Aerobic and anaerobic methane oxidation in a seasonally anoxic basin.

Shallow coastal waters are dynamic environments that dominate global marine methane emissions. Particularly high methane concentrations are found in seasonally anoxic waters, which are spreading in eutrophic coastal systems, potentially leading to increased methane emissions to the atmosphere. Here we explore how the seasonal development of anoxia influenced methane concentrations, rates of methane oxidation, and the community composition of methanotrophs in the shallow eutrophic water column of Mariager Fjord, Denmark. Our results show the development of steep concentration gradients toward the oxic-anoxic interface as methane accumulated to 1.4 µM in anoxic bottom waters. Yet, the fjord possessed an efficient microbial methane filter near the oxic-anoxic interface that responded to the increasing methane flux. In experimental incubations, methane oxidation near the oxic-anoxic interface proceeded both aerobically and anaerobically with nearly equal efficiency reaching turnover rates as high as 0.6 and 0.8 d-1, respectively, and was seemingly mediated by members of the Methylococcales belonging to the Deep Sea-1 clade. Throughout the period, both aerobic and anaerobic methane oxidation rates were high enough to consume the estimated methane flux. Thus, our results indicate that seasonal anoxia did not increase methane emissions.

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

Coastal waters are a major source of marine methane to the atmosphere. Particularly high concentrations of this potent greenhouse gas are found in anoxic waters, but it remains unclear if and to what extent anaerobic methanotrophs mitigate the methane flux. Here we investigate the long-term dynamics in methanotrophic activity and the methanotroph community in the coastal oxygen minimum zone (OMZ) of Golfo Dulce, Costa Rica, combining biogeochemical analyses, experimental incubations and 16S rRNA gene sequencing over 3 consecutive years. Our results demonstrate a stable redox zonation across the years with high concentrations of methane (up to 1.7 µmol L-1 ) in anoxic bottom waters. However, we also measured high activities of anaerobic methane oxidation in the OMZ core (rate constant, k, averaging 30 yr-1 in 2018 and 8 yr-1 in 2019-2020). The OPU3 and Deep Sea-1 clades of the Methylococcales were implicated as conveyors of the activity, peaking in relative abundance 5-25 m below the oxic-anoxic interface and in the deep anoxic water respectively. Although their genetic capacity for anaerobic methane oxidation remains unexplored, their sustained high relative abundance indicates an adaptation of these clades to the anoxic, methane-rich OMZ environment, allowing them to play major roles in mitigating methane fluxes.