Microalgal biomass productivity and dominant species transition in a Korean mass cultivation system

Mass cultivation of microalgae was carried out at a large-scale outdoor raceway facility in Korea from June 2014 to January 2017 and we analyzed the transition of dominant microalgal species and influence of zooplankton predation. Concurrently the microalgae yield, biomass characterization, and resource consumption results were examined from two identical 675.0 m2 raceways with or without a semitransparent film cover to determine which model is a better fit for Korean geoclimatic conditions. Year-round cultivation was possible in the covered raceway, but the average seasonal productivities were lower than those of the non-covered one, probably due to the reduced solar radiation. The addition of a cover did not confer advantages for the production of microalgae, even though the production period was extended over sub-zero temperature winter seasons. Species transitions were observed throughout the cultivation period, and the most dominant microalgal genera found year round were Acutodesmus and Pseudopediastrum. In addition, an algal predator, Vorticella, was present during most of the cultivation periods. The seasonal water temperature fluctuation and presence of predators greatly affected the dominant microalgae and biomass productivity. The maximal productivity, 29.3 g dry weight (DW)/m2-day with 15.0% lipid and a calorific value (CV) of 20.1 MJ/kg, was attained in the non-covered raceway during the summer of 2016. Overall, a yearly average productivity of 8.9 g DW/m2-day was obtained from the raceways, and the biomass had an average lipid content of 12.8% and CV of 17.7 MJ/kg. The grazer-resistant microalgae were allowed to dominate rather than maintaining target strains and the results demonstrated the potential of naturally occurring microalgae as a biofuel source since the CVs of the biomass were close to those of terrestrial energy crops. Also, the mass cultivation of the indigenous isolates could be applied to wastewater treatment due to their high capacity to assimilate nutrients.