A loofah-based microalgae-bacteria symbiosis drives efficient nutrient removal from low-C/N wastewater: high biomass...

This study developed a biodegradable loofah-immobilized microalgae-bacteria (LMB) system to enhance nutrient removal from municipal wastewater through coupled algal assimilation, bacterial transformation, and biomass retention. Operated under a 12 h:12 h light-dark cycle without aeration or external carbon addition, the LMB system achieved high removal efficiencies of 97.71 ± 1.83% for NH₄⁺-N and 93.84 ± 5.32% for PO₄^3--P, with corresponding removal rates of 27.65 ± 0.15 and 2.65 ± 0.32 g m^-3·d^-1, respectively. The total inorganic nitrogen removal efficiency reached 81.84 ± 7.38%, indicating efficient deep nitrogen removal under carbon-limited conditions. Mechanistic investigations showed that the loofah carrier continuously released bioavailable organic carbon, including proteins, polysaccharides, humus and polycyclic aromatic hydrocarbons, which supported microalgal growth and promoted the coupling of photoautotrophic nutrient assimilation with heterotrophic denitrification. Phase-specific nitrogen transformation analysis further revealed that microalgal assimilation dominated nitrogen removal during the light phase, whereas heterotrophic denitrification driven by loofah-derived organic carbon, became the major pathway for deep nitrogen removal in the dark phase. Microbial community analysis indicated that functional bacteria, including SM1A02 (4.53%), Exiguobacterium (6.95%) and Clostridium (36.98%), were enriched together with the dominant microalgal genus Tetradesmus (42.75%), forming a cooperative pollutant-degrading consortium. These results suggest that the LMB system offers a low-energy and carbon-efficient strategy for nutrient removal from wastewater while providing potential for biomass valorization.