**2. Microalgae**

The term 'algae' is used to describe a huge variety of prokaryotic (strictly termed Cyanobacteria) and eukaryotic organisms with a range of morphologies and phylogenies. They represent a wide array of species, inhabiting environments from deserts to the Arctic Ocean, including both salt and fresh water. They vary in colour, shape and size, from picoplankton (0.2 to 2 m) to giant kelp fronds up to 60 m in length (Barsanti & Gualtieri, 2006). Macroalgae (e.g. seaweeds) are generally large (can be seen without the aid of a microscope), multicellular and often show some form of cellular specialisation. Microalgae are usually less than 2 mm in diameter and unicellular or colonial. Microalgae have been investigated for a variety of commercial applications. Annual global microalgal production is currently estimated at about 10 000 metric tons, with the main algae cultivated being *Spirulina* (accounting for roughly half of the worldwide algal production), *Chlorella*, *Dunaliella* and *Haematococcus*.

Algae have been investigated as a source of energy in many different contexts, from direct combustion to the production of hydrogen gas. Anaerobic digestion can be applied for the generation of methane or biogas (Golueke et al., 1957). Algal species with high oil content are particularly attractive as a feedstock for biodiesel production. Research into algae for the mass-production of oil has focused on the microalgae due to their high lipid content compared to macroalgae. Most algal species considered for biodiesel production are either green algae (Chlorophyta) or diatoms (Bacillariophyta) (Sheehan et al., 1998). They are generally photosynthetic, but several species are able to grow heterotrophically or mixotrophically (Barsanti & Gualtieri, 2006).

Microalgae have higher growth rates than land-based plants. Due to their simple cellular structure and existence in an aqueous environment, the entire cell surface is available for light capture and mass transfer, leading to high rates of substrate uptake and photosynthetic efficiency (Miao & Wu, 2006; Sheehan et al., 1998). In contrast to land-based oil crops, where only the seeds are harvested, each algal cell contains lipid and hence the yield of product from biomass is much higher (Becker, 1994). Due to these differences, the oil yield per area of microalgal cultures potentially exceeds that of the best oilseed crops (Table 1).


