Seasonal dynamics of the microbial methane filter in the water column of a eutrophic coastal basin

Abstract

In coastal waters, methane-oxidizing bacteria (MOB) can form a methane biofilter and mitigate methane emissions. The metabolism of these MOB is versatile, and the resilience to changing oxygen concentrations is potentially high. It is still unclear how seasonal changes in oxygen availability and water column chemistry affect the functioning of the methane biofilter, and MOB community composition. Here, we determined water column methane and oxygen depth profiles, the methanotrophic community structure, methane oxidation potential, and water-air methane fluxes of a eutrophic marine basin during summer stratification and in the mixed water in spring and autumn. In spring, the MOB diversity and relative abundance were low. Yet, MOB formed a methane biofilter with up to 9% relative abundance and vertical niche partitioning during summer stratification. The vertical distribution and potential methane oxidation of MOB did not follow the upward shift of the oxycline during summer, and water-air fluxes remained below 0.6 mmol m−2 day−1. Together, this suggests active methane removal by MOB in the anoxic water. Surprisingly, with a weaker stratification, and therefore potentially increased oxygen supply, methane oxidation rates decreased and water-air methane fluxes increased. Thus, despite the potential resilience of the MOB community, seasonal water column dynamics significantly influence methane removal.

Eutrophication and Deoxygenation Drive High Methane Emissions from a Brackish Coastal System

“…We hypothesize that the efficiency of aerobic methane removal was limited because of strong wind-induced mixing of the surface waters. This mixing would not only enhance the upward transport of methane but also hinder the establishment of a stable methanotrophic community . Apparently, such a narrow and turbulent oxic layer of the water column does not provide suitable conditions for efficient aerobic methane oxidation.…”

“…This mixing would not only enhance the upward transport of methane but also hinder the establishment of a stable methanotrophic community. 28 Apparently, such a narrow and turbulent oxic layer of the water column does not provide suitable conditions for efficient aerobic methane oxidation. Assuming that the benthic flux of 2.2 mmol m –2 d –1 captures the total release of methane from the sediment, the efficiency of methane removal at Skurusundet is only 23% with 1.7 mmol m –2 d –1 of methane (77%) escaping to the atmosphere.…”

“…The measurements were performed in triplicate, with each measurement lasting 3–10 min to capture a linear increase in methane concentrations. A detailed description of the floating chamber method and its performance can be found in ref ( 28 ).…”

“…In eutrophic systems, this lack of mixing can lead to oxygen depletion in the bottom water. Changing redox conditions in the water column can influence the microbial filter in various ways, for example, by changing the microbial community structure and its efficiency in removing methane . At present, we still lack sufficient knowledge to predict how methane emissions from coastal systems respond to environmental changes.…”