ENVIRONMENTAL CONTROLS ON AEROBIC METHANE OXIDATION IN COASTAL WATERS

Abstract:

Large quantities of the greenhouse gas CH₄ are produced in anoxic sediments of continental margins and may be liberated to the overlying water column, and later into the atmosphere. Indeed, coastal seas account for more than 75% of global oceanic CH₄ emissions. Yet, aerobic CH₄ oxidizing bacteria (MOB) consume an important part of CH₄ in the water column, thus mitigating CH₄ release to the atmosphere. Coastal oceans are highly dynamic systems, in particular with regard to the variability of temperature, salinity and oxygen concentrations, all of which are potential key environmental factors controlling MOx. To determine the most important controlling factors, we conducted a two-year time-series study with measurements of CH₄, MOx, the composition of the MOB community, and physicochemical water column parameters in a coastal inlet in the Baltic Sea (Eckernförde(E-) Bay, Boknis Eck Time Series Station). In addition, we investigated the influence of temperature and oxygen on MOx during controlled laboratory experiments. In E-Bay, seasonal stratification leads to hypoxia in bottom waters towards the end of the stratification period. Methane is produced year-round in the sediments, resulting in accumulation of methane in bottom waters, and supersaturation (with respect to the atmospheric equilibrium) in surface waters. Here, we will discuss the factors impacting MOx the most, which were a) perturbations of the water column caused by storm events, currents or seasonal mixing, b) temperature and c) oxygen concentration. a) Perturbations of the water column led to a sharp decrease in MOx within hours, probably caused by replacement of ‘old’ water with a high standing stock of MOB by ‘new’ waters with a lower abundance of MOB. b) An increase in temperature generally led to higher MOx rates. c) Even though CH₄ was abundant at all depths, MOx was highest in bottom waters (1-5 nM/d), which usually contain the lowest O₂ concentrations. Lab-based experiments with adjusted O₂ concentrations in the range of 0.2 - 220 µM confirmed a sub-micromolar O₂ optimum. Additionally, the metabolic fate of CH₄-carbon at low (<0.5 µM) and high (~200 µM) O₂ concentrations was investigated during incubations with ¹⁴C-CH₄. The results suggest a differential partitioning of catabolic and anabolic processes at different O₂ concentrations.