Mixotrophic Cultivation of an Algae-Bacteria Consortium in Aluminium Smelter Wastewaters (Quebec,Canada): High Nitrogen Concentration Increases Overall Lipid Production

Abstract

To produce energy for in-house use, an aluminium smelter in Québec launched a study of mixotrophic cultivation of microalgae in its wastewaters with the objective of having an algae production company set up operations on site. To maximize lipid productivity and maintain the biological integrity of the consortium, specific nutrients need to be added to aluminium smelter wastewaters to cultivate the selected algae-bacteria consortium. A 2³ factorial design was used to determine the organic carbon, nitrogen and phosphate inputs needed. Data on biomass and lipid productivity, as well as a “consortium integrity index,” were analyzed using a multiple linear regression model. The highest biomass productivity (0.93 g/L/d) and lipid productivity (0.023 g/L/d) were obtained using the highest tested concentration in nitrogen (0.200 g/L) and the lowest tested concentration in phosphate (0.003 g/L). No significant effect of the organic carbon—tested in concentrations of 1.64 g/L, 2.64 g/L, and 3.64 g/L (glucose) and added in two increments on days 5 and 7—on productivity for a starting cell density of 5 million cells/L was detected. To achieve maximal lipid production, the results suggested that biomass productivity should be prioritized rather than lipid accumulation in the cells through nitrogen starvation. The stability and integrity of the cultured consortium have to be maintained through an appropriate balance of nutrients. A low phosphate concentration increases the stability of the consortium, although part of the variation cannot be explained by the model. Finally, an analysis of fatty acid profiles showed that different concentrations in nitrogen and phosphorus impact the proportion of C18:1(n-9) and other minor fatty acids.