-1)h(-1) after 1h, whereas in the dark the rates remained constant at a rate of 10molg(-1)h(-1) over the first 180min of incubation. The rates of nitrate uptake in the light were high within the first 120min of incubation(7.2-11.1molg(-1)h(-1)) and decreased afterwards to lower values (0.8-3.9molg(-1)h(-1)), whereas in the dark the rates fluctuated around 0.0-11.1molg(-1)h(-1) throughout the whole incubation time (7h). The soluble sugar content of Z.noltei leaves increased significantly with both ammonium and nitrate incubations in the light, indicating the metabolic outcome of photosynthesis. In the dark, there was no significant variation in either the soluble sugar or in the starch content of leaves, rhizomes or roots in either the ammonium or nitrate incubations. However, the total starch content of plants decreased at night whereas the total soluble sugars increased, suggesting a process of starch catabolism to generate energy with the consequent production of smaller monosaccharide products. The starch content of rhizomes decreased significantly during the light incubations with nitrate but not with ammonium. These results suggest that carbohydrate mobilization is necessary for Z.noltei to account for extra energetic costs needed for the uptake and assimilation of nitrate. Furthermore, our results suggest that nitrate uptake, at least during the day, requires the mobilization of starch whereas the uptake of ammonium does not.