Physiologia Plantarum 132: 338–349. 2008 Copyright ª Physiologia Plantarum 2007, ISSN 0031-9317 Bioenergetic changes in the microalgal photosynthetic apparatus by extremely high CO 2 concentrations induce an intense biomass production Aikaterini Papazi a , Pavlos Makridis b , Pascal Divanach b and Kiriakos Kotzabasis a, * a Department of Biology, University of Crete, PO Box 2208, 71409 Heraklion, Crete, Greece b Hellenic Centre for Marine Research, Institute of Aquaculture, PO Box 2214, 71003 Heraklion, Crete, Greece Correspondence *Corresponding author, e-mail: kotzab@biology.uoc.gr Received 22 July 2007; revised 5 October 2007 doi: 10.1111/j.1399-3054.2007.01015.x Unicellular green alga Chlorella minutissima, grown under extreme carbon dioxide concentrations (0.036–100%), natural temperature and light intensi- ties (Mediterranean conditions), strongly increase the microalgal biomass through photochemical and non-photochemical changes in the photosyn- thetic apparatus. Especially, CO 2 concentrations up to 10% enhance the density of active reaction centers (RC/CS o ), decrease the antenna size per active reaction center (ABS/RC), decrease the dissipation energy (DI o /RC) and enhance the quantum yield of primary photochemistry (F v /F m ). Higher CO 2 concentrations (20–25%) combine the above-mentioned photochemical changes with enhanced non-photochemical quenching of surplus energy, which leads to an enhanced steady-state fraction of ‘open’ (oxidized) PSII reaction centers (q p ), and minimize the excitation pressure of PSII (1 2 q p ) under very high light intensities (approximately 1700 mmol m 22 s 21 maximal value), avoiding the photoinhibition and leading to an enormous biomass production (approximately 2500%). In conclusion, these extreme CO 2 concentrations – about 1000 times higher than the ambient one – can be easily metabolized from the unicellular green alga to biomass and can be used, on a local scale at least, for the future development of microalgal photo- bioreactors for the mitigation of the factory-produced carbon dioxide. Introduction The significant rise in atmospheric carbon dioxide is an environmental change that affects the photosynthetic process (Mc Elroy 1994). Photosynthesis is one of the few anti-entropic mechanisms on Earth. It is a compli- cated photochemical process occurring in photosyn- thetic organisms and leads to the transformation of the photon to chemical energy (Papadakis et al. 2005). Increased carbon dioxide concentrations lead to a number of different changes in plant physiology and metabolism, such as the reduction of the stomatal density, the decline of the photorespiration and dark respiration and the increase of the C/N ratio (Bowes 1996). Also, higher plants exposed to elevated carbon dioxide for weeks, months or years accumulate starch and soluble sugars and often have lower levels of Rubisco, lower photorespiration, lower Chl contents and sometimes lower Chl a/b ratios than plants grown in ambient carbon dioxide (Van Oosten et al. 1994). The unicellular green algae demonstrate responses to increased carbon dioxide similar to those of higher plants in terms of biomass increase (Muller et al. 1993). The Abbreviations – (1 2 q p ), excitation pressure of PSII; ABS/RC, antenna size per active reaction center; DI o /RC, dissipation energy per reaction center; ETR, electron transport rate; F v /F m , photosynthetic efficiency; NPQ, non-photochemical quenching; PCV, packed cell volume; q p , photochemical quenching; RC/CS o , density of active reaction centers. 338 Physiol. Plant. 132, 2008