With the aim to optimize the production of exopolysaccharide (EPS) by Porphyridium marinum, cultures in photobioreactors were conducted on a modified Provasoli medium (P) and compared to a new medium (Pm) with an elemental composition of N0.0205S0.0597P0.005. Cultivation on this medium allowed the increase of EPS concentration up to 2.5 g L-1, without modification of the EPS productivity (0.096 g L-1) and EPS structure. In a second time, photosynthetic activity of the strain was monitored as a function of irradiance and temperature, allowing improvement of kinetic parameters of growth and EPS production. A semi-continuous culture, carried out with the Pm medium, an optimal irradiance and temperature of respectively 360 mu mol photons m(-2) s(-1) and 28 degrees C led to an EPS process productivity of 0.031 g h(-1) instead of 0.020 g h(-1) in batch culture. (C) 2016 Elsevier Ltd. All rights reserved.