Microalgal cultivation in porous substrate bioreactor for extracellular polysaccharide production

Abstract

Netrium digitus is a representative of the species-rich class Zygnematophyceae (Streptophyta). Its intensive extracellular polysaccharide (EPS) production makes this alga interesting for biotechnological applications with a focus on cosmetics and food additives. Quantitative data on growth and EPS production in suspension and, for the first time, in immobilized culture using lab-scale porous substrate bioreactors, so-called Twin-Layer (TL) systems, is presented. It is shown that the cell as well as the EPS dry weight content is increased at least sixfold in immobilized compared to suspension culture. Due to the high amount of EPS, the biofilms reach a thickness of more than 8 mm after 27 days at 70 μmol photons m−2 s−1 and with 1.5% CO2 supply. Frequent exchange of the growth medium results in a linear cell biomass increase of 2.02 ± 0.09 g m−2 growth area day−1 compared to 2.99 ± 0.09 g m−2 day−1, when the medium is not exchanged. Under this mode of cultivation, the EPS production is lower and a final concentration of 12.18 ± 1.25 g m−2 compared to 20.76 ± 0.85 g m−2, when medium was exchanged, is reached. It is clearly demonstrated that the relatively slow growing, but excessively EPS producing, microalgal species N. digitus can be grown in porous substrate bioreactors and that this culturing technique is a promising alternative to suspension culture for the Zygnematophyceae.