Microbial abundance and activity of nitrite/nitrate-dependent anaerobic methane oxidizers in estuarine and intertidal wetlands: Heterogeneity and driving factors

Nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) is a crucial link between carbon and nitrogen cycles in estuarine and coastal ecosystems. However, the factors that affect the heterogeneous variability in n-DAMO microbial abundance and activity across estuarine and intertidal wetlands remain unclear. This study examined the spatiotemporal variations in n-DAMO microbial abundance and associated activity in different estuarine and intertidal habitats via quantitative PCR and ¹³C stable isotope experiments. The results showed that Candidatus 'Methylomirabilis oxyfera' (M. oxyfera)-like DAMO bacteria and Candidatus 'Methanoperedens nitroreducens' (M. nitroreducens)-like DAMO archaea cooccurred in estuarine and intertidal wetlands, with a relatively higher abundance of the M. oxyfera-like bacterial pmoA gene (4.0×10⁶-7.6×10⁷ copies g^-1 dry sediment) than the M. nitroreducens-like archaeal mcrA gene (4.5×10⁵-9.4×10⁷ copies g^-1 dry sediment). The abundance of the M. oxyfera-like bacterial pmoA gene was closely associated with sediment pH and ammonium (P<0.05), while no significant relationship was detected between M. nitroreducens-like archaeal mcrA gene abundance and the measured environmental parameters (P>0.05). High n-DAMO microbial activity was observed, which varied between 0.2 and 84.3 nmol ¹³CO₂ g^-1 dry sediment day^-1 for nitrite-DAMO bacteria and between 0.4 and 32.6 nmol ¹³CO₂ g^-1 dry sediment day^-1 for nitrate-DAMO archaea. The total n-DAMO potential tended to be higher in the warm season and in the upstream freshwater and low-salinity estuarine habitats and was significantly related to sediment pH, total organic carbon, Fe(II), and Fe(III) contents (P<0.05). In addition to acting as an important methane (CH₄) sink, n-DAMO microbes had the potential to consume a substantial amount of reactive N in estuarine and intertidal environments, with estimated nitrogen elimination rates of 0.5-224.7 nmol N g^-1 dry sediment day^-1. Overall, our investigation reveals the distribution pattern and controlling factors of n-DAMO bioprocesses in estuarine and intertidal marshes and gains a better understanding of the coupling mechanisms between carbon and nitrogen cycles.