ORIGINAL ARTICLE Photon- and carbon-use efficiency in Ulva rigida at different CO 2 and N levels Received: 18 February 2003 / Accepted: 6 July 2003 / Published online: 21 August 2003 Ó Springer-Verlag 2003 Abstract The seaweed Ulva rigida C. Agardh (Chloro- phyta) was cultured under two CO 2 conditions supplied through the air bubbling system: non-manipulated air and 1% CO 2 -enriched aeration. These were also com- bined with N sufficiency and N limitation, using nitrate as the only N source. High CO 2 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 (/ PSII ) and for oxygen evolution (/ O2 ) decreased at high CO 2 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 CO 2 included reduced investment in processes other than C fixation, as well as in carbon diverted to respi- ration. As a result, quantum yield for new biomass-C production (/ growth ) 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 CO 2 and N-supply conditions studied. Keywords Ulva Æ Absorptance Æ Chlorophyll fluorescence Æ Inorganic carbon Æ Nitrogen limitation Æ Quantum yield Abbreviations A: total thallus absorptance Æ A pig : absorptance due to pigments Æ A str : Absorptance due to non-pigmented structures Æ a*: spectrally averaged in vivo absorption cross-section of chlorophyll a Æ CCM: carbon-concentrating mechanism Æ Chl: chlorophyll Æ DOC: dissolved organic carbon Æ ETR: electron transport rate Æ F v /F m : optimum quantum yield for PSII charge separation Æ GP: gross O 2 evolution rate Æ k pig : specific light absorption coefficient for pigments Æ k str : specific light absorption coefficient for non- pigmented structures Æ OP: optimum O 2 evolution rate Æ PFR: photon fluence rate Æ POC: particulate organic carbon Æ PS: photosystem Æ q N : non-photochemical quenching Æ q P : photochemical quenching Æ / growth : quantum yield for new biomass-C production Æ / O2 : quantum yield for gross O 2 evolution Æ / PSII : quantum yield for PSII charge separation Introduction One of the main queries for depicting future scenarios for ecosystems is whether the expected increase in atmospheric CO 2 will stimulate primary production. In aquatic environments, to answer this question it is necessary to know not only whether photosynthesis is saturated at the present carbon concentration in the oceans, since many aquatic primary producers possess carbon-concentrating mechanisms (CCMs), but also whether there is a corresponding effect on growth (Riebesell et al. 1993). Thus, the majority of the studies on the effects of increasing CO 2 in aquatic photosyn- thetic organisms have mainly focused on the uptake of carbon through CCMs, while little attention has been paid to other processes. This has led to a lag in knowledge of the interaction between high CO 2 , light-harvesting efficiency and nutrient assimilation in aquatic plants with respect to their terrestrial counterparts. For terrestrial plants, it has been proposed that the response to elevated CO 2 would be similar to that of nitrogen limitation (Webber et al. 1994). The hypothesis was developed and tested that elevated CO 2 increases photochemical energy use when there is a high demand for assimilates and decreases usage when demand is low. The acceleration of C assimilation through photosyn- thesis provokes a higher demand for N. If N availability or assimilation rates cannot meet the new demand, a Planta (2003) 218: 315–322 DOI 10.1007/s00425-003-1087-3 Francisco J. L. Gordillo Æ Fe´lix L. Figueroa F. Xavier Niell F. J. L. Gordillo (&) Æ F. L. Figueroa Æ F. X. Niell Departamento de Ecologı´a, Facultad de Ciencias, Universidad de Ma´laga, Campus de Teatinos, 29071 Ma´laga, Spain E-mail: gordillo@uma.es Fax: +34-952-132000