Lipid Production from a Louisiana Native Chlorella vulgaris/Leptolyngbya sp. Co-culture for Biofuel Applications

Abstract

Over the last several decades, microalgae have garnered great interest for biofuel production mainly due to the high lipid productivity and minimum land competition with food crops. In this research, a Louisiana native Chlorella vulgaris/Leptolyngbya sp. co-culture was selected for lipid production. The goal of this work is to improve the lipid productivity and lipid composition of this co-culture via optimizing irradiance and nitrogen levels, lipid extraction method and modeling of pilot photobioreactor (HISTAR). Effects of irradiance and nitrate nitrogen levels on total lipid yield, neutral lipid portion, and fatty acid profile of non-aerated and aerated co-cultures were investigated in bench top scale experiments. The maximum lipid productivities were ~17 and 116 mg L-1d-1 for non-aerated and aerated cultures respectively. The optimal nitrogen level was 2.94 mM and the optimal irradiance was in the range of 400 and 800 ƒÝmol m-2 s-1. Neutral lipids comprised approximately 75% of total lipids for non-aerated culture and 89% for aerated cultures. The major fatty acid components were 16- and 18-carbon fatty acids, and ~35% are saturated fatty acids. The fatty acid profiles did not vary significantly with the irradiance and nitrate nitrogen levels. A method using silver nanofibers to enhance the lipid extraction efficiency was investigated. Silver nanoparticles were added to disrupt microalgal cell walls to improve the lipid extraction efficiency of Folch¡¦s method and microwave assisted lipid extraction. The results showed that 1000 £gg g-1 silver nanofibers had the most significant improvement in the efficiency of lipid extraction (~30% and 50% increase in efficiency for Folch¡¦s method and microwave assisted lipid extraction respectively). Microwave assisted lipid extraction was the optimal method, considering both fatty acid profile and lipid extraction efficiency. For the modeling of HISTAR system, Steele¡¦s model was used for impact of irradiance and Monod model was used for the effects of nitrogen. The final model was able to predict the trend of lipid percentage increase with limited nitrogen level. The results of the simulations indicated the limited nitrogen constrained the lipid productivity despite of a higher lipid percentage. The results matched the conclusions of the experiments in bench top scale.