Effects of temperature on the nitrate reductase activity and growth of Ulva prolifera

To better understand the effect of temperature on the growth and nitrate reductase activity (NRA) of Ulva prolifera and their relationships, the effects of five different temperatures (10, 15, 20, 25, and 30°C) were investigated in a laboratory setup. In this study, an optimization in vitro analysis method for Ulva prolifera NRA was developed. Under different treatments, the NRA, nitrate concentration, pH, the intracellular nitrate and nitrite concentrations, and the POC/PON were evaluated. The results of the in vitro analysis method showed it was optimal for the NRA assay when the extraction time was 6 min, enzymatic reaction time 30 min, volume of phenazine methosulfate (PMS) solution 50 μL, NADH concentration 0.36 mM, and KNO₃ concentration 10 mM. The maximal NRA (NRA_max ) appeared on the 2nd day in the 10, 15, and 20°C (low-temperature) groups and on the 1st day in the 25 and 30°C (high-temperature) groups. The algal growth ended earlier at a high temperature, ending after 5 d at 30 and 25°C and 7 d at 20°C and 9 d at 15°C, and the alga at 10°C had been growing during the incubation period. Ulva prolifera cultivated in a range of 10-20°C had a long growth cycle and the NRA decreased with increasing temperature when exceeded 15°C, a positive correlation between algal growth and NRA was observed. This study supports NRA is a suitable proxy of the effects of temperature changes on the ability of Ulva prolifera to uptake and metabolize nitrogen nutrients.