Abstract
Ammonium is utilized not only as a nitrogen source but also as an energy source for living organisms. Aerobic ammonium-oxidizing microorganisms have been discovered, and their systematic, ecological, and genetic/ biochemical studies are progressing. On the other hand, although the existence of anaerobic ammonium-oxidizing microorganisms has long been predicted based on thermodynamic considerations, our knowledge about their diversity remains limited. This study aimed to explore bacteria that grow anaerobically using ammonium as an electron source from terrestrial hot springs and to clarify their genetic and physiological characteristics. Ammonium-dependent anaerobic growth was identified in sulfate-reducing bacteria (Thermodesulfomicrobium sp.) and anoxygenic photosynthetic bacteria (Chloroflexus sp.). Physiological analyses suggested that ammonium was utilized not only as a nitrogen source but also as an electron source in these isolates; however, the oxidation products of ammonium could not be identified. Genome analysis revealed no known genes related to ammonium oxidation, suggesting the presence of novel enzymes and reactions. Transcriptional analysis of the Chloroflexus isolate suggested that previously unidentified oxidoreductases were active under ammonium-dependent growth conditions.
