Comparative physiological behaviors of Ulva lactuca and Gracilariopsis lemaneiformis in responses to elevated atmospheric CO 2 and temperature

Abstract

Physiological metabolisms of seaweeds usually suffered climate changes in the field. Gracilariopsis lemaneiformis and Ulva lactuca, collected from Nan'ao Island, Shantou, China, were cultured under ambient and elevated CO₂ supply (390 and 800 μl L⁻¹), with low and high temperatures (15 °C and 25 °C) for 2 weeks, aiming to compare the difference of the main physiological metabolism between two seaweed species in response to the elevated CO₂ and high temperature. At 15 °C, the pH reduction in the culture medium caused by elevated CO₂ was larger in G. lemaneiformis than in U. lactuca. At 25 °C, elevated CO₂ significantly increased photosynthetic rates (P_n or P_g) and maintained constant respiratory rates (R_d) in G. lemaneiformis. However, for 25 °C-grown U. lactuca, the increment of CO₂ did not enhance the P_n (P_g) rates but rapidly decreased the R_d rates itself. With the higher R_d/P_g ratios in G. lemaneiformis than U. lactuca, the warming thereby promoted more allocation of photosynthetic products to respiratory consumption in G. lemaneiformis. Both P_g and R_d rates exhibited lower temperature acclimation in two seaweeds. In addition, elevated CO₂ markedly increased the relative growth rate (RGR) and phycobiliprotein (PB) contents at 25 °C, but exhibited no enhancement of chlorophyll a (Chl a), carotenoids (Car), soluble carbohydrate (SC), and soluble protein (SP) contents in G. lemaneiformis, with the reduction of SC when temperature increased only. We suggested that climate changes were probably a more benefit to U. lactuca than to G. lemaneiformis, inherently justifying the metabolism during G. lemaneiformis maricultivation.

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