Pilot outdoor cultivation of an extreme alkalihalophilic Trebouxiophyte in a floating photobioreactor using bicarbonate as carbon source

Abstract

As an alternative carbon supply approach in the microalgae cultivation process, bicarbonate shows many advantages over the conventional method of CO2 sparging and may have great applicability in the future. However, the optimal bicarbonate concentrations for high biomass productivity and carbon utilization efficiency (CUE) are usually different, and a method to simultaneously achieve both of them is lacking. In this study, the CUE and biomass productivity of an isolated alkalihalophilic Trebouxiophyte in bicarbonate-based cultivation condition was investigated. The results showed that CUE decreased as the bicarbonate concentration increased, where the highest CUE of 96.7 ± 0.25% was achieved in a culture with 100 mmol L−1 bicarbonate, while the highest biomass density of 1.40 ± 0.01 g L−1 was produced in a 300 mmol L−1 bicarbonate culture. The bicarbonate concentration of 300 mmol L−1 was selected for a balance between CUE and biomass productivity. Under this concentration, the Trebouxiophyte cultured at high light intensity had higher CUE (46.01 ± 0.65%) and biomass productivity (0.80 g L−1 d−1) than those with 2% CO2. The outdoor cultivation in the floating photobioreactor produced a higher biomass concentration (0.603 g L−1), highest areal biomass productivity (10.1 g m−2 d−1), and CUE (22.61 ± 0.03%) than those in an open pond, which was 0.377 ± 0.01 g L−1, 5.36 g m−2 d−1 and 11.65 ± 0.01%. These results indicated that Trebouxiophyte can be efficiently produced with bicarbonate as the sole carbon source, and provided a good example to select a balanced bicarbonate concentration to simultaneously achieve high biomass productivity and CUE.