Coal formation turned upside down

Some coal deposits may have been formed by microbes instead of pure geochemistry

Charring

Stages of charring: wood, shale coal (xylite), bright lignite and hard coal. © Patrick Mansell/ Penn State

Doctrine disproved? Some coal deposits may have been formed differently than thought, as isotope data now reveal. According to this, the conversion of peaty plant residues into coal did not take place purely geochemically by heat, pressure and acids, but with the help of microbes. The bacteria split off methyl groups from the peat and thus promoted coalification, as the researchers report in the journal "Science". If this is confirmed, textbooks may have to be rewritten.

The fossil fuel coal has its origins in primeval plant remains that died millions of years ago and initially turned into peat in swampy subsoil with the help of anaerobic bacteria. Then comes the actual charring: under the influence of heat, pressure and acids, the cellulose, lignins and humic substances of the peat are gradually converted into coal. The carbon content increases, while the methoxyl groups (-O-CH3) typical of organic compounds are increasingly split off.

According to current doctrine, this transformation of peat into lignite and hard coal takes place purely geochemically. According to this theory, microbes are only involved in the peatification of the dead plant remains.

Coal samples in the isotope test

But this is apparently not always true, as Max Lloyd of the California Institute of Technology and his colleagues have now discovered. For their study, they had analyzed samples of wood, peat, and lignite and bituminous coal from different mining sites for their content of methoxyl groups and for the proportion of the carbon isotope 13C. They wanted to find out whether bacteria were involved in the carbonization of these materials.

"Previously, it was assumed that microbial modification of plant material in nature stopped when peat formed," Lloyd and his colleagues explain. The background to this was the assumption that microbes cannot survive under the heat and pressure of the layers sinking into the depths. But in the meantime, laboratory experiments and the results of deep drilling have proven that bacteria still occur at depths of several kilometers – as part of the deep biosphere.

If bacteria were involved in coalification, this should be evident from an enrichment of the carbon isotope 13C and the way in which the methoxyl groups were split off.

Values do not fit purely abiotic processes

The analyses revealed: Some of the coal samples studied were enriched in carbon variant 13C in terms of isotopic composition. The value in these cases was higher than it would be in the case of classical heat-induced carbonization. "The isotopic composition of the remaining methoxyl groups does not fit a thermally activated reaction," the researchers state.

Incarbonation processes by acid are also unlikely to be a factor, according to the scientists, because the affected lignite formations in Japan and the U.S. were formed under alkaline conditions. Other, as yet unknown abiotic processes would be theoretically conceivable, but unlikely: "There is no obvious reason why the 13C of an as yet undiscovered abiotic process should vary so systematically between samples," Lloyd and his colleagues explain.

Biochemical charring

Coalification with microbial involvement: anaerobic bacteria produce enzymes that break down the methoxyl groups of the coal precursors. © Max Lloyd/ Penn State

Lignite formation by microbial helpers

According to the researchers, their measurement data instead suggest a different explanation: This lignite must have been formed biochemically – with the help of bacteria. "Our results suggest that the separation of methoxyl groups in these samples is due to a microbial reaction," they explain. "These microbes degraded the methoxyl groups and converted the material into charcoal, producing methane in the process."

Specifically, they assume that this biogenic transformation took place in two steps: Initially, the methoxyl groups were still enzymatically degraded by aerobic bacteria. As the layers sank deeper into the subsurface, further methoxyl decomposition followed under anaerobic conditions. "This scenario is fits the geological context of these samples," Lloyd and his team explain.

Coalification by the deep biosphere

But this means: at least in some cases, coal formation may have proceeded differently than conventional wisdom suggests. The charring did not take place purely abiotically after the peat formation, but also biochemically with microbial participation. "Deep biosphere microbial communities contributed to the conversion of plant material into coal over geologic time," researchers state.

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