Combined metals and EDTA control: An integrated and scalable lipid enhancement strategy to alleviate biomass constraints in microalgae under nitrogen limited conditions

Abstract

The commercial realization of microalgal biodiesel production necessitates substantial impulsion towards development of strategies to improve lipid yields upstream. Nitrogen stress is the most widely used lipid enhancement strategy; yet, it is associated with compromised biomass productivity. In this novel approach, combined effect of metals and EDTA on lipid productivity of Acutodesmus obliquus was investigated under nitrogen limited conditions. The effect of metal concentrations, individually and in combination, on microalgal lipids and biomass production is a scarcely exploited area. Combined metal stress alleviates the constraint of low biomass production under nitrogen limitation and improved the overall lipid productivity. Highest lipid productivity of 73.23mgL−1d−1 was achieved with a combination of iron 9mgL−1, magnesium 100mgL−1 and calcium 27mgL−1 at limited nitrogen (750mgL−1). This was 1.72 fold higher than nitrogen stress alone and 1.99 fold higher than BG11 medium. Iron was found to be most significantly influencing metal followed by magnesium in response surface methodology data analysis. The enhanced photosynthetic performance and chlorophyll content further confirmed the significant impact of iron and magnesium on the microalgal biomass. The addition of EDTA to the optimised metal combination further improved the lipid productivity to 80.23mgL−1d−1 (2.18 fold). At 3000 L open cultivation pond this strategy has resulted in an increase of 2.08 fold in lipid productivity. Higher biodiesel conversion rates were also observed with this easy, universally applicable and scalable lipid enhancement strategy.