Comparison of open-air and semi-enclosed cultivation system for massive microalgae production in sub-tropical and temperate latitudes

• Nannochloropsis oculata performed better in semi-enclosed system in colder seasons.
• The semi-enclosed system avoided dilution of culture medium due to the rain.
• The semi-enclosed system allowed higher temperatures than external tanks.
• Microalgae production was higher in semi-enclosed system.
• N. oculata presented tolerance to salinities as high as 130.

This study compared open-air and semi-enclosed production system of the marine microalgae Nannochloropsis oculata in a sub-tropical region (32°S; 52°W) under uncontrolled environmental conditions. The semi-enclosed system was composed of 1.2 m3 circular tanks installed inside of a greenhouse. Water temperature was 4 °C higher in the indoor treatment than in the outdoor, mainly in winter although no difference was observed in warmer seasons. Moreover, variation in salinity was observed in the outdoor treatment due to rainfall (winter) and evaporation (spring), whereas indoor treatment experienced an increase (up to 100 PSU) due to evaporation only in warmer seasons. Light transmission was approximately 20% lower in the indoor treatment although cell densities and biomass yields were higher indoor during winter. As the temperature increased (spring) no differences were observed among treatments. In summary, partial control of temperature and salinity in the semi-enclosed system, especially during the colder and rainy season, allowed higher microalgae biomass production. Further experiments must be conducted with CO2 addition, larger pH range and salinity control.