Anaerobic methane oxidation in a coastal oxygen minimum zone: spatial and temporal dynamics

Area of Science:

• Marine microbial ecology
• Biogeochemistry
• Greenhouse gas mitigation

Background:

• Coastal waters are significant sources of atmospheric methane, a potent greenhouse gas.
• High methane concentrations are found in anoxic waters, but the role of anaerobic methanotrophs in mitigating these fluxes is not fully understood.

Purpose of the Study:

• Investigate long-term dynamics of methanotrophic activity and community structure.
• Assess the role of anaerobic methane oxidation in the Golfo Dulce coastal oxygen minimum zone (OMZ).

Main Methods:

• Combined biogeochemical analyses, experimental incubations, and 16S rRNA gene sequencing over three years.
• Analyzed redox zonation, methane concentrations, and anaerobic methane oxidation rates.
• Examined relative abundance of methanotroph clades.

Main Results:

• Stable redox zonation and high methane concentrations (up to 1.7 μmol L⁻¹) were observed in anoxic bottom waters.
• High anaerobic methane oxidation activity was measured in the OMZ core (rate constant averaging 30 yr⁻¹ in 2018 and 8 yr⁻¹ in 2019-2020).
• OPU3 and Deep Sea-1 clades of Methylococcales were abundant, particularly below the oxic-anoxic interface and in deep anoxic waters.

Conclusions:

• Anaerobic methanotrophs, specifically OPU3 and Deep Sea-1 clades, play a significant role in mitigating methane fluxes in coastal OMZs.
• These microbial clades show adaptation to the anoxic, methane-rich OMZ environment, despite their anaerobic methane oxidation capacity being unexplored.