Optimization of eicosapentaenoic acid production by Phaeodactylum tricornutum in the flat panel airlift (FPA) reactor

Biomass and eicosapentaenoic acid (EPA) productivities were investigated in a flat panel airlift loop reactor ideally mixed by static mixers. Growth with ammonium, urea and nitrate as nitrogen source were performed at different aeration rates. Cultures grew on ammonium but the decay of pH strongly inhibited biomass increase. On urea biomass productivity reached 2.35 g L-1 d(-1) at an aeration rate of 0.66 vvm (24 h light per day, 1000 mumol photon m(-2) s(-1)). Aeration rates between 0.33 vvm and 0.66 vvm and maximal productivities on urea were linearly dependent. Productivity on nitrate never exceeded 1.37 g L-1 d(-1). In the range of maximum productivity photosynthesis efficiency of 10.6% was reached at low irradiance (250 mumol photon m(-2) s(-1)). Photosynthesis efficiency decreased to 4.8% at 1000 mumol photon m(-2) s(-1). At these high irradiances the flat panel airlift reactor showed a 35% higher volume productivity than the bubble column. At continuous culture conditions the influence of CO2 concentration in the supply air was tested. Highest productivities were reached at 1.25% (v/v) CO2 where the continuous culture yielded 1.04 g L-1 d(-1) (16 h light per day, 1000 mumol photon m(-2) s(-1)). The average EPA content amounted to 5.0% of cell dry weight, that resulted in EPA productivities of 52 mg L-1 d(-1) (continuous culture, 16 h light per day) or 118 mg L-1 d(-1) (batch culture, 24 h light per day).