Photon- and carbon-use efficiency in Ulva rigida at different CO 2 and N levels

The seaweed Ulva rigida C. Agardh (Chlorophyta) was cultured under two CO2 conditions supplied through the air bubbling system: non-manipulated air and 1% CO2-enriched aeration. These were also combined with N sufficiency and N limitation, using nitrate as the only N source. High CO2 in U. rigida led to higher growth rates without increasing the C fixed through photosynthesis under N sufficiency. Quantum yields for charge separation at photosystem II (PSII) reaction centres (ϕPSII) and for oxygen evolution (ϕO2) decreased at high CO2 even in N-sufficient thalli. Cyclic electron flow around PSII as part of a photoprotection strategy accompanied by decreased antennae size was suspected. The new re-arrangement of the photosynthetic energy at high CO2 included reduced investment in processes other than C fixation, as well as in carbon diverted to respiration. As a result, quantum yield for new biomass-C production (ϕgrowth) increased. The calculation of the individual quantum yields for the different processes involved allowed the completion of the energy flow scheme through the cell from incident light to biomass production for each of the CO2 and N-supply conditions studied.