Genome Characterization and Metabolic Network Analysis of the Oleaginous Microalgae Chlorella sorokiniana

Chlorella sorokiniana is an oleaginous microalgae which can produce lipids accounting for up to 31.5% of its dry weight. It is considered a promising strain for biodiesel production. Using a combination of high throughput sequencing and bioinformatics, we have assembled and annotated the C. sorokiniana genome and analyzed its metabolic and regulatory network. Its 51 Mb genome has a large quantity of repeat sequences and a GC content of 64%. Lipogenic genes and regulatory factors of lipid synthesis were identified to analyze the lipogenesis pathways. Gene-essentiality and constraint-based analyses were conducted to predict the core metabolic essential genes and reveal key metabolites. In order to understand the metabolic characteristics of C. sorokiniana at a systems level and to examine the potential for enhanced lipid production, a genome-scale compartmentalized metabolic network was reconstructed based on a combination of genome annotation and the detailed biochemical knowledge from multiple databases. Comparative genomic analysis with Chlorella variabilis NC64A and phylogenetic analysis based on housekeeping genes and lipogenic genes revealed new insights into Chlorella genus. Our study provides the first draft genome for C. sorokiniana, and lays the foundation for transcriptome analysis and possible genetic engineering of C. sorokiniana to produce higher levels and diverse contents of lipids.