Sulfate amendment improves the growth and bioremediation capacity of a cyanobacteria cultured on municipal wastewater centrate

A series of experiments were performed to evaluate the growth of the model cyanobacterium Synechocystis sp. PCC6803 cultured on wastewater centrate sourced from a municipal water reclamation facility. Centrate is the liquid removed from sludge during sewage processing. The addition of 304 μM Na2SO4 to centrate diluted by 90% in water yielded final cell concentrations that were over 6 times higher than cultures grown on just centrate. This suggested that S is the primary limiting nutrient for photoautorophic growth on centrate. The expression of the sulfate transporter system encoded by the spbA-cysTWA operon was upregulated when cells were grown in non-amended centrate, confirming that Synechocystis sp. PCC6803 experienced S-limitation during growth on centrate alone. Elemental analysis of centrate further confirmed that the proportion of S relative to other macronutrients is lower in centrate compared to the measured elemental ratios found in cyanobacterial biomass. The cyanobacteria removed 69% of the total soluble nitrogen (TN) in S-amended centrate compared to 25% from cultures grown in centrate alone. It is proposed that S could be recycled from the production of H2S during anaerobic digestion of wastewater, or it could be added from exogenous material such as gypsum. Overall, this work suggests that S-amendment of centrate could improve the sustainability of wastewater remediation and biomass production using photosynthetic microbes.