Modeling of eicosapentaenoic acid (EPA) production from Phaeodactylum tricornutum cultures in tubular photobioreactors. Effects of dilution rate, tube diameter, and solar irradiance

A model for the prediction of eicosapentaenoic acid (EPA) productivity from Phaeodactylum tricornutum cultures that takes into account the existence o f photolimitation and photoinhibition of growth under outdoor conditions is presented. The effects of the external irradiance on the culture surface, the average irradiance inside the culture, and the light regime at which th e cells are exposed on pigments and EPA content are studied. The chlorophyl l content decreases exponentially with the average irradiance, whereas the carotenoids content increases linearly with the external irradiance due to a higher extension of photoinhibition. A decrease in the fatty acid content of the biomass with irradiance on reactor surface is observed when photoin hibition becomes relevant. The average irradiance within the culture mainly influenced the fatty acid profile of the biomass. As the average irradianc e becomes higher, percentages of saturated and monounsaturated fatty acids decrease, increasing the portion of EPA. By taking into account the differe nt relationships among pigment and EPA content with the irradiance, the var iation in EPA productivity over the year can be simulated as a function of average and external irradiance. For the two photobioreactors employed the maximum EPA productivity is attained in spring and fall (30 mg L-1 day(-1) for tube diameter 0.06 m and 50 mg L-1 day(-1) for tube diameter 0.03 m). I n winter, the biomass productivity is limited by low light availability alt hough the EPA content is maximum. In summer, the biomass productivity is hi gher although the EPA content diminished by photoinhibition; the higher the dilution rate, the lower the minimum. Thus, the conditions that increase t he biomass productivity and the polyunsaturated fatty acids content are in opposition, the optimum being reached by operating under photolimitation wi th high growth rates in order to produce a high proportion of polyunsaturat ed fatty acids. (C) 2000 John Wiley & Sons, Inc.