The thermoacidophilic red alga Galdieria sulphuraria is a highly efficient cell factory for ammonium recovery from ultrahigh-NH4+ industrial effluent with co-production of high-protein biomass by photo-fermentation

Abstract

The thermoacidophilic red alga Galdieria sulphuraria is well-suited for nutrient recovery and pathogenic bacteria reduction from urban wastewater, but few investigations were reported for high-NH₄⁺ industrial effluent treatment. The present study evaluated the capacity of NH₄⁺ recovery, bacteria reduction, and high-protein biomass co-production by photo-fermentation of G. sulphuraria. As inoculum, acclimated and non-acclimated seed cultures were applied for non-sterile and sterile cultures in ultrahigh-NH₄⁺ (3,000 ∼ 6,000 mg L^-1) wastewater medium prepared from industrial effluent. The acclimated inoculum showed a higher cell growth rate (0.48 d^-1) and NH₄⁺ recovery rate (0.34 g L^-1 d^-1) than the non-acclimated inoculum in shake flasks. The relative abundance of dominant bacteria (Proteobacteria, Actinobacteria, and Acidobacteria) in the associated microbial community declined sharply in non-sterile cultures due to the extremely low pH value. In repeated fed-batch cultures in 5-L photo-fermenters, the maximum NH₄⁺ recovery rate (2.19 g L^-1 d^-1), biomass production (55.0 g L^-1) and productivity (12.0 g L^-1 d^-1) were achieved with high contents of protein (47.6 %DW) and essential amino acids (18.9 %DW) in sterile culture. Attractively, the maximum NH₄⁺ recovery rate (1.79 g L^-1 d^-1), biomass production (49.5 g L^-1), and productivity (10.8 g L^-1 d^-1) were achieved with the highest protein production (25.3 g L^-1) in non-sterile cultures. This study demonstrates the feasibility of G. sulphuraria as a high-efficient cell factory for recovering NH₄⁺ by photo-fermentation. We developed a novel approach of non-sterile repeated fed-batch culture for cost-effective wastewater treatment and co-production of high-protein biomass, which facilitates algae-based “waste-to-treasure” bioconversion for green manufacturing.