Effects Of Elevated Atmospheric CO₂ On Photosynthesis Of Three Species Of Marine Macroalgae Grown Under Different Conditions

CO2 concentration is the critical environmental factor affecting physiological and biochemical traits of marine macroalgae.The responses of seaweeds to elevated CO2 may vary with various environmental factors under which the seaweeds were grown. This study adopted three species of marine macroalgae, Ulva conglobata(Ulva, Chlorophyta), Porphyra haitanensis(Bangiales, Rhodophyta) and Gracilaria lemaneiformis(Gigartinales,Rhodophyta) as experimental materials. They were cultured under outdoor conditions. The treatments were designated as ambient(390 μL·L-1) and elevated(800 μL·L-1) CO2 concentrations, in order to examine the effects of elevated CO2 on physiological and biochemical traitsin marine macroalgae grown at different N supplies or different incubation densities conditions.The main results were as follows:1. The treatments were designated as low(10 μmol·L-1) and high N(150 μmol·L-1) availability, in order to examine the effects of elevated CO2 on growth and photosynthesis in U.conglobata and G.lemaneiformis grown at different N supplies. The results showed that elevated CO2 significantly enhanced the relative growth rate(RGR) in both the macralgal species regardless of the N levels in culture. However, such enhancement of growth was more pronounced in G.lemaneiformis, but was less pronounced in U.conglobata, under low than high N growth conditions. Short-term exposure to CO2 enrichment stimulated photosynthesis of both the macroalgal species. Such stimulation of photosynthesis in G.lemaneiformis could be fully remained with prolonged growth with enriched-CO2. However, long-term growth condition of CO2 enrichmenthad inhibitory effects on photosynthesis in U.conglobata. Additionally, the results showed that enriched N supply significantly increased photosynthesis and respiration in both the macroalgal species, regardless of the CO2 concentration in culture.2. The treatments were designated as low(1.0 g FW·L-1), middle(2.0 g FW·L-1) and high(4.0 g FW·L-1) incubation densities, to examine the effects of elevated CO2 on growth, nutrient uptake efficiency(NUE), and photosynthesis in P.haitanensis grown at different incubation densities conditions. The results showed that withincreasing incubation density, the RGR,photosynthetic rates(Pn), rates of dark respiration(Rd) and the ratio of respiration to gross photosynthesis(Rd:Pg) of P.haitanensiswere decreased, while NUE was increased. The temperature coefficient values of respiration and photosynthesis(Q10) were not significantly changed, regardless of the CO2 concentration in culture. Long-term growth condition of elevated CO2 significantly enhanced the RGR, NUE, Q10 and Rd:Pg, but inhibited photosynthesis, and had no significant effect on respiration, irrespective of the incubation density. Short-term exposure to CO2 enrichment stimulated photosynthesis, but had no significant effect on respiration, Q10 and Rd:Pg. Low density-grown P.haitanensis was more responsive to CO2 enrichment than highdensity-grown alge. Additionally, higher measuring temperature(23℃) enhanced the Pn and Rd of P.haitanensis relative to lower measuring temperatures(18℃ and 13℃).3. The treatments were designated as low(1.0 g FW·L-1), middle(3.0 g FW·L-1) and high(6.0 g FW·L-1) incubation densities, to examine the effects of elevated CO2 on growth, NUE, biochemical components and chlorophyll fluorescence in U.conglobata and G.lemaneiformis grown at different incubation densities conditions.The results showed thatincubation density and the RGR in U.conglobata and G.lemaneiformis were negatively correlated, but incubation density and the NUE were positively correlated. With increasing incubation densities, the contents of Chla and Car in U.conglobata were increased, while the contents of PE, PC, SP, Chla and Car in G.lemaneiformis were not significantly changed. The increase of incubation densities made a positive effect on maximum photochemical quantum yield values(Fv/Fm) and the photosynthetic light use efficiency of electron transport(α, the initial slope of therapid light curves) of G.lemaneiformis, but made a negligible effect on Fv/Fm and α of U.conglobata, regardless of the CO2 concentration in culture. Elevated CO2 significantly enhanced RGR and NUE of U.conglobata and G.lemaneiformis, but had inhibitory effects on biochemical components of U.conglobata and G.lemaneiformis. Elevated CO2 significantly increased the values of Fv/Fm and α of G.lemaneiformis, but decreased the values of Fv/Fm and α of U.conglobata. Compared with higher incubation densities, the responses of NUE, Fv/Fm, α in U.conglobata and G.lemaneiformis to elevated CO2 were more pronounced under low incubation density, while the responses of growth in U.conglobata and G.lemaneiformis to elevated CO2 were less pronounced under low incubation density.In conclusion, the responses of seaweeds to elevated CO2 were depended not only on species, but also on the conditions such as incubation densities and N supplies under which the seaweeds were grown. Elevated CO2 in the culture enhanced RGR and NUE of U.conglobata, G.lemaneiformis and P.haitanensis. Short-term exposure to CO2 enrichment stimulated photosynthesis of U.conglobata, G.lemaneiformis and P.haitanensis. However, long-term growth condition of CO2 enrichment stimulated photosynthesis of G.lemaneiformis, but suppressed photosynthesis of U.conglobata and P.haitanensis. The relative enhancement of growth in response to elevated CO2 was more pronounced in low-N-grown G.lemaneiformis, but was less pronounced in U.conglobata than high-N-grown alge. Low incubation densities conditions strengthen photosynthesis in response to elevated CO2 in U.conglobata, G.lemaneiformis and P.haitanensis, but weaken growth in response to elevated CO2 in U.conglobata and G.lemaneiformis.