Exploration of changes in the chemical composition of sedimentary organic matter and the underlying processes during biodegradation through advanced analytical techniques

Rationale. Although the scientific community has widely investigated organic matter biodegradation processes, only a limited number of studies have explored the molecular changes of this material, whereas its structure, composition and origin play a key role in these processes. Methodology. We decided to examine the effects of biodegradation on the chemical composition of sedimentary organic matter and to explore the underlying mechanisms. We conceived a laboratory-based degradation experiment utilising organic-rich sediments artificially composed of two contrasting organic matter end-members (i.e. soil and algae) under two oxygen conditions. The sediment samples before and after incubation were then analysed by laser desorption ionisation-Fourier-transform-ion cyclotron resonance-mass spectrometry for molecular characterisation and by thermally assisted hydrolysis and methylation gas chromatography-mass spectrometry in order to offer insights into the mechanisms driving the biodegradation processes. Results. Our results from molecular characterisation unveiled distinct pathways of biodegradation contingent upon the source material. Moreover, they hinted at a predilection for altering high molecular weight compounds like lignin & carboxylic-rich alicyclic molecules (CRAM) and condensed aromatic structures (CAS), manifesting as a conversion into lower molecular weight counterparts. Furthermore, the complementary findings from biomarker analyses underscored the influence of environmental factors - specifically oxygen conditions and microbial communities - on organic matter decomposition. Discussion. Although this study is a controlled laboratory experiment and more studies are needed, it demonstrates the intricate interplay among chemical, biological and environmental factors that profoundly shape the reactivity of organic matter. This study underscores the critical need for persistent inquiry, aimed at unravelling the factors and conditions governing the diverse pathways of biodegradation.