Comparative physiological behaviors of Ulva lactuca and Gracilariopsis lemaneiformis in responses to elevated atmospheric CO2 and temperature.

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 (Pn or Pg) and maintained constant respiratory rates (Rd) in G. lemaneiformis. However, for 25 °C-grown U. lactuca, the increment of CO2 did not enhance the Pn (Pg) rates but rapidly decreased the Rd rates itself. With the higher Rd/Pg ratios in G. lemaneiformis than U. lactuca, the warming thereby promoted more allocation of photosynthetic products to respiratory consumption in G. lemaneiformis. Both Pg and Rd 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.