Transcriptional insights into Chlorella sp. ABC-001: a comparative study of carbon fixation and lipid synthesis under different CO 2 conditions

Background Microalgae's low tolerance to high CO 2 concentrations presents a significant challenge for its industrial application, especially when considering the utilization of industrial exhaust gas streams with high CO 2 content—an economically and environmentally attractive option. Therefore, the objectives of this study were to investigate the metabolic changes in carbon fixation and lipid accumulation of microalgae under ambient air and high CO 2 conditions, deepen our understanding of the molecular mechanisms driving these processes, and identify potential target genes for metabolic engineering in microalgae. To accomplish these goals, we conducted a transcriptomic analysis of the high CO 2 -tolerant strain, Chlorella sp. ABC-001, under two different carbon dioxide levels (ambient air and 10% CO 2 ) and at various growth phases. Results Cells cultivated with 10% CO 2 exhibited significantly better growth and lipid accumulation rates, achieving up to 2.5-fold higher cell density and twice the lipid content by day 7. To understand the relationship between CO 2 concentrations and phenotypes, transcriptomic analysis was conducted across different CO 2 conditions and growth phases. According to the analysis of differentially expressed genes and gene ontology, Chlorella sp. ABC-001 exhibited the development of chloroplast organelles during the early exponential phase under high CO 2 conditions, resulting in improved CO 2 fixation and enhanced photosynthesis. Cobalamin-independent methionine synthase expression was also significantly elevated during the early growth stage, likely contributing to the methionine supply required for various metabolic activities and active proliferation. Conversely, the cells showed sustained repression of carbonic anhydrase and ferredoxin hydrogenase, involved in the carbon concentrating mechanism, throughout the cultivation period under high CO 2 conditions. This study also delved into the transcriptomic profiles in the Calvin cycle, nitrogen reductase, and lipid synthesis. Particularly, Chlorella sp. ABC-001 showed high expression levels of genes involved in lipid synthesis, such as glycerol-3-phosphate dehydrogenase and phospholipid-diacylglycerol acyltransferase. These findings suggest potential targets for metabolic engineering aimed at enhancing lipid production in microalgae. Conclusions We expect that our findings will help understand the carbon concentrating mechanism, photosynthesis, nitrogen assimilation, and lipid accumulation metabolisms of green algae according to CO 2 concentrations. This study also provides insights into systems metabolic engineering of microalgae for improved performance in the future.