Elevated CO2 improves lipid accumulation by increasing carbon metabolism in Chlorella sorokiniana

Abstract

Supplying microalgae with extra CO2 is a promising means for improving lipid production. The molecular mechanisms involved in lipid accumulation under conditions of elevated CO2, however, remain to be fully elucidated. To understand how elevated CO2 improves lipid production, we performed sequencing of Chlorella sorokiniana LS-2 cellular transcripts during growth and compared transcriptional dynamics of genes involved in carbon flow from CO2 to triacylglycerol. These analyses identified the majority genes of carbohydrate metabolism and lipid biosynthesis pathways in C. sorokiniana LS-2. Under high doses of CO2 , despite down-regulation of most de novo fatty acid biosynthesis genes, genes involved in carbohydrate metabolic pathways including carbon fixation, chloroplastic glycolysis, components of the pyruvate dehydrogenase complex (PDHC) and chloroplastic membrane transporters were upexpressed at the prolonged lipid accumulation phase. The data indicate that lipid production is largely independent of de novo fatty acid synthesis. Elevated CO2 might push cells to channel photosynthetic carbon precursors into fatty acid synthesis pathways, resulting in an increase of overall triacylglycerol generation. In support of this notion, genes involved in triacylglycerol biosynthesis were substantially up-regulated. Thus, elevated CO2 may influence regulatory dynamics and result in increased carbon flow to triacylglycerol, thereby providing a feasible approach to increase lipid production in microalgae.

Keywords: Chlorella sorokiniana; carbon dioxide; carbon flow; lipid; photosynthesis; transcriptomics.

Figure 1

Gene ontology classification of assembled unigenes. The 22, 432 matched unigenes were classified into 3 functional categories: molecular function, biological process and cellular component.

Figure 2

KOG functional classification of all unigenes. A total of 5163 unigenes showed significant similarity to the sequences in KOG databases and were clustered into 26 categories.

Figure 3

KEGG classification of assembled unigenes. The 4666 KO annotated unigenes were assigned to 5 KEGG biochemical pathways: metabolism, genetic information processing, organism system, cellular processes and environmental information processing.

Figure 4

Model of the C4‐like pathway in C. sorokiniana LS‐2 based on the transcriptome. The dashed line indicates the route that was not detected in this transcriptome.

Figure 5

Comparison of expression patterns of differentially expressed unigenes identified between high‐dose CO ₂ and air aeration. The red dots represent DEGs, and the blue dots represent non‐DEGs.

Figure 6

Transcriptional dynamics of individual genes related to the carbon flow from CO ₂ to TAG biosynthesis in response to high dose CO ₂ aeration. Relative fold differences were calculated based on the ▵C _t method using the Actin amplification product as an internal standard.

Figure 7

Gene expression changes related to carbon flow from CO ₂ to TAG synthesis during high‐dose CO ₂ aeration compared with air aeration at P3. Differences in fold change are based on log2 scale and the colour scale represents differentially expressed genes. Red (log2^{fold change} ≥1), black (log2^{fold change} = −0.99–0.99), Blue (log2^{fold change}≤−1). Dotted arrows represent undetected genes.