A mixotrophic cultivation strategy was applied for simultaneous improvement of biomass and photosynthetic efficiency in freshwater microalga Scenedesmus obliquus

Fossil energy is associated with environmental issues and worsening global energy crises owing to the depletion of resources. Microalgae serve as third-generation bioenergy owing to higher lipid content per unit area. Microalgae are useful for the production of biofuel, biodiesel, pharmaceutical intermediates, fine chemicals, and bioactive compounds. Low productivity and biomass production pose challenges to the development of microalgae bioenergy. Certain environmental factors may alter the composition of pigments and the pattern of carbon allocation in microalgae. For improving biomass productivity, medium components shall be optimized, and nutrient availability shall be enhanced. High-density cultivation serves as an important tool for industrial development and reduced the cost of production. For rapid microalgae growth, sufficient carbon supply plays a pivotal role. Carbon dioxide, bicarbonate, and other inorganic carbon sources prevent microorganisms from contaminating the growth medium, however, it is difficult for algal cells to achieve high biomass productivity. Organic carbon sources such as acetate promote have been used to increase biomass production due to lower cost. Acetate increases the carbon metabolic flux of the citric acid cycle under the function of Acetyl-CoA synthetase, thus, increasing biomass production. The concentration of acetate is variable in different strains of algae.

In a new study by Dr. Jie Cheng, Dr. Wenxin Fan, Dr. Linggang Zheng from Xiamen University assessed the photosynthetic efficiency and biomass production of a freshwater microalgae Scenedesmus obliquus under a mixotrophic cultivation mode. The authors looked at the effects of different concentrations of sodium acetate on the growth rate and biomass production of S. obliquus. They also used chlorophyll fluorescence technology to study the mechanism and promotion effect of mixotrophic cultivation on the growth rate of S. obliquus. The findings of their study will help develop strategies to improve S. obliquus biomass production using a mixotrophic cultivation mode and improve the industrialization of S. obliquus. Important outcomes. The original research article is now published in the Biochemical Engineering Journal.

At lower concentrations of sodium acetate, the growth of S. obliquus increased significantly. However, growth rate and biomass production decreased upon greater concentrations of sodium acetate. The authors observed maximum biomass productivity and specific growth rate at a sodium acetate concentration of 10 mg/L. The pH of the culture medium also influences the growth rate of S. obliquus with an exponential growth phase characterized by the pH of 8.5. The increased growth rate of the algae in the presence of low concentrations of sodium acetate is attributed to the increased accumulation of photosynthetic pigments leading to increased biomass production. The opposite trend was observed in the presence of greater concentrations of sodium acetate due to the decrease in chlorophyll content, which will led to the accumulation of reactive oxygen species, resulting in toxic to algae cells. Then, exogenous sodium acetate tends to demonstrate a low promotion and high suppression effect on the accumulation of photosynthetic pigments and biomass production of S. obliquus.

The research team also demonstrated contrasting outcomes of different concentrations of sodium acetate on the photosynthetic efficiency of S. obliquus. photosynthetic efficiency was significantly increased under low concentration of sodium acetate, however, when adding the high concentrations of sodium acetate, the oxygen evolving complex in donor side of reaction center was damaged, and electron transport at the donor and receptor sides of the reaction center was also suppressed, accompanied by the changes of the absorption, transfer, and application of light energy.

In summary, Jie Cheng and colleagues found the optimal biomass production, growth rate, and photosynthetic pigment accumulation at a relatively low concentration of 10 mg/L of sodium acetate. While at higher concentrations of sodium acetate, reduced accumulation of photosynthetic pigments leads to increased production of reactive oxygen species that made the algae susceptible to damage. The study paves the way of better methods and techniques that will result in enhancing biomass production by S. obliquus using optimal concentrations of sodium acetate, and would be conducive to further accelerate the industrialization of this strain.