The Physiological Responses Of Two Economic Marine Macroalga Species To Nutrients Supplies In Seawater

At present, there were two serious ecological problems in ocean: increasing atmosphericCO2 and eutrophication of seawater. There were many studies about responses of marinemacroalgae to elevated CO₂, and the effects of nutrient supplies on marine macroalgae werealso concerned by many scholars. However, little was known about responses of marinemacroalgae to nutrient supplies under the background of increasing atmospheric CO₂. On thebasis of reviewing the progresses in the effects of elevated CO₂ or nutrient on marinemacroalgae, studies were carried out on the 1) effects of environmental conditions on theactivity of nitrate reductase in Porphyra haitanensis and the uptake characteristic of Pi inGracilaria lemaneiformis. 2) effects of increased atmospheric CO₂ and nutrients supplies ongrowth, synthesis, biochemical compositions and uptake of nutrients in Gracilarialemaneiformis. The results were as follows:â… The activity of nitrate reductase in Porphyra haitanensis exhibited a daily rhythm andwas under circadian control. In addition, it was enhanced by light and increased nitrate supply,but the activity was not in direct proportion to the concentration of nitrate in culture. Theactivity was reduced if nitrate was substituted by ammonium in culture, suggesting thatammonium could restrain the activity of nitrate reductase in Porphyra haitanensis.â…¡The relationship between uptake rate of Pi in Gracilaria lemaneiformis and the externalconcentration of Pi could be described by the Michaelis-Menten equation. It was shown that thehigher was temperature or the concentration of nitrate, the higher was the uptake capability ofPi, however the uptake efficiency was higher at lower temperature compared to hightemperature. Light could also affect the uptake characteristic of Pi. Both the uptake capabilityand efficiency were higher under the condition of the lower irradiance level than those underthe higher level or absolute darkness. It was concluded that the uptake characteristic of Pi inGracilaria lemaneiformis would be affected by light, temperature or the external concentrationof nutrients.â…¢Either increased atmospheric CO₂ or increased nitrate supply could enhance the relativegrowth rate in Gracilaria lemaneiformis, but they had no synergic effect. Elevated CO₂decreased, but N supply increased, the contents of pigments and souble proteins. Increased atmospheric CO₂ enhanced the activity of nitrate reductase in G lemaneiformis and stimulatedthe uptake of nutrition under the condition of high N supply. Increased N supply also enhancedthe nutrient uptake rates of G. lemaneiformis.â…£In normal air, increased Pi supply had no remarkable effect on the growth in Gracilarialemaneiformis, but when CO₂ was elevated synchronously, the growth was restrained. In naturalseawater, the capability of fixing CO₂, the affinity for Ci and the efficiency of utilizing Ci wererestrained by elevated CO₂, but they could be enhanced by increased Pi supply. The contents ofChla and PC were increased by Pi supply, but they were not remarkably affected by elevatedCO₂. Either elevated CO₂ or Pi supply affected the uptake rate of nutrient in Gracilarialemaneiformis. Elevated Pi supply enhanced nutrients uptake. Elevated CO₂ accelerated theuptake of Pi, but reduced the uptake rate of nitrate under the condition of elevated Pi supply.In conclusion, environmental conditions such as irradiance level, temperature, CO₂ andconcentration of nutrients would affect the physiological characteristic of Porphyra haitanensisand Gracilaria lemaneiformis, especially the uptake characteristic of nutrients. Therefore weproposed that the environmental conditions under which the macroalge are grown should betaken into account when they are considered to be used to solve the ongoing problem of coastaleutrophication.