Water‐soluble phenolic metabolites in lichens and their potential role in soil organic matter formation at the pre‐vascular stage

Abstract

AbstractSoil organic matter (SOM) is the largest reservoir of organic carbon in the biosphere. However, little is known about the processes of its formation at the pre‐vascular stage. Lichens are among the pioneer colonizers on mineral substrates and are possible early land flora. This study is the first report on the identification and quantification of water‐soluble phenolic compounds (PCs), potential precursors of humic substances, in epigeyic lichens from two systematic groups. Results show (Folin–Denis assay) that cyanobiont‐containing lichens (order Peltigerales) possess three to five times more total soluble PCs than Lecanoralean lichens (Cladonia, Cetraria spp.) and mosses. Soluble PCs in lichens occur in the conjugated form. Alkali‐hydrolysable compounds (esters) predominate over acid‐hydrolysable compounds (glycosides). Phenolic complexes with N‐containing compounds or reducing sugars, or both, have been identified by thin layer chromatography (TLC). Benzoic acid derivatives were most common among PCs, detected in lichens by reversed‐phase high‐pressure liquid chromatography (RP‐HPLC). Phenolic acids occur in the order (μg 100 g−1): p‐hydroxybenzoic acid (327–1,007) > syringic acid (87–361) > salycilic acid (135–210) > vanillic acid (12–19) (Peltigeralean lichens); salicylic acid (53–102) > p‐hydroxybenzoic acid (45–54) > caffeic acid (29) > syringic acid (18) > vanillic acid (9–13) (Lecanoralean lichens). Protocatechuic, caffeic and p‐coumaric acids were rare; ferulic acid was not detected. Syringyl and vanillyl aldehydes and ketones occur in much larger amounts than acids. Methoxy‐substituted and ortho‐substituted phenols, detected in lichens, are known for their high reactivity in soils under lignified vegetation, suggesting their important roles in SOM formation under a cryptogam cover.Highlights Phenolic composition of SOM and humification processes at pre‐vascular stage are largely overlooked Soluble phenolic acids, aldehydes and ketones are quantified in lichens for the first time Lichens are depleted in phenylpropanoids and enriched in syringyl structures and monophenols Lichen‐derived phenolic compounds are potential precursors of humic substances under cryptogam cover