Isolation of an Oleaginous microbial strain for biodiesel production and medium reutilization after harvesting

Abstract

instead, carbon dioxide and light supply is required. However, they have slower growth rates and lower final density, meaning that large quantities of water and nutrients are used in microalgae cultivation. In this study, the coagulant $Fe(NO_3)_3$ was used as the coagulant not only to harvest the microalgae, but also to simultaneously provide nitrate into the liquid medium to be utilized in the liquid medium recycle. With 0.8 g/L of $Fe(NO_3)_3$, the nitrogen level after 8 days of cultivation could be replenished back to the original level simply by recycling the liquid medium after microalgae harvesting. The rest of the nutrients had to be supplemented, but only half amount of the nutrients supplied in the BG11 medium was enough to support cultivation of Chlorella sorokiniana in the recycled medium, with the biomass productivity of 370.3 mg/L/day as compared to 325.9 mg/L/day in control culture grown in BG11. The final FAME productivity was 88.3 mg/L/day, and when the entire amount of the nutrients in the BG11 was supplied, the FAME productivity was even higher, with 95.5 mg/L/day. From this study, the potential to use $Fe(NO_3)_3$ as both coagulant and nitrate supply to optimize water and nutrient usage was proven. This method will reduce the water and nutrients footprint of microalgae-based biodiesel production, especially when applied in large-scale commercialization.

The rising carbon dioxide and greenhouse gas emissions are already affecting our lives in many ways, causing harmful effects in terms of health, environment, and climate. In response to this phenomena, many researchers around the world are looking for an alternative energy, which will not only bring environmental benefits, but also will ensure energy security, creation of new jobs, and energy for future generations. There are many types of alternative energy sources, such as solar, wind and water, but biodiesel is receiving more and more attention in both academic and industrial fields. Biodiesel is produced by extracting lipids from biomass, and among various types of biomass used to produce biodiesel, this thesis focuses on microbial oil, which is oil produced by oil-accumulating (oleaginous) microorganisms. More specifically, this thesis includes both yeast and microalgae based biodiesel with investigations on (1) obtaining new oleaginous yeast strains, and (2) recycling the culture medium after microalgae harvesting. Oleaginous yeasts, when grown under appropriate conditions, can be a good source for biodiesel production. In this study, a yeast strain Cyberlindnera saturnus was isolated from fermented seafood. Characterization experiments were carried out with Cyberlindnera saturnus, to find out at what environmental conditions it can accumulate maximum biomass and lipid content. Various culture conditions were tested, including temperature, pH, carbon-source, nitrogen-source, carbon-to-nitrogen ratio, and salt concentrations. The results showed that Cyberlindnera saturnus was able to grow well under a wide pH range of 4 to 9 and was also able to accumulate lipid contents higher than 30% when grown under optimized conditions. Such results indicate that Cyberlindnera saturnus can be called an oleaginous yeast strain, with its strength being the ability to withstand a wide range of pH. In terms of microalgae, they have advantage over yeasts in that they do not require organic substrates to grow