Elucidating the mechanisms associated with the anaerobic biotransformation of the emerging contaminant nitroguanidine

•An enrichment culture biotransformed nitroguanidine (NQ) to nitrosoguanidine (NsoQ).•Lactate was the preferred electron donor for NQ anaerobic biotransformation.•Sulfate reduction competed with NQ reduction as an electron acceptor.•Cupidesulfovibrio oxamicus was the predominant microorganism in the culture.•NsoQ was abiotically transformed to cyanamide.