Accumulation of eicosapentaenoic acid in Nannochloropsis sp. in response to elevated CO2 concentrations

To increase eicosapentaenoic acid (EPA, 20:5, n-3) content in the marine alga Nannochloropsis sp., the effect of CO₂ concentration during cultivation has been investigated. In a batch culture under normal atmospheric conditions (0.037% CO₂), the EPA content per cell increased during the first 1.5 days and then decreased immediately even though the cells were in an exponential growth phase. Increasing the CO₂ concentration to 0.3% and 2% over day 1.5 retained the EPA content at the higher concentration for another 1 and 2 days, respectively, suggesting that the EPA accumulation is enhanced by elevated concentrations of CO₂. EPA accumulation in response to elevated CO₂ concentrations was also observed during a later growth phase when CO₂ was introduced after the decrease of EPA content. The addition of CO₂ caused a slight decrease in the pH of the medium though this was not the cause of the observed EPA accumulation as addition of acidic buffer did not affect the EPA content. The maximum EPA production was obtained when 2% CO₂ was supplied 12 h prior to the end of the exponential growth. The total EPA production during 4-day cultivation was about twice that obtained with ambient air. These results suggest that the available CO₂ concentration affects the EPA content in Nannochloropsis sp.