Two-step cultivation of Neochloris oleoabundans in a novel horizontal thin-layer algal reactor: Interplay of pH and dissolved inorganic carbon

Abstract

This study explored the impacts of culture pH and addition of NaHCO3 on the growth of an oil-rich green alga Neochloris oleoabundans. It was observed that in the absence of aeration, the culture pH fluctuated over wide ranges during the day/night cycles without additional DIC, which was flatted significantly by adding DIC in culture, demonstrating the effectiveness of DIC as a pH buffer. Addition of DIC in the range of 80–320 mM led to an increase in cell size and inhibition of cell division, which became more severe with the increase of DIC concentration in the tested range. Under 160 mM DIC, pH's effects on algal growth were studied at constant levels (7.5, 8.5, and 9.5). Lower pH increased cell division inhibition, linked to rising dissolved CO2. Based on these findings, a two-step cultivation strategy was devised and explored. By growing algae in the first stage in cultures without additional DIC and with culture pH controlled at 7.5, followed by cessation of aeration and addition of 160 mM NaHCO3 on the fourth day of cultivation, similar cell growth rate and lipid productivity were obtained as that in cultures with continuous aeration throughout the 8-day cultivation, with lower cell density but larger cell size. By discontinuing aeration, a reduction of 76.73% in aeration CO2 and 50% of aeration energy were achieved.