Milking exopolysaccharides from Botryococcus braunii CCALA778 by membrane filtration

The aim of this work was to optimize the efficiency of extraction and recovery, also known as ‘milking’ of exopolysaccharides (excreted polysaccharides, EPS) from continuous cultures of Botryococcus braunii CCALA778. First, an indoor process was developed and optimised, ensuring the highest milking efficiency without compromising culture viability. For this, photobioreactors were operated in a photo-chemostat mode under simulated outdoor conditions of a typical summer at AlgaePARC (51°59′44.1”N 5°39′26.2″E) in Wageningen, The Netherlands. Once a steady state was reached, areal productivities of 23 g m⁻² d⁻¹ and 3 g m⁻² d⁻¹ for biomass and EPS were achieved. EPS milking was done by membrane filtration of one reactor volume at the beginning of the dark period. After optimization, the maximum recovery of EPS, without damaging the cells, was 12%; yielding a daily EPS extraction rate of 0.36 g m⁻² d⁻¹. The optimised process was scaled-up and applied outdoors during the summer (at AlgaePARC facilities). Outdoor cultures showed 25% lower biomass productivity (17 g m⁻² d⁻¹) but an 25% higher EPS productivity (4 g m⁻² d⁻¹). The efficiency in the milking, however, decreased as compared to indoor results. Only 3% of the total content of EPS produced outdoors was milked (0.12 g m⁻² d⁻¹). To improve the EPS milking process, future research should focus on increasing the EPS extraction yield without negatively influencing its production by Botryococcus braunii.