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

Abstract

SummarySupplying microalgae with extra CO 2 is a promising means for improving lipid production. The molecular mechanisms involved in lipid accumulation under conditions of elevated CO 2, however, remain to be fully elucidated. To understand how elevated CO 2 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 CO 2 to triacylglycerol. These analyses identified the majority genes of carbohydrate metabolism and lipid biosynthesis pathways in C. sorokiniana LS-2. Under high doses of CO 2 , 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 CO 2 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 CO 2 may influence regulatory dynamics and result in increased carbon flow to triacylglycerol, thereby providing a feasible approach to increase lipid production in microalgae.

Responses of macroalgae to CO2 enrichment cannot be inferred solely from their inorganic carbon uptake strategy

“…Both species invested in growth rather than storing lipids, which supported our hypothesis for C. ramentaceus (the non‐CCM species), but did not support the hypothesis that lipid content and fatty acid composition would change in L. australis (the CCM species). There are several possible explanations for the lack of biochemical response of L. australis : (a) Previous work, mainly on microalgae, has shown that it is the combination of excess carbon and limiting nitrogen (i.e., insufficient nitrogen for protein synthesis) that provides optimal conditions for an increase in carbon flow toward acetyl coenzyme A (acetyl Co‐A) and NADPH for FA synthesis (Sun et al, ). However, the nitrogen concentration used in this experiment was not likely to be limited, as the concentrations were that of ambient seawater and C:N ratios in the algal tissue were in the range consistent with nitrogen sufficiency (Atkinson & Smith, ).…”

“…Such biochemical changes can be observed in the biomass very shortly after exposure (≤7 days) as an acclimation mechanism in membrane and/or storage lipids (Al‐Hasan, Hantash, & Radwan, ; Gosch, Lawton, Paul, Nys, & Magnusson, ). Elevated CO 2 concentrations have been shown to either increase or decrease the lipid production of several species of algae, especially when combined with nutrient limitation (Bermúdez et al, ; Sun, Chen, & Du, ). CO 2 enrichment could therefore not only result in increased growth rates, but also lead to changes in lipid storage.…”

Metabolic regulation of triacylglycerol accumulation in the green algae: identification of potential targets for engineering to improve oil yield

“…Bile acid:Na + symporter) were induced compared to N‐replete cultures. This suggested that an enhanced CO 2 assimilation rate could increase carbon flux towards fatty acid synthesis (Sun et al ., ). Therefore, manipulating the culture C/N ratio by increasing the C source without altering N might be a promising strategy for mimicking the processes of N starvation and boosting lipid production in microalgae.…”

The dilemma for lipid productivity in green microalgae: importance of substrate provision in improving oil yield without sacrificing growth

“…Even under non-nitrogen-limiting conditions, PDC subunits were also observed to be upregulated following transition from starch-rich heterotrophy (dark conditions) to lipid-rich photoautotrophy (light conditions) in the oleaginous C. pyrenoidosa [72]. Under elevated doses of CO 2 which improves lipid accumulation in microalgae, genes involved in carbohydrate metabolic pathways, such as the components of the PDC, were found to be upregulated in Chlorella sorokiniana , despite a downregulation of most FA synthesis genes [102]. This implies that the positive correlation between PDC expression and lipid buildup is not primarily confined to nutrient-depleted conditions.…”