Microbial methane formation in deep aquifers of a coal-bearing sedimentary basin, Germany.

Nontje Straaten,Martin Krã¼Ger,Nãºria Jimã©Nez,Hans-Hermann Richnow,Friederike Grã¼Ndger,Tillmann Lã¼Ders,Thomas Thielemann

doi:10.3389/fmicb.2015.00200

Abstract

Coal-bearing sediments are major reservoirs of organic matter potentially available for methanogenic subsurface microbial communities. In this study the specific microbial community inside lignite-bearing sedimentary basin in Germany and its contribution to methanogenic hydrocarbon degradation processes was investigated. The stable isotope signature of methane measured in groundwater and coal-rich sediment samples indicated methanogenic activity. Analysis of 16S rRNA gene sequences showed the presence of methanogenic Archaea, predominantly belonging to the orders Methanosarcinales and Methanomicrobiales, capable of acetoclastic or hydrogenotrophic methanogenesis. Furthermore, we identified fermenting, sulfate-, nitrate-, and metal-reducing, or acetogenic Bacteria clustering within the phyla Proteobacteria, complemented by members of the classes Actinobacteria, and Clostridia. The indigenous microbial communities found in the groundwater as well as in the coal-rich sediments are able to degrade coal-derived organic components and to produce methane as the final product. Lignite-bearing sediments may be an important nutrient and energy source influencing larger compartments via groundwater transport.

Highlights

The occurrence of methane in coal-bearing sedimentary basins has traditionally been linked to high maturity coals with thermogenic gas potential (Strapocet al., 2011)
To identify the microorganisms responsible for the degradation of complex organic matter of ligniteous coals to substrates for methanogenesis, microbial communities from groundwater and sediment samples were studied using terminal restriction fragment length polymorphism (T-RFLP) fingerprinting combined with pyrosequencing of 16S rRNA gene fragments
Microbial communities in groundwater and coal-rich sediments were dominated by putative hydrocarbon-degrading taxa affiliated to Actinobacteria and Proteobacteria, mostly Gammaproteobacteria belonging to Pseudomonadales, and Betaproteobacteria, including Acidovorax, Burkholderia, Polaromonas, Geofuchsia, and Ralstonia (Figure 4; Table 2)

Summary

Introduction

The occurrence of methane in coal-bearing sedimentary basins has traditionally been linked to high maturity coals with thermogenic gas potential (Strapocet al., 2011). Recent geochemical studies have frequently detected natural occurrence of biogenic methane, identified by its carbon and hydrogen isotopic signatures (Whiticar et al, 1986), in groundwater and pore water from lower maturity coal systems, like those in the Powder River Basin (Strapocet al., 2011 and the references therein). Coal is extremely rich in complex organic matter and a very attractive carbon source for microbial biodegradation (Fakoussa and Hofrichter, 1999) and, eventually, for methanogenesis. Lignite coal is soft brown sediment with high water content and a relatively low heating value. These characteristics place it somewhere between peat and subbituminous coal. Because of the high organic matter concentrations, coal-derived sediments are potential important microbial sources of energy (Fry et al, 2009).

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