CO2 must be supplied to the growth medium to reach high algal productivities. It has been shown that, provided that pH is regulated, the microalgae can be very tolerant to the source of CO2 (Doucha et al., 2005). However, the dissolution efficiency, together with the ability of microalgae to consume this CO2, are very dependent on the cultivation system. The supply rate in the LCA studies ranges from 0.51 to 2.36 kgCO2 kgDM-1. Depending on the studies, CO2 is supplied from compressed and purified gas or from the flue gas of a local power plant, either after capture or directly (Table 13.6). The percentages of CO2 in the flue gas vary from 5% (Stephenson et al., 2010) to 15% (Brentner et al., 2011; Campbell et al., 2011). It is common to point out the lack of knowledge of the long-term consequences on algae and on culture facility due to the use of flue gas. However, Yoo et al. (2010) demonstrated that Botryococcus braunii and Scenedesmus sp. could grow using flue gas as a source of carbon. Energy costs of

injection and head losses are always taken into account. The injection of flue gas in the growth medium without prior enrichment or compression requires compressing higher volumes of gas and reduces the efficiency of the gas-injection system. Hence there is a clear trade-off in terms of energy consumption between prior purification and gas injections. Some authors (Kadam, 2002; Brentner et al., 2011; Clarens et al., 2011) include in their study the costs of purification and transport.