Abstract
The conventional perception that the zone of sulfate reduction and methanogenesis are separated in high- and low-sulfate-containing marine sediments has recently been changed by studies demonstrating their co-occurrence in sediments. The presence of methanogens was linked to the presence of substrates that are not used by sulfate reducers. In the current study, we hypothesized that both groups can co-exist, consuming common substrates (H2 and/or acetate) in sediments. We enriched butyrate-degrading communities in sediment slurries originating from the sulfate, sulfate–methane transition, and methane zone of Aarhus Bay, Denmark. Sulfate was added at different concentrations (0, 3, 20 mM), and the slurries were incubated at 10 °C and 25 °C. During butyrate conversion, sulfate reduction and methanogenesis occurred simultaneously. The syntrophic butyrate degrader Syntrophomonas was enriched both in sulfate-amended and in sulfate-free slurries, indicating the occurrence of syntrophic conversions at both conditions. Archaeal community analysis revealed a dominance of Methanomicrobiaceae. The acetoclastic Methanosaetaceae reached high relative abundance in the absence of sulfate, while presence of acetoclastic Methanosarcinaceae was independent of the sulfate concentration, temperature, and the initial zone of the sediment. This study shows that there is no vertical separation of sulfate reducers, syntrophs, and methanogens in the sediment and that they all participate in the conversion of butyrate.
Highlights
Coastal marine ecosystems receive a regular input of organic matter from primary production of plankton, macroalgae, and vascular plants and the influx of rivers, and they are important in the remineralization of organic matter [1,2]
The sediment core was divided into three pieces representing the sulfate zone (SZ) (15–120 cm), the sulfate–methane transition zone (SMTZ) (120–170 cm), and the methane zone (MZ) (170–300 cm) (Figure S1)
This study demonstrates that methanogenic archaea and sulfate-reducing bacteria are present and active in sediments of the sulfate zone, the sulfate–methane transition zone, and methane zone of Aarhus Bay and that there is no vertical separation of both groups in the sediment profile
Summary
Coastal marine ecosystems receive a regular input of organic matter from primary production of plankton, macroalgae, and vascular plants and the influx of rivers, and they are important in the remineralization of organic matter [1,2]. High microbial activity in the sediment layers leads to the formation of distinct biogeochemical zones. The depth range of each zone varies strongly depending on chemical changes in the sediment pore water, the rates of sediment accumulation, and replenishment of electron acceptors from overlying seawater [3]. The thickness of the oxic surface layer can be just a few millimeters [2]. In the anoxic part of the sediment, nitrate, manganese, iron, sulfate, and carbon dioxide, in an order of decreasing energy gain, serve as terminal electron acceptors for the mineralization processes [4,5]
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