Abstract
BackgroundThe colonial microalga Botryococcus braunii has been brought to people’s attention for its conspicuous ability to accumulate a variety of lipids including hydrocarbons. B. braunii strains are classified into 3 races based on the types of hydrocarbons. A and B races are known to accumulate high level of lipids. However, their extreme slow growth rate has impeded its application for renewable biofuel production.ResultsIn this study, we report the transcriptomic response of a moderately growing subisolate from the culture of Botryococcus sp. CCALA-779 upon nitrogen deprivation (ND). We show that the subisolate has an average growth rate of 0.52 g l−1 day−1 under photoautotrophic growth conditions and lipid content is enhanced to 75 % of CDW upon ND. Both rDNA sequence and hydrocarbon composition analyses indicate that the subisolate belongs to A race B. braunii. Hence, it is designated as B. braunii 779. We show that B. braunii 779 transcriptome shares homology to majority of the A race but not B race B. braunii ESTs, suggesting that transcriptomes of A race differ from that of B race. We found that many homologous ESTs between A races 779 and Bot-88 are unknown sequences, implying that A race contains many unknown genes. Pathway-based transcriptomic analysis indicates that energy metabolisms are among the top expressed functions in log-phase cells, indicating that the slow growth rate is a result that energy flow is directed to lipid biosynthesis but not population growth. Upon ND, reconfiguration of metabolisms for reducing power is apparent, suggesting that B. braunii 779 is rapidly adapting under ND condition by transcriptomic reprogramming.ConclusionsTaken together, our result shows that the subisolate B. braunii 779, similar to the Gottingen strain, is useful for biofuel production. Difference between transcriptomes of A and B races implies that different races of B. braunii strains belong to different sub-species. Furthermore, there are many novel genes that are unique to A race, suggesting that sequences of many enzymes involved in hydrocarbon biosynthesis are not currently known. We propose that B. braunii transcriptomes provide a rich source for discovery of novel genes involved in hydrocarbon biosynthesis.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0307-y) contains supplementary material, which is available to authorized users.
Highlights
The colonial microalga Botryococcus braunii has been brought to people’s attention for its conspicuous ability to accumulate a variety of lipids including hydrocarbons
We examined the content of lipid and hydrocarbon in log-phase cells that were extracted by methanol/chloroform solution and hexane, respectively, using gravimetric methodologies
Taken together, our results show that the subisolate B. braunii 779 exhibits a moderate growth rate and high lipid content, attractive for biofuel production
Summary
The colonial microalga Botryococcus braunii has been brought to people’s attention for its conspicuous ability to accumulate a variety of lipids including hydrocarbons. A and B races are known to accumulate high level of lipids Their extreme slow growth rate has impeded its application for renewable biofuel production. Oleaginous green colonial microalga Botryococcus braunii has been brought to people’s attention for its conspicuous ability to accumulate a variety of lipids including hydrocarbons that are found in petroleum deposits and can be refined directly without the need of esterification because they are not fatty acids [4, 6]. B. braunii has suffered from an extreme slow growth (i.e., population doubling time is approximately 5–7 days) that has hampered from its application for biofuel production [6]. Several genes encoding enzymes such as triterpene methyltransferases TMTs [8], 1-deoxy-D-xylulose 5-phosphate synthase DXS [10], squalene synthase-like SSL botryococcenes synthase [9, 17], Botryococcus squalene synthase BSS [18], and squalene epoxidases SQE [11] have been cloned and their enzymatic activities have been tested
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