Mixotrophic cultivation of green microalgae Scenedesmus obliquus on cheese whey permeate for biodiesel production

• A microalgae species that is able to grow on lactose was selected for biodiesel production.
• Cheese whey permeate highly stimulated Scenedesmus obliquus growth and biomass yield.
• Lactose hydrolysis induced glucose and galactose accumulation in the culture medium.
• Significant neutral lipid accumulations were obtained under specific culture conditions.
• A quick and simple method to estimate neutral lipid cellular content is proposed.

Microalgae mass cultivation for biodiesel production might very well become the next marketable biofuel. The main challenge to overcome however is the development of high efficiency strategies for the large-scale production of oleaginous microalgae at low costs. In the present work, the use of cheese whey permeate (WP) in mixotrophic microalgae cultures is proposed. Pure lactose, the main constituent of WP (> 80% w/w of the total dissolved solids), can support Scenedesmus growth under heterotrophic culture conditions (absence of light). Substituting 40% (v/v) of the culture medium with WP significantly stimulates Scenedesmus obliquus growth under mixotrophic (μmax = 1.083 ± 0.030 day− 1) and heterotrophic (μmax = 0.702 ± 0.025 day− 1) conditions, compared to photoautotrophic control cultures (μmax = 0.267 ± 0.083 day− 1). As growth occurs in the presence of lactose, a significant reduction of its concentration is observed, while the galactose and glucose concentrations actually increase in the culture medium. Culture medium analyses showed complete exhaustion of extracellular nitrogen (nitrate and ammonium), while intracellular lipid analyses showed neutral lipid (NL) accumulation, particularly under conditions of high pH (> 9.5). Photoautotrophic control cultures accumulated more lipids (per dry weight) than WP-supplemented cultures, an aspect which is discussed in the context of lipid enrichment strategies. A fast and simple method for NL cellular content estimation is also described.