Large-size plate bioreactors were used to compare the production of extracellular polysaccharide by the red microalga Porphyridium cruentum when grown in suspension and in a foam sheet. A well-defined illuminated area and unidirectional light propagation allowed us to generate information that is better quantified when expressed in terms of illuminated area. This is essential for meaningful comparison of data, especially considering that for a well-designed and managed bioreactor, the culture production rates are believed to be light limited. At the same level of illumination, the culture immobilized in foam showed double the production rate of extracellular polysaccharide compared with the culture in suspension. The saturation level of biomass density per unit of illuminated area was eight times higher for the immobilized culture compared with the culture in suspension. Despite the increased biomass density for the immobilized culture, an increase in the light level above the optimum found for the culture in suspension reduced the extracellular polysaccharide production, suggesting that the photoinhibition light level was surpassed.