Denitrifying anaerobic methane oxidation in intertidal marsh soils: Occurrence and environmental significance

Denitrifying anaerobic methane oxidation (DAMO), consisting of nitrite-dependent DAMO (nitrite-DAMO) and nitrate-dependent DAMO (nitrate-DAMO), has recently been discovered and considered an important link between carbon and nitrogen cycles. In this work, we investigated biodiversity, abundance, and potential methane (CH₄) oxidation activity of nitrite-DAMO bacteria and nitrate-DAMO archaea in intertidal marsh soil cores, based on molecular and stable isotope tracing methods. Results evidenced the co-occurrence and vertical stratification of DAMO bacteria and archaea in intertidal marsh soils, with higher biodiversity of DAMO archaea compared with DAMO bacteria. The abundance of DAMO bacterial pmoA gene (8.2 × 10⁵–3.0 × 10⁷ copies g⁻¹ dry soil) was approximate to that of DAMO archaeal mcrA gene (3.0 × 10⁵–3.9 × 10⁷ copies g⁻¹ dry soil) in the intertidal marsh soil cores. Stable isotope experiments showed that DAMO bacteria and archaea were both active in the intertidal marshes, with CH₄ oxidation potential of 0.1–3.8 nmol ¹³CO₂ g⁻¹ dry soil day⁻¹ and 0.1–4.1 nmol ¹³CO₂ g⁻¹ dry soil day⁻¹, respectively. The relative importance (including the abundance and CH₄ oxidation activity) of DAMO bacteria and archaea showed significant vertical variations, with more contribution by DAMO bacteria at the soil-tidal water interface and soil-groundwater interface layers of the soil cores, which are largely due to the influence of tidal dynamics. In addition to playing an important role as CH₄ sink, DAMO process was also a non-negligible pathway of nitrogen removal in intertidal marsh soils, with an estimated nitrogen removal rate of 0.4–10.1 nmol N g⁻¹ dry soil day⁻¹. Overall, these results illustrated the occurrence and environmental significance of DAMO bioprocess in intertidal marshes.