- Abstract The feasibility of growing Porphyridium biomass outdoors was investigated. Laboratory studies indicated that although the optimum temperature for growth is 25° C no damage to the photosynthetic activity can be detected after exposure of the organism to higher temperatures, up to 35° C. Also, high O 2 evolution activity was observed even at relatively high cell concentrations. No inhibition of O 2 evolution was detected at high light intensity, ie 2000 μE m− 2 s− 1. In cultures grown outdoors an output rate of up to 22 g dry wt m− 2 day− 1 was obtained for several weeks. The biomass contained, on a dry weight basis, up to 40% polysaccharides and about 1· 5% arachidonic acid. The cultures were maintained essentially monoalgal for over 3 months. The biomass can be readily removed from the medium by autoflocculation.