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The microalgae Chlorella vulgaris produce lipids that after extraction from cells can be converted into biodiesel. However, these lipids cannot be efficiently extracted from cells due to the presence of the microalgae cell wall, which acts as a barrier for lipid removal when traditional extraction methods are employed. Therefore, a microalgae system with high lipid productivity and thinner cell walls could be more suitable for lipid production from microalgae. This study addresses the effect of culture conditions, specifically carbon dioxide and sodium nitrate concentrations, on biomass concentration and the ratio of lipid productivity/cellulose content. Optimization of culture conditions was done by response surface methodology. The empirical model for biomass concentration (R(2)  = 96.0%) led to a predicted maximum of 1123.2 mg dw L(-1) when carbon dioxide and sodium nitrate concentrations were 2.33% (v/v) and 5.77 mM, respectively. For lipid productivity/cellulose content ratio (R(2)  = 95.2%) the maximum predicted value was 0.46 (mg lipid L(-1)  day(-1) )(mg cellulose mg biomass(-1) )(-1) when carbon dioxide concentration was 4.02% (v/v) and sodium nitrate concentration was 3.21 mM. A common optimum point for both variables (biomass concentration and lipid productivity/cellulose content ratio) was also found, predicting a biomass concentration of 1119.7 mg dw L(-1) and lipid productivity/cellulose content ratio of 0.44 (mg lipid L(-1)  day(-1) )(mg cellulose mg biomass(-1) )(-1) for culture conditions of 3.77% (v/v) carbon dioxide and 4.01 mM sodium nitrate. The models were experimentally validated and results supported their accuracy. This study shows that it is possible to improve lipid productivity/cellulose content by manipulation of culture conditions, which may be applicable to any scale of bioreactors.

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Micro-algae offer potentially significant advantages over other approaches to overcome the current challenges of energy shortages and for pollution control. In addition major new directions in the genetic manipulation of algae and in new bioreactor design have been initiated in an effort to design new systems and approaches for this purpose. However, significant barriers for the commercialization of microalgae still exist including economic barriers, the level of technology readiness and lack of established co-products to make the overall process of energy from algae attractive. Mixotrophic algae cultivation offers the benefits of high biomass productivity and allows for integrated approaches which combine both photosynthetic and heterotrophic components during the diurnal cycle. This chapter focuses mainly on mixotrophic algae, the different methods of cultivation and different roles of algae in energy production. The large number of algal species and their versatile habitats and adaptability make mixotrophic microalgae serious candidates for applied research and development and for commercialization of new technologies. Approaches based on mixotrophic cultivation of algae will continue to play a role in efforts to mitigate the above mentioned challenges.