The Effect of Biochar Application in Microalgal Culture on the Biomass Yield and Cellular Lipids of Chlorella vulgaris

Microalgal culture may have the potential as a source of raw materials for biodiesel to provide renewable energy services for regional and remote communities. However, current culturing systems face many technical hurdles to be economically feasible, such as high costs for microalgae harvesting and difficulties to make algal cell accumulating lipids. Biochar is an inherent product of biomass pyrolysis; it is a carbon rich and porous substance with high surface area and many surface charges. Biochar could thus be utilised to absorb nutrients and attract and immobilise algae to its surfaces, providing an effective method to alleviate the algae harvesting challenge. In this study, an Oil Mallee biochar of varying particle sizes were added to Chlorella vulgaris culture in a Tris-Acetate-Phosphate (TAP) medium to investigate the biochar – algae interaction. Algae growth were severely inhibited when biochar was added at inoculation phase, whereas, adding biochar at later stages of the exponential or stationary growth phase has less or no effect on the algal culture. Based on these observations biochar was tested as filtration aid for algae harvesting. When C. vulgaris was gravity-filtrated using a filter paper of 11 μm pore size, the algal mass on filter was increased from 4.7 % of total biomass in the control sample to 8.2 % in a 0.2 g/100mL biochar treated sample. Interestingly, the cellular lipids contents (determined in the form of fatty acid methyl ester) increased by 40 % in the biochar treated culture as compared to the control. The lipid accumulation was thought to have resulted from the stress induced by biochar stripping phosphorus from the medium. These findings provided a scientific basis for an innovative use of biochar to improve the efficiency of the algal culture system and further research to reveal the detailed mechanism of biochar – algae interaction was discussed. Keywordsalgae; biochar; cellular lipids; Chlorella vulgaris; phosphorus