Interactive effects of elevated CO2 and nitrogen-phosphorus supply on the physiological properties of Pyropia haitanensis (Bangiales, Rhodophyta)

The ongoing increases in atmospheric CO2 concentrations and coastal eutrophication have affected the coastal environment and marine macroalgae. In this study, the interactive effects of CO2 (390 and 1000 mu L L-1) and nutrient levels (nitrogen and phosphorus supplied simultaneously) on the physiological properties of the maricultured macroalga Pyropia haitanensis (Bangiales, Rhodophyta) were investigated. The results showed that elevated CO2 significantly enhanced P. haitanensis growth and NO3 (-) uptake, but lowered the pH compensation points, regardless of nutrient levels, and enhanced photosynthesis and apparent photosynthetic efficiency (alpha) when the nutrient levels were high. However, CO2 had little effect on photosynthetic rates at low nutrient levels. At each nutrient level, CO2 elevation lowered both the phycobiliproteins (PB) and soluble protein contents, but enhanced biomass accumulation. Chlorophyll a (Chl a) and carotenoid (Car) contents were markedly increased by high CO2 concentrations at low nutrient levels. Increasing nutrient supply significantly enhanced growth, pH compensation points, and photosynthesis in P. haitanensis at each CO2 level, but the differences of the effects between intermediate and high nutrient levels on this alga were not significant for photosynthesis, pigment content, and nutrient uptake, regardless of CO2 levels. Our results suggest that the growth and physiological responses of P. haitanensis to CO2 levels are largely dependent on the supplement of nutrients. However, the interactive effects of elevated CO2 and nitrogen-phosphorus supplies on the physiological properties of P. haitanensis were limited through respective regulation by the CO2 levels in the atmosphere and the nutrient concentrations in the seawater.