Production of eicosapentaenoic acid by Nannochloropsis sp. cultures in outdoor tubular photobioreactors

Autotrophic microalgae cultures have been proposed as an alternative source of EPA, a nutritionally important polyunsaturated fatty acid that plays a key role in the prevention and treatment of several human diseases and disorders. The technology currently available is however, considered commercially not viable because of the low degree of control of algae cultures in outdoor open ponds. The use of closed reactors could overcome these limitations and bring EPA production by microalgae closer to becoming a reality. In this study, we have demonstrated the feasibility of outdoor cultivation of Nannochloropsis sp. in tubular reactors and the potential of this eustigmatophyte as an alternative source of EPA. Nannochloropsis sp. was cultivated in NHTRs of different sizes (from 10.2 to 610 l) from spring to autumn under the climatic conditions of central Italy. EPA productivity essentially reflected the productivity of the culture and reached its maximum in May–June (mean monthly value: 32 mg l⁻¹ day⁻¹). Although the fatty acid composition of the biomass varied significantly during the cultivation period, EPA content remained rather stable around the value of 4% of dry biomass. The transfer of the cultures from laboratory to outdoor conditions, the exposure to natural light–dark cycles, along with lowering the salt concentration from 33 g l⁻¹ (seawater salinity value) to 20 g l⁻¹, factors that caused lasting modifications in the fatty acid content and composition of Nannochloropsis sp., did not significantly affect the EPA content of the biomass.