Abstract
Trehalose, mannitol and arabitol are the main saccharides of extant fungal metabolism, but their occurrence and distribution in geological materials have rarely been considered. Here, we identify these sugars in Miocene lignites and for the first time in Late Cretaceous mudstones and coals. The co-occurrence of trehalose, mannitol and arabitol in the sedimentary rocks investigated suggests their fungal origin, because these three saccharides are major compounds present in most modern fungi, including the very common mycorrhizal and wood-rotting groups. Therefore, we conclude that these sugars should be treated as new fungal biomarkers (biomolecules) present in geological rocks. Trehalose and mannitol are major compounds in total extracts of the samples and a sum of their concentration reaches 4.6 μg/g of sample. The arabitol concentrations do not exceed 0.5 μg/g, but in contrast to trehalose, the concentration correlates well with mannitol (R2 = 0.94), suggesting that they have the same, translocatory role in fungi. Based on the trehalose vs. mannitol and arabitol distributions in Cretaceous samples and their comparison with data for modern fungi, we preliminarily conclude that the coal seams from the Rakowice Małe (SW Poland) section were formed during warmer climatic periods than the overlying sediments. Furthermore, no DNA could be isolated from the samples of lignites and overlying sediments, whereas it was abundant in the control samples of maple, birch and oak wood degraded by fungi. This indicates an absence of recent fungi responsible for decay in lignites and implies that the saccharide origin is connected with ancient fungi.Other sugar alcohols and acids like D-pinitol, quinic acid and shikimic acid, were found for the first time in sedimentary rocks, and their source is inferred to be from higher plants, most likely conifers. The preservation of mono- and disaccharides of fungal origins in pre-Palaeogene strata implies that compounds previously thought as unstable can survive for tens to hundreds of millions of years without structural changes in immature rocks unaffected by secondary processes.
Highlights
Biomolecules, natural products of living organisms are relatively rare in sedimentary organic matter (OM)
We demonstrate the co-occurrence of mannitol, arabitol, trehalose, and other sugars in the Late Cretaceous sections from the Rakowice Małe area (SW Poland), as well as their presence in some Miocene detritic lignites and xylites. We propose that these compounds, when present as dominant saccharides, can be used as indicators of fungal metabolism, as is typical for many modern fungi (e.g. Koide et al, 2000; Simoneit et al, 2004; Hybelbauerová et al, 2008)
Such values are characteristic for lignites to sub-bituminous coals, where the maximum temperature influence on the OM never exceeded 50 °C (Hunt, 1995)
Summary
Biomolecules, natural products of living organisms are relatively rare in sedimentary organic matter (OM). This is mainly connected with the fact that their low stability leads to early metabolic and diagenetic destruction or conversion. In favorable conditions (low maturation, reducing redox potential, no secondary processes) biomolecules can be preserved hundreds of millions of years, and were occasionally reported from sediments across the Mesozoic and Cenozoic In euxinic conditions polysaccharides can be preserved through sulfurization processes. Experimental study has shown that the reaction of glucose and cellulose with H2S leads to formation of organic sulfur compounds (Moers et al, 1988). Van Kaam-Peters et al (1998) and Sinninghe Damsté et al (1998) demonstrated using isotope and International Journal of Coal Geology 201 (2019) 51–61 molecular studies that sulfurized carbohydrates can constitute an important part of kerogen
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