Uncovering nitrogen removal in algal-bacterial processes for domestic wastewater treatment

The synergy between microalgae and bacteria in photobioreactors is complex, with the competition between assimilation and nitrification critically influencing nutrient removal efficiency. This study investigated the interplay between nutrient and carbon removal pathways in a single-stage microalgal-bacterial photobioreactor treating domestic wastewater by selectively inhibiting nitrification with allylthiourea (ATU). The inhibition successfully suppressed bacterial activity, reducing the NH 4 + removal rate by 20% and increasing the contribution of microalgal assimilation to total nitrogen removal from 59% to 68%. Moreover, ATU addition induced a change in microbial population structure, as suggested by the decline in relative abundance of Proteobacteria and Bacteroidota phyla, which ultimately mediated a shift in the share of carbon fate mechanisms. The total organic carbon removal efficiency decreased from 94 ± 1% to 88 ± 1%, while the carbon assimilation efficiency into biomass was increased from 65% to 80%. Thus, mixotrophic microalgae like Scenedesmus sp. became the dominant genus, alongside the cyanobacterium Nodosilinea . Phosphate removal remained consistently high (97–98%) and unaffected by ATU addition, indicating its decoupling from nitrification. The results demonstrated that nitrogen removal was mainly dominated by microalgae assimilation complemented by bacterial pathway, consistent with nitrification-denitrification to achieve complete nitrogen removal. • Nitrification suppression rerouted 8% of total nitrogen to microalgal assimilation. • Scenedesmus and Nodosilinea dominated the consortium after ATU addition. • ATU addition reduced Proteobacteria and Bacteroidota, changing microbial functions. • Scenedesmus drives efficient carbon and nitrogen removal post-inhibition.