Outdoor cultivation of Ulva lactuca in a recently developed ring-shaped photobioreactor: effects of elevated CO2 concentration on growth and photosynthetic performance

Land-based cultivation of marine macroalgae may open up the possibility to produce high quality algal biomass as required in various application areas all year round. In this context, the potential of a recently developed ring-shaped cultivation system with algae moving in a circular way, simulating the movement pattern in a standard tank cultivation vessel was evaluated using the green alga Ulva lactuca. Plants were cultivated under outdoor conditions at ambient (37 mu mol CO2 kg(-1) seawater) and increased CO2 concentration (152 mu mol CO2 kg(-1) seawater). Biomass growth and photosynthetic performance of algae were analyzed over a test period of 7 d. Elevated CO2 concentration significantly stimulated algal growth and also helped to compensate the effects of environmental stress conditions. This was indicated by the predominant stability of photosynthetic competence and represented by maximum photosynthetic electron transport rates, efficiency of light-harvesting and photon fluence rates (PFR) saturating photosynthetic electron transport at low PFR. At high PFR, no difference in photosynthetic competence was detected between algae cultivated at the high CO2-concentration and those grown at ambient CO2. Under elevated CO2 concentrations, photochemical energy dissipation decreased more distinctly at low PFR. This may reflect a declining energy demand necessary to maintain photosynthetic capacity. Under elevated CO2, the apparent changes in the quantum yields of regulated and unregulated non-photochemical energy dissipation of PS II at high PFR possibly reflected the enhanced capacity of photoprotection under the prevailing environmental conditions.