Enhancement of lipid yield in <em>Chlorella</em> sp. under different stress conditions for the production of biodiesel

Abstract

Microalgae derived bio-lipids can be potentially converted to biofuel using appropriate technologies. However, optimum conditions for enhancing the growth of microalgae often lead to relatively low accumulation of bio-lipids. Alternatively, stress conditions favor the storage of lipids despite the low biomass yield. For commercial production of microalgae derived biofuel, the unit cost of production of biodiesel should be lower than that of petro diesel. The aim of the present study was to enhance the lipid yield of <em>Chlorella</em> sp. by introducing various stress conditions such as nitrogen deficiency, zinc deficiency and excessive salinity in the growth media. Conventional Bold’s Basal Media (BBM) was used as the culture media in all the experimental conditions. Five treatments were used namely; normal Bold’s basal media, three times concentration enhanced Bold’s basal media, Nitrogen deficient media, Zinc deficient media and Salinity stress media. Light intensity was kept constant in all the experimental conditions. Air mixing was provided using spargers in all test conditions to keep the microalgae cultures in suspension. All the reactors were maintained in similar growth conditions in the first phase and the stress conditions were introduced in the second phase after 7 days. The microalgae biomass was harvested on 4th, 8th, 11th, 14th and 17th day of cultivation for the determination of lipids. A Chloroform: Methanol mixture with 1:1 ratio was used to extract lipids from dried biomass using Soxhlet extractor. A considerable increase in the lipid content was observed in all the test conditions compared to control samples (highest was 16.9%). Highest lipid content of 23.3% was observed in Zn stressed conditions. However, in terms of lipid productivity, salinity stress treatment produced the highest average lipid productivity of 0.008±0.005 mgL^-1d^-1. After certain period of time (14th day), lipid content of stressed samples decreased. Oleic acid and linoleic acid were found to be abundant in all of the treatments. The study suggests that imposing stressful conditions can result in a higher amount of lipid yield from the algae biomass with a higher rate of productivity that contains important fatty acids which can be a feasible opening for biofuel production.