Responses of elevated CO2 on photosynthesis and nitrogen metabolism in Ulva lactuca (Chlorophyta) at different temperature levels

Ulva lactuca was cultured in a laboratory at two CO2 levels (390 and 700 μl l−1) and at low (15°C) and high (25°C) temperature levels. Growth, biochemical composition, photosynthesis and nitrogen metabolism were examined to evaluate the impacts of elevated CO2 and temperature on this species. Elevated CO2 had little effect on the relative growth rate (RGR) in both low and high temperature levels. The levels of Chl a, carotenoids, soluble protein (SP) and soluble carbohydrates (SC) were also unaffected by CO2 enrichment. Elevated CO2 decreased the content of total fatty acids (FAs) but the higher temperature increased them. At 15°C, the light-saturated maximal photosynthetic rate (Pm), photosynthetic efficiency (α), and dark respiration (Rd) were barely affected by the increase in CO2 concentration. However, at 25°C the Pm, α and Rd were pronouncedly enhanced by elevated CO2. Additionally, high temperature increased the slopes of curves of electron transport rates (ETR) versus gross photosynthesis (Pg), indicating the loss of correlation between ETR and linear photosynthetic flow. The elevation of temperature significantly increased the nitrogen uptake rates (NUR) during the dark and light period, while elevated CO2 increased NUR only during the light period. Increased CO2 had no marked effect on nitrate reductase (NR) activity at 15°C, but high CO2 could remarkably improve the NR activity at 25°C. These results suggest that the responses to elevated CO2 in U. lactuca were temperature-dependent, and elevated CO2 alone was less effective than temperature levels.

Liu C. & Zou D., in press. Responses of elevated CO2 on photosynthesis and nitrogen metabolism in Ulva lactuca (Chlorophyta) at different temperature levels. Marine Biology Research. Article (subscription required).


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