RESUMO
BACKGROUND: Rivers and streams are important components of the global carbon cycle and methane budget. However, our understanding of the microbial diversity and the metabolic pathways underpinning methylotrophic methane production in river sediments is limited. Dimethylsulfide is an important methylated compound, found in freshwater sediments. Yet, the magnitude of DMS-dependent methanogenesis nor the methanogens carrying out this process in river sediments have been explored before. This study addressed this knowledge gap in DMS-dependent methanogenesis in gravel and sandy river sediments. RESULTS: Significant methane production via DMS degradation was found in all sediment microcosms. Sandy, less permeable river sediments had higher methane yields (83 and 92%) than gravel, permeable sediments (40 and 48%). There was no significant difference between the methanogen diversity in DMS-amended gravel and sandy sediment microcosms, which Methanomethylovorans dominated. Metagenomics data analysis also showed the dominance of Methanomethylovorans and Methanosarcina. DMS-specific methyltransferase genes (mts) were found in very low relative abundances whilst the methanol-, trimethylamine- and dimethylamine-specific methyltransferase genes (mtaA, mttB and mtbB) had the highest relative abundances, suggesting their involvement in DMS-dependent methanogenesis. CONCLUSIONS: This is the first study demonstrating a significant potential for DMS-dependent methanogenesis in river sediments with contrasting geologies. Methanomethylovorans were the dominant methylotrophic methanogen in all river sediment microcosms. Methyltransferases specific to methylotrophic substrates other than DMS are likely key enzymes in DMS-dependent methanogenesis, highlighting their versatility and importance in the methane cycle in freshwater sediments, which would warrant further study.
RESUMO
BACKGROUND: In anoxic coastal and marine sediments, degradation of methylated compounds is the major route to the production of methane, a powerful greenhouse gas. Dimethylsulphide (DMS) is the most abundant biogenic organic sulphur compound in the environment and an abundant methylated compound leading to methane production in anoxic sediments. However, understanding of the microbial diversity driving DMS-dependent methanogenesis is limited, and the metabolic pathways underlying this process in the environment remain unexplored. To address this, we used anoxic incubations, amplicon sequencing, genome-centric metagenomics and metatranscriptomics of brackish sediments collected along the depth profile of the Baltic Sea with varying sulphate concentrations. RESULTS: We identified Methanolobus as the dominant methylotrophic methanogens in all our DMS-amended sediment incubations (61-99%) regardless of their sulphate concentrations. We also showed that the mtt and mta genes (trimethylamine- and methanol-methyltransferases) from Methanolobus were highly expressed when the sediment samples were incubated with DMS. Furthermore, we did not find mtsA and mtsB (methylsulphide-methyltransferases) in metatranscriptomes, metagenomes or in the Methanolobus MAGs, whilst mtsD and mtsF were found 2-3 orders of magnitude lower in selected samples. CONCLUSIONS: Our study demonstrated that the Methanolobus genus is likely the key player in anaerobic DMS degradation in brackish Baltic Sea sediments. This is also the first study analysing the metabolic pathways of anaerobic DMS degradation in the environment and showing that methylotrophic methane production from DMS may not require a substrate-specific methyltransferase as was previously accepted. This highlights the versatility of the key enzymes in methane production in anoxic sediments, which would have significant implications for the global greenhouse gas budget and the methane cycle. Video Abstract.
