Mineral composition, C/N ratio, and growth rate of microalgae vary naturally according to environmental conditions (light and temperature), availability of nutrients, or occurrence of stress. For instance, the application of nitrogen starvation induces, for some species, the stor­age of lipids (Ketchum and Redfield, 1949). However the increase of lipid content is done to the detriment of cell division, and consequently the mass productivity is lower. Therefore, it should be highlighted that all these properties are correlated and cannot be determined on the basis of independent assumptions or sources.

As shown on Table 13.5, a large variability of productivity, lipid fraction, or nutrient re­quirement is observed among the various studies. In four publications (Lardon et al., 2009; Batan et al., 2010; Stephenson et al., 2010; Khoo et al., 2011), authors suggest to impose nitrogen deprivation on the algae. To overcome the problem of the growth-rate reduction under nutrient stresses, some authors suggest cultivating microalgae in two steps. First, microalgal biomass is cultivated in nitrogen-replete conditions in order to reach a high growth rate. Then microalgae are submitted to nitrogen deprivation to increase their lipid content.

Growth rate is known to be species dependent and strongly influenced by light and tem­perature (Falkowski and Raven, 1997). It can be strongly reduced by the stress protocol used to induce lipid accumulation by nutrient deprivation (Lacour et al., 2012). Depending on the location, cultivation system, species, and protocol, growth rate and biomass concentration can therefore vary by more than an order of magnitude. The hypotheses made in LCA studies reflect this large spectrum. In ORW, growth rates vary from 25 (Batan et al., 2010; Collet et al., 2011) to 40.6 g m-2 d-1 (Clarens et al., 2010). In PBR, productivities are much higher and vary from 270 (Jorquera et al., 2010) to 1536 g m-3 d-1 (Brentner et al., 2011). The PBR conception has a strong influence on the growth rate (Jorquera et al., 2010). Microalgae con­centrations range from 0.5 (Lardon et al., 2009) to 1.67 g L — (Stephenson et al., 2010) in an OR, and from 1.02 (Jorquera et al., 2010) to 8.3 g L-1 (Stephenson et al., 2010) in a PBR. Expected lipid contents vary broadly between authors: from 17.5% (Lardon et al., 2009) to 50% (Kadam, 2002) without nitrogen deprivation and from 25% (Khoo et al., 2011) to 50% with nitrogen deprivation (Batan et al., 2010; Stephenson et al., 2010).