Outdoor cultivation of newly isolated tropical marine microalgae Nannochloropsis sp. UHO3 and Skeletonema sp. UHO29 in raceway ponds for biodiesel feedstock production

The microalgae Nannochloropsis sp. UHO3 and Skeletonema sp. UHO29 are newly isolated marine microalgae from coastal areas in Kendari, Southeast Sulawesi, Indonesia. Indoor studies of the microalgae showed potential as biodiesel feedstock due to their high growth rates, high biomass and lipid productivity. However, for commercial uses, especially for biodiesel feedstock, the species should have the ability to perform well under real outdoor conditions. This study aimed to determine the growth, biomass, and lipid productivity of the Nannochloropsis sp. UHO3 and Skeletonema sp. UHO29 in outdoor raceway ponds. The species were cultured using 1 m² raceway ponds at 30 cm depth. The two species were cultured using water-based sea media enriched with f/2 nutrients for Skeletonema sp. and Walne media for Nannochloropsis sp. Cultures were initially operated in batch mode until they reached the stationary phase before operating in semi-continuous mode. The culture duration was about three months. Cell counting was done every two days, while sampling for biomass (dry weight-DW, and ash-free dry weight-AFDW) and lipids were done every four days. The results showed that the two species of microalgae could grow well in outdoor raceway ponds. The specific growth rate of the Nannochloropsis sp. UHO3 ranged from 0.105 to 0.447 d^-1 while the specific growth rate of Skeletonema sp. UHO29 ranged from 0.127 to 0.457 d^-1. Nannochloropsis sp. UHO3 and Skeletonema sp. UHO29 have high lipid content ranging from 15 to 44% AFDW and from 14 to 53% AFDW, respectively. The biomass productivity of Nannochloropsis sp. UHO3 and Skeletonema sp. UHO29 range between 10-122 g m^-2 d^-1 and 13-107 g m^-2 d^-1 respectively, while the lipid productivity value for Nannochloropsis sp. UHO3 ranges from 4 to 31 g m^-2 d^-1 and for Skeletonema sp. UHO29 range from 5 to 25 g m^-2 d^-1. The results of this study indicate that the two microalgae species are potentially developed as biodiesel feedstocks due to their fast growth rate, high lipid content, high biomass, and lipid productivity under outdoor conditions.