Mixotrophic Production of Omega-3 Fatty Acid-rich Alga Phaeodactylum tricornutum on Biodiesel-derived Crude Glycerol

Crude glycerol is the major byproduct of the biodiesel industry. There is an abundance of this byproduct and purifying it for use in industries such as food, pharmaceutical, or cosmetic is prohibitively expensive. Developing an alternative use for crude glycerol is needed. Utilizing it as a carbon source in the fermentation of algae is one potential method for using this under-utilized byproduct.

In this research, crude glycerol is used in the mixotrophic production of the alga, Phaeodactylum tricornutum, which is an eicosapentaenoic acid (EPA) producing diatom. Mixotrophic growth is when cells perform autotrophic and heterotrophic modes of growth concurrently. EPA is an omega-3 polyunsaturated fatty acid that has been demonstrated to have a multitude of beneficial health effects, including maintaining human cardiovascular health, treating cancer and human depression diseases, and an anti-obesity effect.

In this study, the potential of using crude glycerol in batch mode mixotrophic culture of P. tricornutum was investigated. Once the mixotrophic culture was established, parameters involved in increasing the biomass and EPA production were optimized. These included nitrogen source, level of supplemental carbon dioxide, and concentration of crude glycerol. Using nitrate, 0.08 M crude glycerol, and 3% (vol/vol) carbon dioxide led to the highest biomass productivity of 0.446 g L?? day?? and the highest EPA productivity of 16.9 mg L?? day?? in batch mode culture.

The continuous culture of the mixotrophic culture was then performed following the batch culture optimization. The effects of dilution rate were observed in continuous culture with the parameters of nitrate as the nitrogen source, 0.08 M crude glycerol, and 3% (vol/vol) carbon dioxide held constant. The highest biomass productivity of 0.612 g L?? day?? was obtained at D = 0.24 day??. The highest EPA productivity of 16.5 mg L?? day?? was achieved at both D = 0.15 day?? and D = 0.24 day??. The maximum specific growth rate was estimated from the washing out dilution rate and was determined to be around 0.677 day??.

Overall, it was found that crude glycerol increases the biomass and EPA productivity of Phaeodactylum tricornutum. Continuous culture with the use of crude glycerol can further increase these measurements. The potential for scaling up studies is demonstrated by these results and can help lead to a market for this abundant, little-used byproduct of the biodiesel industry.