Biodiesel is a diesel fuel consisting of mono-alkyl esters of long-chain fatty acids that are generally made by the transesterification of lipids in animal fat or vegetable oils such as soybean, sunflower, rapeseed, and oil palm (Hankamer et al. 2007; Li et al. 2007; Ma and Hanna 1999). As an alternative, microalgae have become popular for the renewable generation of hydrocarbon-based biofuels with high biofuel yields relative to those from plants (Eroglu and Melis 2009; Li et al. 2007).

Immobilization of the hydrocarbon-rich microalgae, Botryococcus braunii and Botryococcus protuberans, in alginate beads yielded a significant increase in the chlorophyll, carotenoids, cellular growth, and lipid contents of the cells during their stationary growth phase (Singh 2003). Bailliez et al. (1985) also observed enhanced hydrocarbon production for the B. braunii cells immobilized in calcium alginate gel as a result of enhanced photosynthetic activity.

In a study by de-Bashan et al. (2002a), C. vulgaris and C. sorokiniana micro­algal cells were individually co-immobilized with A. brasilense growth-promoting bacterium in alginate beads. They found that the presence of the growth-promoting bacterium within the immobilization matrix significantly enhanced the metabolism of Chlorella strains and yielded higher lipid and fatty acid production.

Li et al. (2007) used immobilization technology for the transesterification of algal oils, using immobilized lipase enzyme from Candidia sp. Initially, they grew Chlorella protothecoides cells in large-scale photobioreactors at three different sizes (5, 750, 11,000 L) yielding high lipid contents in the range 44-49 % per dry cell weight. Then, immobilized lipase enzyme from Candidia sp. was used to catalyze the transesterification of the lipids from C. protothecoides, yielding biodiesel production rates of 7.02, 6.12, and 6.24 g L-1 from 5, 750, and 11,000 L biore­actors, respectively. They also highlighted that the quality of this Chlorella biodiesel was comparable to that for conventional diesel fuels (Li et al. 2007).