Nitrogen source is very important in mixotrophic and heterotrophic cultures of mic­roalgae. Adequate concentration of nitrogen is required for cell growth, while nitrogen limitation is often used to enhance lipid accumulation. Inorganic nitrogen, organic nitrogen, and various waste products have been investigated for biodiesel oil pro­duction (Becker 1994). The use of ammonium as a nitrogen source for Ellipsoidion sp. resulted in higher growth rate and lipid content than when urea and nitrate were used (Xu et al. 2001). On the other hand, Neochloris oleoabundans grew faster and accumulated higher lipid with nitrate than with urea (Li et al. 2008a), but the cell grew poorly in medium with ammonium as the nitrogen source. Complex nitrogen sources are expected to be more effective than simple nitrogen sources in the heterotrophic culture of microalgae, since most of them contain amino acids, vitamins, and growth factors. However, the effectiveness of the nitrogen source depends on the species. For example, nitrate was the best, followed by urea, for the growth of Chlorella vulgaris, while peptone and beef extract did not improve cell growth. Furthermore, ammonium sulfate and ammonium nitrate were less effective than nitrate and urea (Kong et al.

2011) . The type of nitrogen source affects not only the cell growth, but also lipid accumulation. The lipid content of Chlorella vulgaris in mixotrophic culture was highest for peptone, followed by beef extract, but the lipid productivities were low because of low biomass concentration. Ammonium sulfate and ammonium nitrate gave the least lipid content. Potassium nitrate and urea gave intermediate lipid content, yet had the highest productivity as a result of the high biomass content (Kong et al. 2011).

Among the organic nitrogen sources, urea is a promising nitrogen source for large — scale production because it is relatively cheap (Becker 1994; Danesi et al. 2002; Matsudo et al. 2009). With urea as the nitrogen source, the lipid contents of Chlorella sp. decreased with the increase in urea concentration (Hsieh and Wu 2009). The optimal concentrations differ with the nitrogen source. The optimal sodium nitrate and yeast extract concentrations for growth and lipid production by Tetraselmis sp. in mixotrophic culture were 4.70 and 0.93 g/L, respectively (Iyovo et al. 2010). Industrial wastewater rich in nitrogen, such as monosodium glutamate waste, has been reported to be a good and cheap source of nitrogen for cultivation of Rhodotorula glutinis for the production of biodiesel (Xue et al. 2006; Becker 1994). The type of nitrogen source affects the pH of the culture broth. The pH was stable when urea or potassium nitrate were used, but dropped sharply when other nitrogen sources were used.