PHOTOLIMITATION AND PHOTOINHIBITION AS FACTORS DETERMINING OPTIMAL DILUTION RATE TO PRODUCE EICOSAPENTAENOIC ACID FROM CULTURES OF THE MICROALGA ISOCHRYSIS-GALBANA

Eicosapentaenoic acid (EPA) productivity from continuous cultures of t he marine microalga Isochrysis galbana was studied, taking into accoun t the irradiance on the reactor surface, that is, the photolimitation/ photoinhibition regime to which the cells are exposed. Experiments wer e conducted under a wide variety of operating conditions. The dilution rate ranged from 0.005 h(-1) to 0.040 h(-1) at five external irradian ces (820, 1620, 2050, 2450 and 3270 mu mol photons m(-2) s(-1)) coveri ng photolimited to photoinhibited growth. Under these conditions, the specific growth rate (mu) was found to be the main factor influencing EPA content (ranging from 2.35% to 5.23% dryweight) and productivity ( up to 0.88 mg l(-1) h(-1)). The fatty acid content was not significant ly affected by the external irradiance, but was influenced by the stat e of growth of the microalga, depending on whether the light regime wa s photolimiting or photoinhibiting. It might be suggested that light s hould no longer be considered an isolated factor affecting EPA synthes is, but an indirect influence through the photolimitation/photoinhibit ion regime and growth rate. At a given dilution rate, EPA content and biomass concentration are lower under photoinhibiting external irradia nces than those corresponding to photolimiting conditions, and consequ ently EPA productivity decays. Since the effect of photoinhibition is less marked at high biomass concentration, a strategy to optimize EPA productivity from microalgal cultures could consist of reducing the di lution rate when the external irradiance increases above the phoinhibi tion threshold.