A microbial consortium couples anaerobic methane oxidation to denitrification. Researchers identified a microbial consortium that can oxidize methane anaerobically while simultaneously denitrifying nitrate. This consortium, enriched from anoxic sediments, oxidized methane to carbon dioxide coupled to denitrification in the absence of oxygen. The consortium consisted of two microorganisms: a bacterium from an uncultured phylum and an archaeon related to marine methanotrophic archaea. The presence of these microorganisms in various freshwater ecosystems suggests that this reaction may significantly contribute to biogeochemical methane and nitrogen cycles. Global biogeochemical cycles are driven by microorganisms that use one-carbon compounds like methane or carbon dioxide. These microorganisms form ecological guilds, sharing similar lifestyles. Thermodynamic calculations show that most of these guilds have been discovered, but the microorganisms that couple anaerobic methane oxidation (AOM) to denitrification are considered missing in nature. However, AOM coupled to denitrification is possible both thermodynamically and biochemically through reverse methanogenesis. The study found that AOM coupled to denitrification can occur in anoxic sediments, where the process is likely to occur near the oxic/anoxic interface, making it difficult to detect. The researchers successfully enriched a consortium of microorganisms capable of AOM coupled to denitrification using anoxic sediment from the Twentekanaal. Methane was supplied as the only electron donor, and nitrate, nitrite, bicarbonate, and trace elements were present in the medium. Over 16 months, the consortium consumed nitrite and nitrate, and methane consumption was observed after stopping the media and methane supply. The study also showed that methane can be oxidized anaerobically in this system, coupled to denitrification. The participation of oxygen was excluded through various experiments. The consortium was found to consist of an archaeon and a bacterium, with the archaeon being related to marine methanotrophic archaea. The bacterial and archaeal 16S rRNA gene sequences were analyzed, and the archaeal sequence was found to be related to sequences from Lake Biwa and contaminated groundwater. The bacterial sequence was related to sequences from Lake Michigan and contaminated soils in Japan. The study highlights the importance of this microbial consortium in biogeochemical cycles and suggests that this process may contribute significantly to global methane and nitrogen cycles. The findings provide new insights into the microbial guilds involved in biogeochemical cycles and the potential for this process to counteract worldwide increases in methane production associated with intensive agriculture.A microbial consortium couples anaerobic methane oxidation to denitrification. Researchers identified a microbial consortium that can oxidize methane anaerobically while simultaneously denitrifying nitrate. This consortium, enriched from anoxic sediments, oxidized methane to carbon dioxide coupled to denitrification in the absence of oxygen. The consortium consisted of two microorganisms: a bacterium from an uncultured phylum and an archaeon related to marine methanotrophic archaea. The presence of these microorganisms in various freshwater ecosystems suggests that this reaction may significantly contribute to biogeochemical methane and nitrogen cycles. Global biogeochemical cycles are driven by microorganisms that use one-carbon compounds like methane or carbon dioxide. These microorganisms form ecological guilds, sharing similar lifestyles. Thermodynamic calculations show that most of these guilds have been discovered, but the microorganisms that couple anaerobic methane oxidation (AOM) to denitrification are considered missing in nature. However, AOM coupled to denitrification is possible both thermodynamically and biochemically through reverse methanogenesis. The study found that AOM coupled to denitrification can occur in anoxic sediments, where the process is likely to occur near the oxic/anoxic interface, making it difficult to detect. The researchers successfully enriched a consortium of microorganisms capable of AOM coupled to denitrification using anoxic sediment from the Twentekanaal. Methane was supplied as the only electron donor, and nitrate, nitrite, bicarbonate, and trace elements were present in the medium. Over 16 months, the consortium consumed nitrite and nitrate, and methane consumption was observed after stopping the media and methane supply. The study also showed that methane can be oxidized anaerobically in this system, coupled to denitrification. The participation of oxygen was excluded through various experiments. The consortium was found to consist of an archaeon and a bacterium, with the archaeon being related to marine methanotrophic archaea. The bacterial and archaeal 16S rRNA gene sequences were analyzed, and the archaeal sequence was found to be related to sequences from Lake Biwa and contaminated groundwater. The bacterial sequence was related to sequences from Lake Michigan and contaminated soils in Japan. The study highlights the importance of this microbial consortium in biogeochemical cycles and suggests that this process may contribute significantly to global methane and nitrogen cycles. The findings provide new insights into the microbial guilds involved in biogeochemical cycles and the potential for this process to counteract worldwide increases in methane production associated with intensive agriculture.