Accumulation of Exopolysaccharides in Liquid Suspension Culture of Nostoc flagelliforme Cells

The liquid suspension culture of dissociated Nostoc flagelliforme cells was investigated. It was found that the growth rate of N. flagelliforme cells and the accumulation of exopolysaccharides (EPS) increased prominently when NaNO₃ and KH₂PO₄ were added in the liquid BG-11culture medium though phosphate had little effect on EPS yield for specific mass of cells. N. flagelliforme cells grew well at 25 °C and neutral pH, however, a lower or higher temperature and weak alkaline can promote EPS accumulation. With the increase of the light intensity, the growth rate of N. flagelliforme cells and the EPS accumulation increase accordingly. When N. flagelliforme cells was cultured in BG-11 medium added with 2.5 g L⁻¹ of NaNO₃ and 0.956 g L⁻¹ of KH₂PO₄ at 25 °C with 60 μmol photon m⁻² s⁻¹ of light intensity, 1.05 g L⁻¹ cell density and 89.9 mg L⁻¹ EPS yield were achieved respectively. Adopting the optimal conditions established in flask culture, the liquid culture of N. flagelliforme cells in 20-L photobioreactor for 16 days was conducted and a maximum biomass of 1.32 g L⁻¹ was achieved, which was about 17.6-fold of that in the initial inoculation. The yield of EPS was 228.56 mg L⁻¹and about 2.23-fold of that in flask culture. Moreover, the polysaccharides’ material was released into the culture medium during cell growth. These released polysaccharides (RPSs), which can be easily recovered from the medium, are favorable for industrial applications.