Modulation of photosynthesis, biomass and lipid production by sulfur and CO₂ supplementation in Chlorella vulgaris

Biofuels are essential to combat climate change. Microalgae have a huge potential to generate green fuel, biodiesel. In the current study, the impact of sulfur availability on growth, photosynthetic capacity, lipid production, and its composition of the freshwater green microalga Chlorella vulgaris grown in ambient air or 5% CO₂ in a photo-bioreactor was analysed. Low sulfur decreased the photosynthesis rates of the algal cells; their whole chain electron transport, partial PSI and PSII reactions, and biomass production. Conversely, S-limited growth resulted in intracellular accumulation of lipids. S-deficiency in the growth medium of Chlorella channelled the unused carbon skeletons for the synthesis of lipids instead of amino acids, resulting in increased lipid content per dry cell weight. The increased lipid content of the alga in S-deficiency was due to up-regulation of expression of genes coding for diacyl and triacylglycerol synthesis. 5% CO₂ increased the chlorophyll content and photosynthesis and respiration rates of C. vulgaris. In 5% CO₂ media, photosynthesis rates increased, ensuing increased biomass accumulation. In S-deficient media flushed with 5% CO₂, the increased photosynthates generated augmented amounts of carbon skeletons that were utilized for lipid synthesis. The S50 (50% S) and 5% CO₂ culture had only a 13% reduction in biomass and a 41% increase in lipid content per dry cell weight could be ideal for biodiesel production. In S-deficient (S50) and 5% CO₂ culture, biodiesel of C. vulgaris had ~ 91% saturated and monounsaturated fatty acid methyl esters having high cetane number, essential for biofuel generation.