Uptake kinetics of nitrogen and phosphorus by Zostera marina

Aims Nutrient availability is an external factor that affect the growth of seagrasses. However, the demand for and absorption of different forms of nitrogen and phosphorus by different tissues of seagrasses are unclear. In this study, the uptake of nitrogen and phosphorus by Zostera marina was studied to determine the nutrient uptake kinetics. The objectives of this research are to: 1) investigate the absorption characteristics of ammonia nitrogen, nitrate nitrogen and phosphorus in Z. marina; 2) evaluate the differences in absorption between the different forms of nitrogen; and 3) analyse the differences in absorption between the different tissues of Z. marina.Methods Equipment was used to separate the aboveground and belowground tissues of Z. marina. Six concentration levels of ammonia nitrogen, nitrate nitrogen and phosphorus were established to experimentally test the uptake kinetics of nutrients by Z. marina. The nutrient concentrations in different parts of seawater column were measured to determine the nutrient changes and calculate the kinetic characteristics of nutrient uptake.Important findings 1) The absorption of ammonia nitrogen, nitrate nitrogen and phosphorus by Z. marina was consistent with the characteristics of saturated absorption kinetics. The relationship between the absorption rate and the nutrient concentrations in water could be described by the Michaelis-Menten equation. 2) The maximum absorption rate (V_max) of ammonia nitrogen by Z. marina (52 μmol·g^-1·h^-1) was significantly higher than that of nitrate nitrogen (39 μmol·g^-1·h^-1). 3) Both aboveground and belowground tissues of Z. marina could absorb nutrient, but the V_max of leaves (aboveground tissues) for ammonia nitrogen, nitrate nitrogen and phosphorus absorption were 43.1, 30.5 and 15.6 μmol·g^-1·h^-1, respectively, which were 2.6-fold, 1.2-fold and 6-fold higher than the corresponding V_max of belowground tissues. The results show that the absorption capacity of Z. marina for ammonia nitrogen is higher than that for nitrate nitrogen, and the absorption of nitrogen and phosphorus may depend primarily on the aboveground tissues (leaves). The results provide a theoretical basis for the study of the mechanisms of nitrogen and phosphorus uptake and utilization by Z. marina and the evaluation of marine ecological impacts.