Extracellular polysaccharide production in outdoor mass cultures of Porphyridium sp. in flat plate glass reactors

This work concerns an attempt to develop large scale cultivation of Porphyridium sp. outdoors. The impact on cell growth and production of soluble sulphated polysaccharides of light-path length (31LP) was studied in flat plate glass reactors outdoors. The LP of the plate reactors ranged from 1.3-30 cm, corresponding to culture volumes of 3-72 L. The side walls of all reactors were covered, ensuring similar illuminated surfaces for all reactors. Maximal day temperature was maintained at 26±1 °C. Growth conditions of pH (7.5), stirring (with compressed air) and mineral nutrients, were optimal. Maximal volumetric concentration of the soluble sulfated polysaccharide (1.32 g L^-1) was obtained in winter with the smallest light-path reactor (1.3 cm) at a cell density of 1.37 x 10¹¹ cells L^-1. Under these conditions, the viscosity of the culture medium was also highest, being inversely proportional to the culture's light-path. Highest areal concentration of soluble polysaccharides (60 g m^-2) and areal cell density (3.01 x 10¹²m^-2) was recorded in the 20 cm LP reactor, progressively lower values being obtained as the light path became shorter. A similar pattern was obtained for the areal productivity of polysaccharides, the highest being 4.15 g m^-2 day^-1 (considering the total illuminated reactor surface), produced in the 20-cm LP reactor. The main sugar composition (i.e. xylose, galactose and glucose) of the sulfated polysaccharides was similar in all reactors. As viscosity increased with time during culture growth, there was a substantial decline in bacterial population. Cultivation throughout most of the year provided good evidence that a light path length of 20 cm in flat plate reactors under these conditions is optimal for maximal areal soluble polysaccharide production of Porphyridium sp.