Microalgae are considered one of the most promising alternatives for biodiesel production and $CO_2$ fixation. However, the low biomass and lipid productivities are the major bottleneck for the commercialization of microalgae biotechnology.
The purpose of this study is to optimize the lipid production and $CO_2$ fixation through developing the processes and system for the microalgal production based on the isolated oleaginous microalgae. The microalgae showing high growth rate and lipid productivity under high $CO_2$ concentration was successfully isolated from environments samples. This isolate strain was identified as Ettlia sp. using an 18S rDNA-based phylogenetic analysis and morphological comparison. In the comparison of its biomass and lipid productivities with three microalgae, Scenedesmus sp., Chlorella vulgaris, and Botryococcus braunii, the biomass productivity of Ettlia sp. YC001 was comparable to that of Scendesmus sp. and its lipid productivity was around 4 times higher than those of three microalgae.
The optimal pH of Ettlia sp. for high biomass productivity and an auto-flocculaton activity was at pH 8.5 and 10.5, respectively. These differentiated optimal pH regimes were established using $CO_2$ and light irradiation or only light irradiation, respectively. For the mass cultivation, the high efficient $CO_2$ supplying system was developed with the Venturi tube. This system was confirmed to be effective for pH control and mixing the mass microalgal culture in outdoor raceway-ponds. As results of this study, Ettlia sp. YC001 is appropriate for mitigating $CO_2$, and a suitable candidate for producing biodiesel. The proposed simple strategy and $CO_2$ supplying system were cost-effective and flexible for the design and operation of microalgae cultivation-harvest systems. Consequently, the processes and system developed can improve the lipid productivity and $CO_2$ fixation, and contribute to increase the economic viability of microalgae biotechnolgy.