Algal-bacterial synergy in marimo granules enables low-carbon advanced nitrogen removal from wastewater

Marimo, a natural microalgal-bacterial consortium, represents a low-carbon solution for nitrate pollution control in low-nutrient waters. However, the adaptation of its nitrogen-cycling network to fluctuating nitrate levels remains poorly understood. This study reveals that elevated nitrate drives water matrix-dependent metabolic shifts, enhancing denitrification and assimilatory nitrate reduction performance in oligotrophic lake water, while strengthening dissimilatory nitrate reduction to ammonium in carbon-richer wastewater treatment plant effluent. Metagenomic analysis shows that this reprogramming is coordinated by a functionally specialized core consortium, with Rhodobacteraceae as the keystone driver of nitrate reduction. Meanwhile, correlation analysis further demonstrates that nitrate functions as an environmental filter, enriching this consortium to sustain high nitrate removal efficiency, endogenous nitrogen recycling, and symbiotic stability. Overall, these findings offer insights into the microbial mechanisms underlying the stable nitrate removal performance of marimo and can guide the design of low-carbon marimo-based treatment systems under fluctuating nitrate conditions.