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Abstract
Chlorella sorokiniana is one of the most productive microalgal species with a high potential for the production of biofuels and other high value-added molecules. Many studies have focused on its capability of mixotrophic growth using reduced organic carbon and growth pattern shift between autotrophic and mixotrophic conditions. In this study, we investigated growth patterns of a novel isolate, C. sorokiniana G32, under mixotrophic growth conditions supplemented with a low level (1.25 g L–1) and a high level (5 g L–1) of glucose. Physiological, transcriptomic (i.e., RNA-seq), and metabolomic (i.e., LC-MS/MS) methods were used. We showed that peak growth based on OD680nm absorbance is ∼4-fold higher with high glucose vs. low glucose supplementation. Photosynthetic efficiency (Fv/Fm) in G32 mixotrophic cultures with high or low glucose supplementation remains identical to that of G32 phototrophic growth. We also found that the conversion rate between absorbance-based cell density and cell dry weight with high glucose supplementation was lower than with low glucose. This suggests that more cell biomass is produced under high glucose treatment than with low glucose. The result was confirmed via sucrose density gradient centrifugation. It is likely that accumulation of high concentration of starch may account for this effect. Transcriptomic analysis of G32 cultures (i.e., via RNA-seq) in response to reciprocal change of glucose levels reveals that expression of a subset of differentially expressed genes (DEGs) is correlated with the amount of glucose supplementation. These DEGs are designated as glucose-specific responsive (GSR) genes. GSR genes are enriched for a number of energy metabolic pathways. Together with metabolomics data (i.e., LC-MS/MS), we show that under high-level supplementation, glucose is preferentially oxidized through an oxidative pentose phosphate pathway. Collectively, our results indicate the mechanism of regulation of glucose assimilation and energy metabolism in G32 under mixotrophic conditions with different levels of glucose supplementation revealed by transcriptomic and metabolomic analyses. We propose that C. sorokiniana G32 has the potential for the production of high value-added molecules.
Highlights
There is renewed interest in microalgae for their potential in the production of biofuels and other high value-added molecules
We show that C. sorokiniana G32 accumulates energy-rich molecule starch without impeding growth rate and biomass productivity when supplemented with high-level glucose
By combining the transcriptome and metabolome profiling, we show that assimilation of glucose in cell with high glucose supplementation is preferentially undertaken via oxidative pentose phosphate pathway
Summary
There is renewed interest in microalgae for their potential in the production of biofuels and other high value-added molecules. Microalgae are considered to have high photosynthetic efficiency and growth rate compared with other photosynthetic organisms, allowing for rapid, high biomass accumulation (Herrero et al, 2006; Spolaore et al, 2006; Chisti, 2007; Unkefer et al, 2017; Randhir et al, 2020) Some microalgae species such as Chlorella sorokiniana are capable of mixotrophic growth using both photosynthetic carbon and supplemented organic carbons from molecules such as glucose, glycerol, ethanol, and acetate (Wan et al, 2011; Li et al, 2014; Cecchin et al, 2018). An individualized assessment of each C. sorokiniana strain for potential performance in cultivation systems is required (Hovde et al, 2018)
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