Enhancing cultivation of biodiesel-promising microalgae Chlorella pyrenoidosa using plant hormones in municipal wastewater

This study was aimed to screen a plant hormone that can increase the microalgae growth and lipid accumulation when cultured in municipal wastewater. The effects of four plant hormones on the biomass and lipid yield of the microalgae Chlorella pyrenoidosa cultured in sewage were studied. It was found that 2,4-dichlorophenoxyacetic acid (2,4-D) with 1.00 mg/L had the optimum effect. The microalgae biomass and lipid content with 2,4-D treatment were 0.86 and 0.46 g/L, which were enhanced by 89.7 and 76.5% compared with the control, respectively. In the outdoor scale-up test, the biomass and lipid in the treatment group were 0.75 and 0.38 g/L, respectively. The increase of total lipid was due to the increase of microalgae biomass because the lipid content in the microalgae cell did not change much. The fatty acids composition analysis for the microalgae lipid showed that the main components of the lipid were 16-corbon fatty acids and 18-corbon fatty acids, which were suitable for biodiesel production. Meanwhile, the nitrogen and phosphorus indexes of effluent decreased significantly, including 87.3% removal of total nitrogen, 94.1% removal of ammonia nitrogen, and 90.0% removal of total phosphorus. The results of relative membrane permeability showed that plant hormone 2,4-D changed the cell permeability of microalgae. The relative membrane permeability of treatment group was 69.5%, 56% higher than that in the control group, which promoted the propagation of microalgae. The results showed that the addition of plant hormone improved the microalgae growth, while degraded nitrogen and phosphorus pollutants in sewage, which provided ideas for the industrial production of microalgae biodiesel and municipal wastewater reuse. Graphical abstract