Many strains of microalgae are known to grow either heterotrophically or mixo — trophically. They include Haematococcus pluvialis, Chlamydomonas reinhardtii, Chlamydomonas globosa, Scenedesmus acutus, Selenastrum capricornutum, Scenedesmus bijuja, Ankistrodesmus sp., and many strains of Chlorella (Salim 2013; Ogbonna et al. 1998; Chojnacka and Marquez-Rocha 2004; Chojinacka and Noworyta 2004; Chen 1996). Growth of cyanobacteria can also be enhanced by mixotrophic culture, depending on the carbon source used (Lodi et al. 2005).

However, only a few of these strains that can utilize organic carbon sources accumulate more than 20 % oil under normal growth conditions. The oleaginous strains (those that accumulate more than 20 % oil) include Chlamydomonas, Dunaliella, Botryococcus, Chlorella, Phaeodactylum, Thalassiosira, Nannochlor- opsis, and Isochrysis. Out of these, many are known to be capable of growing heterotrophically. The choice of the strain to be used for heterotrophic or mixo — trophic oil production depends on the oil productivity (a product of the cell growth rate and oil contents of the cells), and the quality of the oils.

Many reports have shown that photoautotrophic and heterotrophic culture conditions result in different biomass and lipid yields for the same microalga strain (Xu et al. 2006; Cheng et al. 2009). Chlorella emersonii and C. protothecoides gave the highest average lipid and biomass yield among many strains of microalgae tested (Suali and Sarbatly 2012). Liang et al. (2009) also reported very high oil productivities with Chlorella vulgaris, while Mandal and Mallick (2009) reported that Scenedesmus obliquus has very high potential for oil production. Heterotro — phically cultivated C. protothecoides was reported to be composed of 40-60 % lipid, 10-28 % protein, 11-15 % carbohydrate, and 6 % ash (Xu et al. 2006; Miao and Wu 2004; Zhang et al. 2008). In view of their lipid content, C. vulgaris, C. protothecoides, and C. zofingiensis were reported as candidates for biodiesel production under photoautotrophic or heterotrophic culture conditions (Liu et al. 2008; Miao and Wu 2006; Hsieh and Wu 2009). Park et al. (2012) found that under mixotrophic conditions, oleic acid is comprised of 41-62 % fatty acid in many strains of microalgae, but in some Chlamydomonas isolates, oleic acid comprised only 9-16 % fatty acid, while palmitic and linoleic acid constituted 47-49 % of the total fatty acid content. Although C. vulgaris and C. minutissima are capable of producing high lipid contents, the triglyceride content is low, making them unsuitable for biodiesel production (Stephenson et al. 2010). The biodiesels pro­duced from some Chlorella species were acid methyl esters, linoleic acid methyl esters, and oleic acid methyl esters (Gao et al. 2009). Unsaturated fatty acid methyl esters comprised over 82 % of the total biodiesel content of Chlorella species (Xu et al. 2006; Cheng et al. 2009). Therefore, the properties of the biodiesel produced from Chlorella comply with ASTM 6751, the US Standard for biodiesel (Li et al. 2007). From various reports on the potentials of several strains of mic­roalgae for oil production, no strain can be selected as the best for biodiesel oil production, since both their oil contents and the composition of the oils vary with culture conditions. The choice, therefore, depends on the culture condition and the composition of the culture medium.