Responses of Ulva prolifera to short-term nutrient enrichment under light and dark conditions

• Ulva prolifera can regulate the uptake rates of inorganic N and P rapidly.
• U. prolifera can utilize multiple nutrient sources.
• Enzyme activity of NR, AcP and AP showed little changes in a short term.

To define responses of short-term nutrient uptake in Ulva prolifera, we measured uptake rates, enzyme activity, and tissue nutrient content in lab experiments where we manipulated nutrient supply and irradiation. Nitrate uptake of U. prolifera was significantly impacted by the external nitrate concentrations, and ammonium uptake was mainly determined by the light availability. The measured nitrogen contents in tissues were higher than the calculated values from the uptake of dissolve inorganic nitrogen, indicating that U. prolifera might use multiple nitrogen sources. High external phosphate concentrations and sufficient light can accelerate the phosphate uptake of U. prolifera, while the measured phosphorus contents in tissues were lower than the calculated values from the uptake of phosphate, suggesting a possibility of internal phosphorus release. The enzymatic activities of nitrate reductase (NR), acid phosphatase (AcP) and alkaline phosphatase (AP) showed little changes, indicating that enzymatic activity might not a direct factor determining the short-term nutrient uptake of U. prolifera.