The nutrient requirement is known to depend on the species but also on the stress that has been induced to stimulate lipid or carbohydrate storage. The nitrogen and phosphorus quota can strongly vary during a starvation period (Geider and La Roche, 2002). The hypotheses on required fertilizers strongly vary according to the species and between the publications for the same species (Lardon et al., 2009; Stephenson et al., 2010; Yang et al., 2011). Needs in ni­trogen vary from 10.9 g kgDM-1 (Lardon et al., 2009) to 20.32 g kgDM-1 (Stephenson et al.,

2010) in limiting conditions and from 9.41 g kgDM-1 (Kadam, 2002) to 77.6 g kgDM-1 (Clarens et al., 2011) without stress. Needs in phosphorus vary from 2.4 g kgDM-1 (Lardon et al., 2009) to 2.58 g kgDM-1 (Khoo et al., 2011) in limiting conditions, and from 0.02 g kgDM-1 (Kadam, 2002) to 71 g kgDM-1 (Yang et al., 2011) without stress. All the au­thors agree on high nutrient consumption for the culture of microalgae, but they differ on the ways to provide them (see Table 13.3). Some authors, such as Sander and Murthy (2010) and Clarens et al. (2010,2011), consider that the needs in nitrogen and phosphorus can be totally or partially covered by the addition of wastewater to the growth medium. But in most of the publications, nutrients are provided by chemical fertilizers. To reduce the nutrient consump­tion, several authors suggest recycling the digestates resulting from the anaerobic digestion of oilcakes (Stephenson et al., 2010; Brentner et al., 2011; Campbell et al., 2011; Clarens et al.,

2011) or of bulk microalgae (Clarens et al., 2011; Collet et al., 2011).

Figure 13.4 shows environmental impacts of various fertilizer sources. As previously dem­onstrated for the energy mix, the source of nutrients can have important consequences on the environmental balance of the energy production from microalgae. Climate-change impact and endpoint impacts on human health and ecosystem can vary by a factor of two, based on the chosen nitrogen fertilizer. For these three impacts, ammonium nitrate is the worst one, and the impacts of ammonium sulphate, calcium nitrate, and urea are quite the same. Concerning resource consumption, urea is the worst, mainly because of the high amount of natural gas used for its production. Clarens et al. (2010, 2011) and Sander and Murthy (2010) suggest using wastewater to grow algae. This assumption allows reducing the con­sumed quantities of freshwater and chemical fertilizers. However, mineral elements’ content in wastewater can strongly vary depending on the place and the period of the year. For these reasons, from our point of view it seems very difficult to rely on such fertilizers.