The long-chain PUFAs arachidonic acid, EPA and DHA (Fig. 13.1) are of nutritional importance and also play important roles in the prevention of various diseases. Fish oil is the conventional source of EPA and DHA. However, fish oils contain a crude mixture of several different fatty acids and the omega-3 PUFAs need to be extracted and purified before they can be used for dietary supplementation in pharmacological applications. It should be noted that fish cannot produce the long-chain omega-3 PUFAs by themselves and the fatty acids that can be extracted from fish fat derive originally from marine microalgae. Many methods have been used to purify EPA and DHA from fish oil including, for example, chromatographic methods, distillation, enzymatic splitting and crystallisation. However, these processes are expensive and new methods and sources need to be identified. Microalgae produce omega-3 PUFAs and several studies have been performed to develop a commercially feasible technology to produce EPA directly from these marine plants (Wen and Chen, 2003). Even though there are a large number of EPA-containing microalgae, few have demonstrated industrial production potential. The major problems have been low growth rate, low cell density and high cost in the conventional photoautotrophic conditions. An alternative method is a heterotrophic cultivation system, where an organic carbon source such as sugar can be used as the sole energy source. This mode of culture eliminates the requirement for light and may increase productivity (Wen and Chen, 2003).
Recent progress has been made using another approach to produce long-chain PUFAs, which involves the use of transgenic plants (Napier et al., 2004, Qi et al., 2004). Long-chain omega-3 PUFAs such as arachidonic acid and EPA were produced and accumulated in plants that had been trans-fected with genes encoding enzymes participating in the omega-3/omega-6 biosynthetic pathway for formation of long-chain PUFAs. This is a new promising method for alternative production of these essential fatty acids.
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