Abstract
Freshwater wetlands are a major source of the greenhouse gas methane but at the same time can function as carbon sink. Their response to global warming and environmental pollution is one of the largest unknowns in the upcoming decades to centuries. In this review, we highlight the role of sulfate-reducing microorganisms (SRM) in the intertwined element cycles of wetlands. Although regarded primarily as methanogenic environments, biogeochemical studies have revealed a previously hidden sulfur cycle in wetlands that can sustain rapid renewal of the small standing pools of sulfate. Thus, dissimilatory sulfate reduction, which frequently occurs at rates comparable to marine surface sediments, can contribute up to 36–50% to anaerobic carbon mineralization in these ecosystems. Since sulfate reduction is thermodynamically favored relative to fermentative processes and methanogenesis, it effectively decreases gross methane production thereby mitigating the flux of methane to the atmosphere. However, very little is known about wetland SRM. Molecular analyses using dsrAB [encoding subunit A and B of the dissimilatory (bi)sulfite reductase] as marker genes demonstrated that members of novel phylogenetic lineages, which are unrelated to recognized SRM, dominate dsrAB richness and, if tested, are also abundant among the dsrAB-containing wetland microbiota. These discoveries point toward the existence of so far unknown SRM that are an important part of the autochthonous wetland microbiota. In addition to these numerically dominant microorganisms, a recent stable isotope probing study of SRM in a German peatland indicated that rare biosphere members might be highly active in situ and have a considerable stake in wetland sulfate reduction. The hidden sulfur cycle in wetlands and the fact that wetland SRM are not well represented by described SRM species explains their so far neglected role as important actors in carbon cycling and climate change.
Highlights
Freshwater wetlands comprise diverse habitats ranging from peatlands with and without permafrost, freshwater marshes, freshwater swamps including riparian zones, constructed wetlands for waste water treatment, and agricultural wetlands (Mitsch et al, 2009; Kögel-Knabner et al, 2010; Vymazal, 2010)
Apparent sulfate half-saturation concentrations, K m, for sulfate reducers in pure culture (Pallud and Van Cappellen, 2006 and references therein) and in sediment slurries (Tarpgaard et al, 2011) have been reported to be as low as 2–5 μM indicating no kinetic limitation from the electron acceptor side as well. These findings indicated that the identified peatland Desulfosporosinus sp. has the potential to contribute considerably to peatland sulfate reduction despite its very low abundance
The measured rates were often comparable to or partly exceeded sulfate reduction rates (SRR) in marine environments (Jørgensen, 1982; Howarth and Jørgensen, 1984; Skyring, 1987), the importance of sulfate reduction to anaerobic carbon mineralization in wetlands is still often neglected. This can be mainly attributed to the current inability to quantitatively explain how the small sulfate pools are rapidly replenished to maintain the high SRR that can be repeatedly measured in different wetland types and over prolonged periods of time
Summary
Freshwater wetlands (from here on referred to as wetlands) comprise diverse habitats ranging from peatlands with and without permafrost (e.g., ombrotrophic bogs and minerotrophic fens), freshwater marshes (e.g., the Everglades or the Okavango delta), freshwater swamps including riparian zones, constructed wetlands for waste water treatment, and agricultural wetlands (Mitsch et al, 2009; Kögel-Knabner et al, 2010; Vymazal, 2010). Sulfate reduction in freshwater wetlands change in the upcoming decades to centuries due to global warming, which is connected to rising atmospheric carbon dioxide levels and changes in precipitation amount and frequency (IPCC, 2007; Dise, 2009) Such variations of environmental conditions on short- and long-term scales have important implications for the biogeochemical cycling of elements in wetlands and govern transitions between synergistic and antagonistic trophic interactions among wetland microorganisms, which determine the extent of carbon mineralization that is channeled through methanogenesis. Despite successful efforts to reduce aerial sulfur pollution in developed countries, global SO2 emission is predicted to increase in the decades due to increasing untreated combustion of coal and other fossil fuels in developing countries situated mainly in Asia
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
