Microalgae may utilize one or more of the three major metabolic pathways depending on light and carbon conditions: photoautotrophy, heterotrophy, and mixotrophy [6]. Most microalgae are capable of photoautotrophic growth. Photoautotrophic cultivation in open ponds is a simple and low — cost way for large-scale production; however the biomass density is low because of limited light transmission, contamination by other species or bacteria, and low organic carbon concentration [7]. Some microalgae can make use of organic carbons and O2 to undergo rapid propagation through heterotrophic pathway. Heterotrophic cultivation has drawn increasing attention and it is regarded as the most practical and promising way to increase the productivity [8-10]. Currently, research on heterotrophic cul­tivation of microalgae is mainly focused on Chlorella. Cell densities as high as 104.9 gL-1 (dry cell weight, Chlorella pyrenoidosa) have been

reported [11]. Microalgae can adapt to different organic matters such as sucrose, glycerol, xylan, organic acids in slurry after acclimatization [12]. The ability of heterotrophic microalgae to utlize a wide variety of organic carbons provides an opportunity to reduce the overall cost of microalgae biodiesel production since these organic substrates can be found in the waste streams such as animal and municipal wastewaters, effluents from anaerobic digestion, food processing wastes, etc. On the basis of hetero­trophic cultivation, researchers have carried out studies of mixotrophic cultivation which can greatly enhance the growth rate because it realizes the combined effects of photosynthesis and heterotrophy. After examin­ing the biomass and lipid productivities characteristics of 14 microalgae, Park et al. [13] found that biomass and lipid productivities were boosted by mixotrophic cultivation. Andrade et al. [14] studied the effects of mo­lasses concentration and light levels on mixotrophic growth of Spirulina platensis, and found the biomass production was stimulated by molasses, which suggested that this industrial by-product could be used as a low-cost supplement for the growth of this species. Bhatnagar et al. [15] found the mixotrophic growth of Chlamydomonas globosa, Chlorella minutissima and Scenedesmus bijuga resulted in 3-10 times more biomass production compared to that obtained under phototrophic growth conditions. The max­imum lipid productivities of Phaeodactylum tricornutum in mixotrophic cultures with glucose, starch and acetate in medium were 0.053, 0.023 and

0. 020 gL-1day-1, which were respectively 4.6-, 2.0-, and 1.7-fold of those obtained in the corresponding photoautotrophic control cultures [16].