Enhanced lipid accumulation of photoautotrophic microalgae by high-dose CO2 mimics a heterotrophic characterization

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Abstract
Microalgae possess higher photosynthetic efficiency and accumulate more neutral lipids when supplied with high-dose CO2. However, the nature of lipid accumulation under conditions of elevated CO2 has not been fully elucidated so far. We now revealed that the enhanced lipid accumulation of Chlorella in high-dose CO2 was as efficient as under heterotrophic conditions and this may be attributed to the driving of enlarged carbon source. Both photoautotrophic and heterotrophic cultures were established by using Chlorella sorokiniana CS-1. A series of changes in the carbon fixation, lipid accumulation, energy conversion, and carbon-lipid conversion under high-dose CO2 (1–10 %) treatment were characterized subsequently. The daily carbon fixation rate of C. sorokiniana LS-2 in 10 % CO2 aeration was significantly increased compared with air CO2. Correspondingly, double oil content (28 %) was observed in 10 % CO2 aeration, close to 32.3 % produced under heterotrophic conditions. In addition, with 10 % CO2 aeration, the overall energy yield (Ψ) in Chlorella reached 12.4 from 7.3 % (with air aeration) because of the enhanced daily carbon fixation rates. This treatment also improved the energetic lipid yield (Ylipid/Es) with 4.7-fold, tending to the heterotrophic parameters. More significantly, 2.2 times of carbon-lipid conversion efficiency (ηClipid/Ctotal, 42.4 %) was observed in 10 % CO2 aeration, towards to 53.7 % in heterotrophic cultures, suggesting that more fixed carbon might flow into lipid synthesis under both 10 % CO2 aeration and heterotrophic conditions. Taken together, all our evidence showed that 10 % CO2 may push photoautotrophic Chlorella to display heterotrophic-like efficiency at least in lipid production. It might bring us an efficient model of lipid production based on microalgal cells with high-dose CO2, which is essential to sustain biodiesel production at large scales.

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