Methane production by microphytobenthos and its contribution to the benthic methane flux from the coastal zone of the Baltic Sea

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Increasing natural emissions of the greenhouse gas methane (CH4) substantially impact the Earth climate. In this context, near-shore waters play a key role because water column CH4 concentrations in these areas are much higher when compared with the open waters. However, little is known about CH4 emitters in shallow coastal zones or their contribution to the atmospheric CH4 flux. Moreover, a series of recent studies indicate that microbial CH4 production is not limited to strictly anoxic conditions and might be widespread in the oxic water column. This oxic CH4 production places the CH4 source close to the water surface, thereby intensifying fluxes to the atmosphere. Based on these recent investigations and a proposed linkage between primary production and CH4 formation, we hypothesize that microphytobenthos (MPB) communities, as major primary producer in many coastal systems, play a significant role in shallow-water CH4 dynamics. To explore MPB-associated CH4 production, we will investigate the potential of this CH4 source, by conducting incubation experiments and determining the rates of both primary production and CH4 production by abundant benthic diatom species. The use of a novel cavity ring-down spectroscopy-based semi-continuous method in closed incubation experiments will enable sensitive measurements of CH4 production in high temporal resolution over day/night cycles. The experiments will be undertaken under controlled temperature and light conditions and will use axenic/xenic clonal cultures of selected MPB diatoms to identify the main effectors and those species with the highest CH4 production rates. Experiments using 13C-labeled precursor substrates will be performed to identify and characterize MPB-associated CH4 production pathways. Previous studies of oxic CH4 production were mostly mechanistic laboratory studies that used culture samples; whether the measured CH4 production rates were representative of those in natural systems was not determined. To overcome these limitations in our study, the incubation experiments with clonal cultures will be accompanied by natural MPB communities sampled from the shallow waters at Askö island (Swedish Baltic Sea) and from the inner coastal waters of the Darss-Zingst Bodden Chain (German Baltic Sea), thus covering different sediment types, hydrodynamic conditions, and MPB communities. To align benthic CH4 fluxes from MPB communities with overall benthic and sea-to-air fluxes of CH4, we will determine CH4 fluxes between the sediment, water column, and atmosphere in both study areas.