Genetic engineering of microalgae can provide important and significant improve­ments for algal-biofuel production, by increasing the yields of TAGs to facilitate more efficient biodiesel conversion [187]. In the 1960s, the genome of Anabaena PCC7120 (Chloroxybacteria) was successfully cloned to produce a model organism for academic research [92] . However, for eukaryotic algae there is still a lack of understanding of the detailed molecular biological and regulation of lipid body

TRENDS in Biotechnology

Fig. 5 Basic design of an enclosed horizontal tubular photobioreactor (Adapted from Chisti [39]). Two main sections, airlift system and solar receiver array. The degassing column allows for the transfer of O2 out of the systems and transfer of CO2 into the system as well as providing a means to harvest the biomass. The solar receiver provides a platform for growth enhancement by giving a high surface area to volume ratio (With permission from Elsevier Ltd.)

metabolism [187]. Therefore, genetic manipulation (transgenics) remains limited to a few selected algal laboratory models, i. e. Chlamydomonas reinhardtii, Volvox car — teri, Cyanidioschyzon merolae, Emiliania huxleyi, and the diatoms Phaeodactactylum tricornutum, and Thalassiosira pseudonana. The expanding research interest in biofuel-directed microalgae has led to genetic engineered model organisms [13, 217] and general advances in algal transgenics. Additional genome sequencing for strains with suitable characteristics for biofuels and more universal genetic transformation tools that could enable further development of trangenetic based microalgae-derived biofuel production are considered to be necessary for further advancement [13]

The application of genetic engineering has progressed in several paths, with most recently, the direct manipulation of the microalgal lipid synthesis through gene expres­sion and shunting of photosynthetic carbon partitioning to TAG synthesis [119]. The high expression of acetyl-coA carboxylase gene, which has a role in controlling the level of lipid accumulation, has lead to an improvement of lipid content in the engi­neered microalgae cells [92]. Another genetic engineering process for enhancing the lipid yields that has been tested is the shunting of pathways from starch production to lipid synthesis, by freeing precursor metabolites for desired biofuel products [187]. Both Li et al. [119] and Wang et al. [219] successfully induced inactivation of ADP — glucose pyrophosphorylase in a C. reinhardtii starchless mutant (sta6). Li et al. [119] reported a tenfold increase in lipid synthesis, while Wang et al. [219] reported a 30-fold increase in lipid synthesis, ultimately leading to a high concentration of TAG per cell. Therefore, future use of transgenics to improve TAG content of biofuel-directed microalgae is potentially an important bearing in the quest for economic production.