Contiguous supplementation of bicarbonate and carbon dioxide to improve carbon utilization efficiency and biodiesel production from a single-cell alga Chlamydomonasreinhardtii

This work established a promising hybrid carbon absorption-biofixation of CO2 approach, that benefit of having an adequate carbon supply allowed for the creation of an efficient buffer system that maintained a constant pH and enough dissolved inorganic carbon (DIC) for photosynthesis to improve biomass and lipid production in the single-cell green alga Chlamydomonas reinhardtii. In a 5L photobioreactor, the microalga C. reinhardtii exhibited better performance with 6 % CO2 enriched air (0.4 vvm flow rate) and NaHCO3 concentration of 5 mM on 3rd day of supplementation in nutrient sufficient medium. This included a maximum specific growth rate of 0.462 d−1, a biomass yield of 3.79 g L−1, a biomass productivity of 0.63 g L−1 d−1, and a total lipid content of 43.7 % (dcw). This competitive growth rates might only be possible, when the medium contained high amount of DIC up to 103 mg L−1. The pH data demonstrated that bicarbonate consumption can effectively raise pH levels, which can be countered by CO2 supplementation to keep the medium within the ideal range for algal growth throughout the whole cultivation period. It was found that the maximal rate of CO2 bio-fixation reached up to 1.18 g L−1 d−1. This study demonstrates that 6 % CO2 with 5 mM NaHCO3 (supplemented on 3rd day of incubation) as the inorganic carbon sources followed by nitrogen deficiency induces synthesis of high levels of lipid up to 1.67 g L−1 (57 % dcw) in C. reinhardtii. The presence of suitable fatty acid profile including palmitic, palmitoleic, oleic, linoleic, linolenic and stearic acid methyl esters demonstrated that the algal lipids could fulfil the requirements for standard fuel properties of biodiesel.