Abstract:Eutrophication caused by mariculture has become a serious environmental problem. Seaweeds would play an important role for decreasing marine eutrophication due to the cost saving, low sensitivity to environmental and impurity factors, the possible contaminant recovery from the biomaterial and its elevated adsorption capacity. Seaweeds are able to absorb large quantities of nitrogen, phosphorus and carbon dioxide, produce large quantities of oxygen, and have excellent effect on decreasing eutrophication. In this paper, ammonium nitrogen removing efficiency and absorption rate by Gracilaria asiatica with different culture densities were determined, and dynamics character of ammonium nitrogen uptaking at N-limited and the N-replete situation and the effect of different initial concentration of ammonium nitrogen on absorption rate were studied. Gracilaria asiatica was collected at Xiangshan harbor, Zhejiang province, southern China, and cultivated in the laboratory. Samples of the alga were maintained in a flask with 250 mL seawater of 21 salinity. The alga were cultured in culture boxes at (20±1) ℃, under 60 μmol·m-2·s-1 illumination, and on 12 h:12 h light/dark cycle. The result showed that the ability of ammonium nitrogen removing was enhanced by seaweed culture density (in 2-24 g·L-1) and experiment time (in 5 hours) increasing. The highest efficiency of ammonium nitrogen removing with highest density (24 g·L-1) of G. asiatica was up to 99.77% after 5 hours. Ammonium nitrogen concentration declined from 300 μmol·L-1 to zero. The ammonium nitrogen removing efficiency with lowest density (2 g·L-1) was only 20%. Higher ammonium nitrogen absorption rate (30-41 μmol·g-1·h-1) was kept in different density groups during initial culture period, especially in 16 g·L-1 density group, then decreased with density increase. The maximum uptake rates reached as high as 28.33 μmol·g-1h-1and 18.85 μmol·g-1·h-1 at 3 h and 5 h, respectively, in 2 g·L-1 minimum density group. The lowest ammonium nitrogen uptake rate of 11.70 μmol·g-1·h-1 and 1.90 μmol·g-1·h-1 were obtained at 3 h and 5 h with the highest density group (24g/L). In initial concentration experiment, maximum uptake rates (Vmax) and half-saturation constant(Km) of G. asiatica at N-limited and the N-replete situation reached the highest value, 116.47, 159.40 μmol·g-1·h-1 and 439.70, 913.61 μmol·L-1 in the first hour, then they declined from 24.29, 23.22 μmol·g-1h-1 and 166.87, 255.30 μmol·L-1 with culture prolonging from 1 h to 5 h. In initial concentration experiment, ammonium nitrogen removing efficiency were increased with initial concentration reduced in the range of 100-300 μmol·L-1, especially with lowest ammonium nitrogen concentration (100 μmol·L-1). Ammonium nitrogen removing efficiency kept in 56.7%-67.4% with 300-500 μmol·L-1 of ammonium nitrogen concentration. There was no difference for ammonium nitrogen absorption of G. asiatica at between N-limited and the N-replete situation. Within 300-500 μmol·L-1 of ammonium nitrogen concentration, a fast absorption rate of G. asiatica at N-half hungry situation presented in the first stage(40.7-102.1 μmol·g-1·h-1), and almost had a positive relation with concentration, which was not fit the Micheal dynamics saturation equations. But, within 100-200 μmol·L-1 of ammonium nitrogen concentration, the uptake rates of G. asiatica were as low as 17.8-40.8 μmol·g-1·h-1. When the concentration decreased in some limited range, the absorption rate reached the maximum and fit the Micheal dynamics saturation equations. It would provide important theory data for cleaning water and bioremediation by cultivating G. asiatica, and large-scale cultivation of G. asiatica could be a good solution to the problem of eutrophication due to their capability of removing nutrients.