Vertical distribution of nitrite-dependent anaerobic methane-oxidising bacteria in natural freshwater wetland soils.

Nitrite-dependent anaerobic methane oxidation (n-damo) is a recently discovered process that is catalysed by "Candidatus Methylomirabilis oxyfera". In the present study, the vertical distribution (0-10, 20-30, 50-60 and 90-100 cm) of M. oxyfera-like bacteria was investigated in Xiazhuhu wetland, the largest natural wetland on the southern Yangtze River (China). Phylogenetic analyses showed that group A of M. oxyfera-like bacteria and pmoA genes occurred primarily at depths of 50-60 and 90-100 cm. Quantitative PCR further confirmed the presence of M. oxyfera-like bacteria in soil cores from different depths, with the highest abundance of 5.1 × 10(7) copies g(-1) dry soil at depth of 50-60 cm. Stable isotope experiments demonstrated that the n-damo process occurred primarily at depths of 50-60 and 90-100 cm, with the potential rates ranging from 0.2 to 14.5 nmol CO2 g(-1) dry soil d(-1). It was estimated that the methane flux may increase by approximately 2.7-4.3% in the examined wetland in the absence of n-damo. This study shows that the deep wetland soils (50-60 and 90-100 cm) are the preferred habitats for M. oxyfera-like bacteria. The study also highlights the potential importance of these bacteria in the methane and nitrogen cycles in deep wetland soils.