Abstract
BackgroundLight/dark cycling is an inevitable outdoor culture condition for microalgal biofuel production; however, the influence of this cycling on cellular lipid production has not been clearly established. The general aim of this study was to determine the influence of light/dark cycling on microalgal biomass production and lipid accumulation. To achieve this goal, specific causative mechanisms were investigated using a metabolomics approach. Laboratory scale photoautotrophic cultivations of the oleaginous green microalga Chlamydomonas sp. JSC4 were performed under continuous light (LL) and light/dark (LD) conditions.ResultsLipid accumulation and carbohydrate degradation were delayed under the LD condition compared with that under the LL condition. Metabolomic analysis revealed accumulation of phosphoenolpyruvate and decrease of glycerol 3-phosphate under the LD condition, suggesting that the imbalance of these metabolites is a source of delayed lipid accumulation. When accounting for light dosage, biomass yield under the LD condition was significantly higher than that under the LL condition. Dynamic metabolic profiling showed higher levels of lipid/carbohydrate anabolism (including production of 3-phosphoglycerate, fructose 6-phosphate, glucose 6-phosphate, phosphoenolpyruvate and acetyl-CoA) from CO2 under the LD condition, indicating higher CO2 fixation than that of the LL condition.ConclusionsPhotoperiods define lipid accumulation and biomass production, and light/dark cycling was determined as a critical obstacle for lipid production in JSC4. Conversions of phosphoenolpyruvate to pyruvate and 3-phosphoglycerate to glycerol 3-phosphate are the candidate rate-limiting steps responsible for delayed lipid accumulation. The accumulation of substrates including ribulose 5-phosphate could be explained by the close relationship of increased biomass yield with enhanced CO2 fixation. The present study investigated the influence of light/dark cycling on lipid production by direct comparison with continuous illumination for the first time, and revealed underlying metabolic mechanisms and candidate metabolic rate-limiting steps during light/dark cycling. These findings suggest promising targets to metabolically engineer improved lipid production.
Highlights
Light/dark cycling is an inevitable outdoor culture condition for microalgal biofuel production; how‐ ever, the influence of this cycling on cellular lipid production has not been clearly established
Alternations in biomass and lipid production under light/ dark cycling and continuous illumination To examine the influence of light/dark cycling on lipid accumulation and biomass production, JSC4 was cultivated under continuous light (LL) and light/dark (LD) conditions
Cultured cells were harvested every 12 h at dawn and dusk to discriminate the influences of light periods and dark periods on nitrate concentration, biomass production, carbohydrate content, and lipid content
Summary
Light/dark cycling is an inevitable outdoor culture condition for microalgal biofuel production; how‐ ever, the influence of this cycling on cellular lipid production has not been clearly established. The general aim of this study was to determine the influence of light/dark cycling on microalgal biomass production and lipid accumulation. To achieve this goal, specific causative mechanisms were investigated using a metabolomics approach. In the cultivation process, microalgae are required to achieve high lipid content together with high biomass production [6, 7]. Outdoor cultivation under photoautotrophic conditions using sunlight as an energy source and CO2 as a main carbon substrate is a cost-effective strategy for biofuel production. Periodic changes in light intensity throughout the day and night (hereafter, light/dark cycling) are an impactful factor for photosynthetic organisms including microalgae as these cycles are directly linked to biomass production under photoautotrophic conditions [8]
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