Outdoor continuous culture of Porphyridium cruentum in a tubular photobioreactor: quantitative analysis of the daily cyclic variation of culture parameters

The present work reports on the daily cyclic variation of oxygen generation rates, carbon consumption rates, photosynthetic activities, growth rates and biochemical composition of the biomass in a pilot plant continuous outdoor culture of the microalgae Porphyridium cruentum. A linear relationship between the external irradiance and the average irradiance inside the culture was found. In addition, the oxygen generation and carbon consumption rates were found to be a function of the average irradiance inside the culture. A reduction in photosynthetic activity of the cells at noon and recovery in the afternoon was also observed. Therefore, the cells showed a short-term response of parameters such as oxygen generation rate as well as carbon consumption rate with external and average irradiance; a model of photosynthesis rate considering photoinhibition is proposed. This model is a useful tool for the operation and scaleup of tubular photobioreactors, and can be used for determining CO₂ requirements of the system. The higher the photosynthesis rates, the lower the carbon losses, ranging from 25% at noon to 100% during the night. The growth rate showed a linear relationship with the daily mean average irradiance inside the culture with a long-term response. Likewise, a linear relationship among the oxygen generation rate and the growth rate was obtained. With respect to the biochemical composition of the biomass, the cells showed a long-term response of metabolic routes to mean daily culture conditions. During the illuminated period, energy was stored as carbohydrates and synthesis of proteins was low. During the night, the stored carbohydrates were consumed. The fatty acid dry weight (DW) content decreased during the daylight period, whereas the fatty acid profile, as total fatty acids, was a function of growth rate. A short-term variation of exopolysaccharides synthesis with solar irradiance was also observed, i.e. the higher the external irradiance the higher the excretion of exopolysaccharides as a protection against adverse culture conditions.