Open Access

For centuries, peatlands have received little attention or recognition due to their perceived insignificance, invisibility, and negative associations. However, they are now receiving increasing attention due to the ecosystem services they provide as carbon and water reservoirs, as well as their rich biodiversity of rare and endangered animals and plants. In the discussion of greenhouse gas (GHG) emissions and their impact on the climate, peatlands are also recognized as being of significant importance. To comprehend the source of these GHG, it is necessary to examine the microbiome of peatlands and the effects of lowering (draining) and raising (rewetting) the water level. Peatlands store large amounts of soil organic carbon (SOC) in form of peat. Dead plant material is preserved in the water-saturated soils and the carbon dioxide (CO2) absorbed from the atmosphere through photosynthesis is stored as peat. However, when the water table in peatlands is lowered, atmospheric oxygen enters the soil, and the soil microbiome changes, leading to the microbial decomposition of SOC and the release of large quantities of CO2. Rewetting can halt aerobic decomposition processes by blocking oxygen. Anaerobic conditions are not conducive to aerobic organisms. Complex plant polymers, such as lignin, can no longer be broken down, preserving the peat body and allowing the peatland to once again serve as a long-term carbon sink. Under these anaerobic conditions, methanogenic archaea produce methane from metabolic products of fermentation processes. Methane is emitted in much smaller quantities, but has about 30 times the warming potential of CO2, making it the second most important GHG after CO2. To gain a better understanding of the processes involved in methanogenesis, it is essential to conduct a detailed investigation of the individual groups of archaea involved. The metabolic pathways of hydrogenotrophic methanogenesis from CO2 and hydrogen, as well as acetoclastic methanogenesis from acetate cleavage, have already been extensively researched and are considered the most significant pathways of methanogenesis. In recent years, additional organisms have been discovered that carry out methylotrophic methanogenesis from methanol and methylated compounds. The methylotrophic order of Methanomassiliicoccales was first described in 2012, but only one pure culture, Methanomassiliicoccus luminyensis, has been isolated. Further research into Methanomassiliicoccales, commonly found in mires, can enhance our understanding of methanogenesis not only in mires but also in other habitats, such as the gastrointestinal tracts (GIT) of animals or water body sediments. This study investigates the peat soil microbiomes, with a focus on methanogenic Archaea of the order Methanomassiliicoccales, at the six study sites of the WETSCAPES project. The sites are located in the three important fen types of Mecklenburg-Western Pomerania (alder forest, percolation fen, and coastal flooded fen), each in drained and rewetted areas. The first article investigates the microbial composition of prokaryotes and eukaryotes in the sites using Illumina MiSeq Amplicon sequencing of 16S rRNA and 18S rRNA genes. The aerobic microorganisms responsible for decomposing plant biomass are present in drained peatlands. In rewetted sites, the relative abundance of these organisms was significantly lower, while other groups, such as fermenting bacteria and methanogenic archaea, occurred in greater numbers. Quantitative polymerase chain reaction (qPCR) was used to determine the number of methanogenic archaea per gram soil. The results indicate that methanogens are at least 10 times more abundant in rewetted peatlands than in drained sites, suggesting increased methanogenesis in rewetted peatlands. These findings were confirmed by GHG measurements. The second article presents an overview of the interactions between water transport, soil chemistry, primary production, peat formation, material conversion and transport, microorganisms, and GHG exchange. The findings are based on state-of-the-art methods in the relevant research areas and combine initial results from all project partners of the WETSCAPES project. The third article focuses on an application-oriented approach and tests different rewetting strategies. The objective of this study was to decrease methane emissions by removing the topsoil and additional planting of peat mosses. The results of the study, which included measurements of GHG emissions and qPCR analyses of methanogenic archaea, showed a significant reduction in methane emissions after the top 30 cm of soil were removed compared to the untreated area. The fourth article deals with the methylotrophic order Methanomassiliicoccales, which belongs to the group of methanogenic archaea. Over a period of 2.5 years, at 12 time points, at three soil depths, 16S rRNA gene sequencing was used to produce a seasonal and spatial analysis of the Methanomassiliicoccales phylotypes present. The results revealed a distribution of the phylotypes based on soil depth, redox potential, and season, which were related to their physiological characteristics. In the fifth article, the 1.52 Mbp genome of "Ca. Methanogranum gryphiswaldense U3.2.1", a Methanomassiliicoccales strain, enriched to > 80% from peat soil of the rewetted percolation fen, was presented. The sixth article presented the microscopic, physiological, and genomic studies of Ca. Methanogranum gryphiswaldense U3.2.1. This hydrogen-dependent, methylotrophic strain from the family Methanomethylophilaceae, uses methanol and trimethylamine as electron acceptors. Despite various attempts, a pure culture has not yet been achieved. The study demonstrates the impact of rewetting on the soil microbiome, revealing its complexity. The development of the microbiome was influenced by various biotic and abiotic factors, depending on the location and water level. The abundance of methanogenic archaea was significantly higher in the rewetted sites compared to the drained ones. The Methanomassiliicoccales were found to be a significant contributor to the methanogenic archaea in the investigated sites. It remains to be examinated whether they also significantly contribute to methane formation. Enrichment cultures and genome analyses were used to investigate the substrate spectrum of Ca. Methanogranum gryphiswaldense U3.2.1 obtained from peat soil. The extraction of a pure culture from peat soils has not yet been successful and therefore remains a goal of future investigations.