Cellular biomass of oleaginous yeasts and filaments fungi (Miao and Wu, 2006) has also been evaluated as a cheap source of renewable raw materials for biodiesel production. In addition to being cheap, using microalgae to produce biodiesel will not compromise production of food and other products derived from crops. Above that, oil crops, waste cooking oil and animal fat cannot realistically satisfy the demand required to achieve the target of replacing all current transport fuel consumed with biodiesel. This scenario changes dramatically, if microalgae are used to produce biodiesel. It has been reported that for microalgae of 30% oil content per weight of biomass, the oil yield per hectare is estimated at 58.7 m3 per hectare (Chisti, 2007). This is almost ten times the yield of palm oil, and the difference becomes even higher if compared to microalgae of 70% oil content per weight of biomass. It appears therefore that microalgae are the only source of biodiesel that has the potential to completely displace fossil diesel. Microalgae commonly double their biomass within 24 h. However, during exponential growth period, doubling times are as short as 3.5 h (Chisti, 2007). In addition, oil content in microalgae may exceed 80% by weight of dry biomass (Spolaore et al., 2006).

Besides microalgae, other oil producing heterotrophic oleaginous microorganisms have been used to produce biodiesel (Ratledge and Wynn, 2002). Nevertheless, heterotrophic production is not as efficient as using photosynthetic microalgae, because the renewable organic carbon sources required for growing heterotrophic microorganisms are produced ultimately by photosynthesis, usually in crop plants, which brings us back to square one.

The use of oils from microalga, Chlorella protothecoids for large-scale biodiesel production using immobilized Candida sp. lipase has been reported (Li et al, 2007). Algal oils have been largely produced through substrate feeding and heterotrophic fermentation. In their work, Li et al. (2007) achieved an increase in the lipid content up to 48% of the cell dry weight of the microalga. The oils were then used as raw material to produce biodiesel using immobilized Candida sp. lipase.