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

Abstract

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 (P_m), photosynthetic efficiency (α), and dark respiration (R_d) were barely affected by the increase in CO₂ concentration. However, at 25°C the P_m, α and R_d were pronouncedly enhanced by elevated CO₂. Additionally, high temperature increased the slopes of curves of electron transport rates (ETR) versus gross photosynthesis (P_g), 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.

Key words:

• Elevated CO₂
• growth
• nitrate assimilation
• photosynthesis
• temperature
• Ulva lactuca

Editorial responsibility: Mirta Teichberg

Additional information

Funding

This study was supported by the National Natural Science Foundation of China (grant number 41276148, grant number 41076094).