De novo transcriptomic analysis of the oleaginous alga Botryococcus braunii AC768 (Chlorophyta)

The oleaginous alga Botryococcus braunii (Chlorophyta) is known to accumulate hydrocarbons up to 80% of cell dry weight. Races A, B, and L of B. braunii species accumulate alkadiene/triene, botryococcene, and lycopadiene, respectively. Transcriptome analyses in race A and race B have identified various transcripts encoding enzymes involved in the biosynthesis of alkadiene/triene and botryococcene. However, the transcriptome of B. braunii race L has not been analyzed. In this study, we report the de novo assembly of the transcriptome of B. braunii AC768 race L. AC768 culture in 2×BB medium reaches a maximum density (cell dry weight) of 5.5 g L−1, of which, 6% is lycopadiene. Transcriptional profiles of AC768 show that ESTs involved in energy metabolisms are most abundantly expressed, suggesting an energy requirement for lipid accumulation. Transcriptomic analysis indicates the presence of a putative lycopaoctaene synthase that synthesizes the precursor of lycopadiene in AC768. Comparative analysis between races reveals that botryococcene synthase and lycopaoctaene synthase are specific to races B and L of B. braunii, respectively. Taken together, our results indicate that hydrocarbon chemicals accumulated in different races of B. braunii are determined by their transcriptional profiles.