PhD defence: Jonas Thomsen

Jonas Thomsen defends his thesis:

Sphagnum peat: Chemical environment, composition and enzymatic degradation

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Supervisors:
Professor Katja Salomon Johansen, formerly with IGN
Professor Lisbeth Garbrecht Thygesen, IGN
Assistant professor Signe Lett, IGN

Assesment Committee:
Associate professor Mirijam Kabel, University of Wageningen, Netherlands
Assistant professor Tomáš Hájek, University of South Bohemia, Czechia
Professor Per Gundersen (chair), IGN

Summary:
Peatlands store biologically bound carbon corresponding to 20-30% of the global soil carbon (C) stock and the majority is stored in Northern peatlands. Unfavourable environmental conditions for microbial decomposition is normally used for explaining peat C accumulation. However, the primary producer of peat in Northern peatlands, Sphagnum mosses, possess intrinsic properties that might contribute to slow degradation of peat. However, which exact properties of Sphagnum lead to resistance against degradation is debated. The primary aim of this thesis was to investigate some of these properties that affect degradation of Sphagnum and Sphagnum derived peat. This was approached through a large-scale field study that described carbon and metal dynamics in a Sphagnum mire, and by detailed laboratory experiments on Sphagnum cell wall composition and enzymatic degradability.
The aim of the field study was to describe a Sphagnum mire in terms of total content of carbon, iron (Fe) and lead (Pb). Iron plays an important role in carbon stabilization and degradation in peatlands. A strong and linear correlation was found between annual export of DOC and Fe from the mire that is in line with observations from other peatlands. Element analysis of peat cores revealed the mire had previously been exposed to anthropogenic Pb pollution. The top 50 cm of the bog contained Pb in concentrations that could affect the vitality of microorganisms and living vegetation. In particular, the hummock topography contained high concentrations of Pb compared to the hollow topography. This finding could have implications for interpretation of mass loss studies comparing hollow and hummock species collected in peatlands exposed to anthropogenic pollution. Also, the high concentrations of Pb in the mire system calls for further investigation of how climate change affect the environmental status of mires.
Degradation of a laboratory grown Sphagnum fuscum and environmental samples of S. fuscum moss and peat collected from a bog was investigated directly in laboratory experiments. Sphagnum peat was subjected to a 180°C hydrothermal pretreatment, a method developed to overcome the recalcitrance of plant biomass. Biomass of Sphagnum moss and Sphagnum peat was subjected to chemical treatment for extraction of a pectin, hemicellulose and cellulose rich fraction. This method allows characterization of the monosaccharide composition and enzymatic degradability of cell walls and the extracted polysaccharide fractions. The recalcitrance of Sphagnum moss and Sphagnum peat was assessed with a commercial enzyme cocktail optimised for cellulose saccharafication.
The hydrothermal pretreatment of Sphagnum peat resulted in a substrate containing cellulose that was susceptible to enzymatic saccharafication similar to that of higher plants. Specifically the hydrothermal pretreatment, resulted in loss of peat cell wall integrity, relocated lignin-like compounds and released non-cellulosic glucose-rich polysaccharides. During saccharification of the pretreated Sphagnum peat, abiotic oxidative reactions substantially acidified the substrate and inactivated the commercial enzyme cocktail. The enzymatic saccharification of Sphagnum cell wall was not influenced by the content of lignin-like compounds. Also, the extracted cellulose rich fraction of Sphagnum and peat samples were resistant to enzymatic degradation, but monosaccharide analysis revealed that 50% of the extracted cellulose was non-cellulosic polysaccharides. The main finding in this thesis is that Sphagnum recalcitrance is related to restricted access of enzymes to Sphagnum cell wall polysaccharides. This thesis present results that in part support previous hypotheses on Sphagnum recalcitrance and contribute to the understanding of the degradability of 20-30% of the global soil C pool.

A digital version of the PhD thesis can be obtained from the PhD secretary at phd@ign.ku.dk before the defence. After the defence the thesis will become available from the Royal Danish Libary at kb@kb.dk.