Zero-valent iron boosts nitrate-to-ammonia bioconversion via extracellular electron donation and reduction pathway complementation

•Bio-hybrid of S. oneidensis and ZVI was developed for nitrate-to-ammonia conversion.•ZVI directly donate electrons via an unknown outer membrane pathway to the cells.•ZVI efficiently reduces nitrite to alleviate the intermediate accumulation.•Bio-hybrid exhibited one-order-of-magnitude higher activity than individual systems.•Bio-hybrid achieved nearly 100% NRA selectivity and good stability for cyclic operation.