Permeabilization of the cell wall of Chlorella sorokiniana by the chitosan-degrading protease papain

Abstract

To enhance the nutritional value of Chlorella microalgae for use in food and feed, it is crucial to improve the bioavailability of nutrients and bioactive compounds from whole-cell biomass in a cost-effective manner. In this study, papain – a plant-derived cysteine protease with additional chitosanase-like activity – was applied to partially degrade the cell wall of Chlorella sorokiniana at ambient temperature, whereas the cell wall of the closely related species Chlorella vulgaris remained unaffected and generally appeared more stable. Three commercial enzymes - cellulase, lysozyme and papain - were evaluated for their suitability to digest the fresh C. sorokiniana and C. vulgaris biomass suspended in acetate buffer at pH 4.8. The accessibility of algal carotenoids to acetone extraction, performed on analytical scale, proved to be a simple and reliable measure for the degree of cell wall degradation, while on the contrary the enzyme-induced release of reducing sugars did not strictly correlate with the actual permeabilization of the cell. Ultimately, papain was found to be more active than lysozyme, while cellulase showed almost no effect. This is in line with the knowledge about the C. sorokiniana cell wall composition with regard to its framework of chitosan-like fibers and high protein content. Papain concentration, enzyme-to-biomass ratio and duration of the enzyme treatment were varied. Papain was equally efficient in acidified BG11 medium, therefore enzymatic permeabilization of the cell wall directly after harvesting the microalgae seems possible– either for valorization of whole-cell biomass or as an energy-efficient process step to increase yields in biorefinery approaches. Main drawback of the approach is the high enzyme dose of estimated 5 g papain per 100 g of C. sorokiniana dry mass, that was needed to gain sufficient effect. Future research is required to optimize dosage in combination with synergistically acting enzymes and under the conditions of the actual application.