The hidden Sulfate-Reducing Prokaroytes

Sulfate-reducing prokaryotes (SRPs) are widespread key players in the cycling of sulfur and carbon on our planet. Environmental inventories of dsrAB, genes that encode major subunits of the dissimilatory (bi)sulfite reductase and are diagnostic for SRPs, have recently demonstrated that the diversity of this guild is far greater than previously recognized by cultivation-based surveys. For example, we could show that the SRP community in an acidic fen soil system is largely represented by novel dsrAB sequence types which form novel, deep-branching lineages in the phylogenetic tree and thus most likely derive from yet unknown SRPs. Similar dsrAB sequences were also found in other wetland ecosystems, indicating a widespread distribution of these novel SRP lineages in the environment. However, apart from their molecular signature nothing is currently known about these previously hidden SRPs. This project proposal aims at considerably extending our current knowledge of these presumably important microorganisms, which have so far resisted cultivation, by using the acidic fens as a model system. Based on already available sequence data, novel cultivation-independent methods such as DNA microarrays will be developed and applied for monitoring changes in the diversity and cellular activity of SRPs thriving in the acidic fen system, and for yielding insights into their metabolic properties. A further goal is the selective enrichment, which is also a requirement for future (meta)genomic studies, and subsequent taxonomic identification of the novel SRPs. The combined application of these complementary techniques should allow one to unravel important features of the biology of these novel SRPs, for which currently nothing more than partial sequences of a key enzyme are known.

Emission of the greenhouse gas methane from peatlands contributes substantially to global warming but is significantly reduced by sulfate reduction, a microbiological process that is fuelled by globally increasing aerial sulfur pollution. For the first time, the identity, abundance, and long-term temporal and spatial community changes of key players for this process were determined at a model peatland site by using molecular and isotope-labeling methods. Uncultivated species, represented by novel sequence variants of the sulfate reducer key genes dsrAB, were relatively abundant core members of the indigenous peatland microbiota. A biogeography survey additionally showed that several of these novel dsrAB-carrying microorganisms are widespread in different peatlands in Central Europe and thus not limited by dispersal. Substrate-mediated isotope labeling surprisingly indicated that a new Desulfosporosinus species, which constitutes only 0.006% of the total microbial community, is a major sulfate reducer in the model peatland. The low-abundant but highly potent Desulfosporosinus species potentially accounts for a significant part of sulfate reduction in the peat soil. These data show that the identified Desulfosporosinus species, despite being a member of the `rare biosphere`, could contribute considerably to an important biogeochemical process that suppresses methane emissions from peatlands and, thus, alters their contribution to global warming.