The biopolymer lignin – one of the most important carbon reservoirs in plants and soils – not only decays through fungi or bacteria but can also be chemically attacked by iron linked with reactive oxygen species, without the involvement of microorganisms. When this happens, the lignin is converted into methanol and subsequently into formaldehyde. That is shown by a study conducted at Heidelberg University's Institute of Earth Sciences. The researchers also proved that this conversion takes place in natural soils – solely through wetting and without adding chemicals. This form of lignin degradation shines a light on a previously overlooked abiotic process that is related to Earth's carbon cycle and can have implications for trace gas fluxes and atmospheric chemistry.
Lignin constitutes a considerable part of terrestrial non-fossil organic carbon, and gives wood and other plant matter their solidity. Characteristic of the biopolymer are so-called methoxy groups (–OCH₃). The compounds methanol and formaldehyde are, above all, known as important industrial chemicals but they also develop naturally and, in the soil, serve as carbon and energy sources for microorganisms. It was previously not known that an iron-induced process of lignin decay can be an important source of both compounds. Iron minerals are natural components of many soils and also reactive oxygen species – oxygen-containing molecules with very high chemical reactivity – can form there, particularly under changing environmental conditions.
When reactive iron-oxygen species attack lignin without the involvement of microorganisms, the methoxy groups (–OCH₃) are directly released from the biopolymer. Previously known decay pathways only liberate the methyl share of such groups. In the course of demethoxylation, methanol and subsequently formaldehyde are already formed under natural conditions. "The reaction takes place at ambient temperature and under normal pressure in water, and will only intensify at higher temperatures. So far it has only been known from industry – using temperatures of over 285 degrees Celsius, with higher pressure and in a hydrogen atmosphere," explains Dr. Jonas Hädeler from the Institute of Earth Sciences, who carried out the laboratory experiments in the biogeochemistry research group led by Prof. Dr. Frank Keppler.
The researchers followed the reaction pathway directly with isotope-marked molecules, where individual atoms are replaced by heavier variants. The methoxy group is cleaved as a unit and thereby forms the methanol – unlike with processes in which only methyl groups are removed. Quantum chemical calculations by Prof. Dr. Peter Comba's working group support the results of the laboratory experiments. "They indicate the lowest energy barrier for cleavage of the whole methoxy group – which is, in chemical terms, the most favorable of the reactive pathways investigated. That exactly matches the experimental observations," explains Prof. Comba, whose research is based at Heidelberg University's Institute of Inorganic Chemistry.
Experiments by the Heidelberg researchers in sterilized soil samples show that the process occurs in natural soils: methanol and then formaldehyde formed through wetting alone – without the addition of iron or reactive oxygen species. Methanol formation also continued through several wetting-drying cycles. It did not occur when the methoxy groups had been deliberately removed in advance. By contrast, considerably less methanol is found in untreated, biologically active soils, as microorganisms rapidly consume it. "Hence it primarily functions as an intermediate product linking abiotic chemistry and microbial metabolism," Dr. Hädeler says.
"It surprised us that the compounds methanol and formaldehyde can also be released entirely chemically from lignin-rich organic materials," Prof. Keppler underlines. If they enter the atmosphere, they influence photochemical reactions there and the atmospheric chemistry. Experiments at differing temperatures show that their formation increases with rising temperatures. "What this means in the climate context is something further field studies will have to clarify," says the Heidelberg earth scientist, whose research focuses on where small carbon compounds like methanol originate from in the environment. Furthermore, the current research findings might be interesting in the near future for using lignin as a by-product of the paper industry.
Also participating in the investigations were, besides the Institute of Earth Sciences and the Institute of Inorganic Chemistry, the Heidelberg Center for the Environment and the Interdisciplinary Center for Scientific Computing of Heidelberg University. The research was funded by the German Research Foundation and in the context of the Max Planck School "Matter to Life", which is sponsored by the Federal Ministry of Research, Technology and Space and the Max Planck Society. The results appeared in "Nature Communications".
