Chemical characterization of lignin preparations of fresh and decomposing Spartina anglica by pyrolysis mass spectrometry

Abstract

In order to determine terrestrial contributions to the organic matter pool in the marine environment, a reliable chemical tracer is required. Lignin, an abundant component of vascular plant tissue is often used as such. In this study the lignin moiety of both fresh and degraded tissue of the model halophyte Spartina anglica was purified and chemically characterized to investigate its suitability as tracer. The study can be split into three parts. In the first part several enzymatic lignin purifications of fresh S. anglica tissue were compared with each other. In the second part pyrolysis gas chromatography mass spectrometry was applied to analyse untreated and on-line methylated milled wood lignin preparations. The previous characterizations of the lignin moiety of S. anglica were used in the third part to monitor modifications in the chemical composition induced by natural aerobic decomposition in the field. The temperature at which lipids were extracted from the original tissue was not critical, although low temperatures improved the purification to some extent. For enzymatic purification of lignin the use of a polysaccharidase turned out to be the most important prerequisite. The use of a protease did not have much influence on the lignin residue. After 2 yr of aerobic decomposition, the lignin character of untreated S. anglica tissue was no longer identifiable with direct temperature-resolved mass spectrometry, but it still could be retrieved in the residue obtained after enzyme treatment. With continuing decomposition the chemical structure of the lignin moieties became partly modified. The very complicated spectra obtained after longer decomposition periods suggest the formation of a heterogeneous organic fraction, probably representing “humic substances”. The gradual loss of lignin characteristics in the litter makes its suitability as tracer for terrestrial material in a complex mixture of substances doubtful, at least with the analytical technique applied in this study.