Microbial transformation of a seaweed polysaccharide preparation enhances denitrification potential in low-carbon municipal tailwater.

Low-carbon secondary effluent presents significant challenges for effective nitrogen removal due to limited available organic carbon. In this study, a laboratory-scale microbial system integrating a polysaccharide-based external carbon source (MP) with the nitrate-assimilating bacterium Enterobacter hormaechei EN-1 was developed to enhance aerobic nitrogen removal. The tested co-culture model increased total nitrogen removal efficiency from 63.46% to 85.86%. In batch tailwater microcosms, MP supplementation combined with EN-1 inoculation improved nitrate transformation and nitrogen removal. In the 7-day semi-continuous shake-flask microcosms, treatment-associated changes in microbial community composition and the abundance of the denitrification-related genes nirS and nosZ were observed. Biomass-associated nitrogen measurements indicated that growth-related assimilation was an important measured route of nitrate removal by EN-1. Time-course HPLC analysis showed dynamic changes in soluble monosaccharides and organic acids during EN-1 cultivation. Cell-free medium conditioned by EN-1 during cultivation with the dialyzed high-molecular-weight MP fraction enhanced nitrate transformation by DE6 relative to the matched abiotic control. An HPLC-informed organic-acid mixture also altered nirS and nosZ transcription in DE6 under the tested conditions. These results support a potential soluble-product-mediated facilitation, but do not establish direct metabolite-specific carbon transfer from EN-1 to DE6. Overall, EN-1 associated transformation of the tested MP preparation was linked to improved short-term nitrogen transformation in laboratory tailwater microcosms and to denitrification-associated community and functional-marker responses. These findings provide a laboratory-scale proof of concept and a basis for future validation in long-term continuous-flow systems.