Enhanced extracellular polysaccharide production and growth by microalga Netrium digitus in a porous substrate bioreactor

Desmids (Zygnematophyceae) often secrete enormous amounts of extracellular polysaccharides with potential biotechnological applications. By using a porous substrate bioreactor (PSBR), the effects of light, pH and nitrogen availability on extracellular polysaccharide (EP) production of Netrium digitus, a desmid with excessive EP secretion, were investigated in immobilized cultures. The cultivation of a microalga in a PSBR, in contrast to conventional suspension culture, separates growth medium from biomass and therefore allows exchange and manipulation of the growth medium without physical disturbance of the biofilm. Over a cultivation period of at least 12 days, cellular biomass growth and EP concentration per growth area were monitored under various abiotic conditions. Growth and EP production were positively influenced by light intensity as an increased growth rate of 3.2 g ∙ m− 2 of growth area ∙ d− 1 compared to 1.8 g ∙ m− 2 ∙ d− 1 and an EP concentration of 32.4 g ∙ m− 2 versus 21.4 g ∙ m− 2 were observed after 18 days at 330 and 70 μmol photons ∙ m− 2 ∙ s− 1, respectively. Further, a correlation between the pH of the culture medium and the ratio of EP to cell dry weight was observed. The lower the pH, the higher this ratio was with a maximum of 0.61 at a pH of 4.5. Omitting the nitrogen source after 6 days of growth did not affect the growth rate of cell dry weight compared to a medium with nitrogen, but EP production ceased completely. In contrast, increasing the nitrate concentration to 3.85 mM, increased the EP concentration by 18% and resulted in an EP concentration of 25.0 g ∙ m− 2 at 330 μmol photons m− 2 ∙ s− 1 after 6 days with increased nitrogen supply. The results provide a roadmap to improve the yield of EP in immobilized cultivation of desmids and thus make them amenable for future biotechnological applications.