8.3.1 General remarks

When various sugars or similarly metabolized compounds (e. g. glycerol, polysaccharides, etc.) are utilized for the production of SCO, accumulation of lipid in the microbial cells or mycelia (the so-called ‘de novo’ lipid accumulation process) is triggered by exhaustion of nitrogen from the growth medium, which allows the conversion of sugar to storage lipid (Ratledge, 1988, 1994; Ratledge and Wynn, 2002; Wynn and Ratledge, 2006; Papanikolaou and Aggelis, 2009;

Fakas et al, 2009b). In contrast, when growth is conducted on hydrophobic carbon sources (e. g. fats, oils), accumulation of storage lipids (the so-called ‘ex novo’ lipid accumulation process) is a primary anabolic process occurring simultaneously with the production of lipid-free material, being independent from the nitrogen exhaustion in the medium (Fickers et al., 2005; Papanikolaou and Aggelis, 2010).

In the case of SCO utilization for biodiesel production, research interest is focused only upon the process of de novo lipid accumulation. In this case, there is continuously increasing interest upon the potentiality of transforming abundant renewable materials (like waste glycerol, flour-rich waste streams, cellulose and hemicellulose hydrolysates, etc.) into SCO that will be further transformed into biodiesel. The process of ex novo lipid accumulation aims at adding value to low-cost fatty materials so that speciality high-value lipids (e. g. cocoa-butter or other exotic fats substitutes) will be produced (Papanikolaou et al., 2001; 2003; Papanikolaou and Aggelis 2003a, 2003b, 2010).

The lipids produced by oleaginous microorganisms are mainly composed of neutral fractions [principally triacylglycerols (TAGs) and to lesser extent steryl — esters (SEs)] (Ratledge, 1994; Ratledge and Wynn, 2002). As a general remark it must be stressed that when growth is carried out on various hydrophobic substances, the microbial lipid produced contains lower quantities of accumulated TAGs compared with growth elaborated on sugar-based substrates (Koritala et al., 1987; Guo et al., 1999; Kinoshita and Ota, 2001; Papanikolaou et al., 2001, 2002a; Fakas et al. 2006, 2007, 2008a). In any case, accumulation of storage lipids is accompanied by morphological changes in the oleaginous microorganisms, since ‘obese’ cells with large lipid globules can generally appear during the lipid-accumulating phase (Figure 8.1). Storage lipids, unable to integrate into

phospholipid bi-layers, cluster to form the hydrophobic core of the so-called ‘lipid bodies’ or ‘oil bodies’ (Mlickova et al., 2004a, 2004b). Lipid bodies of the oleaginous Y. lipolytica yeast are illustrated in Figure 8.2. As previously stressed, the biochemical pathways of de novo and ex novo lipid accumulation process present fundamental differences. These differences will be presented, explained, clarified and comprehensively discussed in the following sections.