Bacterial and abiotic decay in waterlogged archaeological Picea abies (L.) Karst studied by confocal Raman imaging and ATR-FTIR spectroscopy

Abstract

Bacterial and abiotic decay in waterlogged archaeological Picea abies (L.) Karst studied by confocal Raman imaging and ATR-FTIR spectroscopyAbstract: Waterlogged archaeological Norway spruce [Picea abies (L.) Karst] poles were studied by means of confocal Raman imaging (CRI) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) analysis to determine lignin and polysaccharide composition and distribution in the cell wall. The waterlogged archaeological wood (WAW) was submerged under anoxic conditions for approximately 400 years and solely decayed by erosion bacteria (EB). CRI showed that decayed tracheids contain a residual material (RM) with heterogeneous lignin distribution; within the same tracheid RM often contained regions with intensities lower than sound S2 layers up to intensity values as high as the compound middle lamella (CML). CRI revealed strong depletion of carbohydrates in RM which indicated that EB are able to utilise the carbohydrate fraction of the cell wall effectively. Raman bands assigned to lignin did not show any difference between RM and sound S2. This is a hint that EB do not modify the lignin structure. Sound WAW free from EB decay showed evidence of loss of acetyl groups in glucomannan, loss of un-conjugated ester linkages in the lignin-carbohydrate complexes between xylan and lignin, and minor oxidation of the lignin polymer compared to recent reference material. This is evidence for abiotic decay in the course of waterlogging.

Even Visually Intact Cell Walls in Waterlogged Archaeological Wood Are Chemically Deteriorated and Mechanically Fragile: A Case of a 170 Year-Old Shipwreck

“…The dramatic intensity decline at both 1730 cm −1 and 1234 cm −1 indicated the loss of carboxyl groups in glucuronic acids of O-acetyl-(4-O-methylgulcurono) xylan, which was also proved by the absence of signal 16 at 21 ppm and signal 1 at 172 ppm, ascribed to CH 3 -COO-methyl carbon in hemicellulose acetyl groups and the carbonyl of carbohydrates, respectively, in the 13 C solid state NMR curve of WAW [17,[23][24][25] ( Figure 4A and Table S2). The loss of carboxyl group of glucuronic acid residues in hemicellulose probably indicates the partial loss of unconjugated ester linkages in lignin-carbohydrate complexes (LCCs) of VICWs in archaeological wood according to a previous report [14]. It is known that the covalent link between the carboxyl group of glucuronic acid residue in hemicelluloses and α-hydroxyl group of the lignin represents one kind of covalent links in LCCs [26,27].…”

“…It should be emphasized that the Raman region from 1420 cm −1 to 1510 cm −1 was not taken into account due to the influence of the embedding material PEG. The lignin specific band at 1660 cm −1 , attributed to the conjugated C=C stretching of coniferyl alcohol or the C=O stretch of coniferaldehyde [14,36,37] (Table S4), exhibited substantial differences. CW and CML spectra of WAW revealed decrease of this respective band.…”

“…WAW is usually divided into severely, moderately and slightly deteriorated wood according to the maximum water contents (MWC) of the WAW [8,11,12]. Cell walls in these deterioration states can be easily differentiated into VICWs and decayed cell walls based on visible cell wall erosion [13,14]. Wood cell wall mechanics are highly dependent on the chemical composition and cell wall structure, including porosity [15,16], thus cell wall alteration in deterioration processes should be reflected in the decrease of mechanical properties of cell walls in WAW.…”

Hygroscopicity of Waterlogged Archaeological Wood from Xiaobaijiao No.1 Shipwreck Related to Its Deterioration State

“…However, next to the deterioration of cellulose and hemicellulose, the modification of lignin may also lead to an increase of adsorbent functional groups in WAW [7,54,69], responsible for an increased uptake of water vapor. In detail, lignin in archaeological wood generally undergoes modifications such as the partial breakage of β-O-4 interlinks, an alternation of lignin structure, and demethylation/demethoxylation [7,54,70,71]. Although there is few studies on the relationship between WAW and the number of available OH groups, it was reported that the amount of available OH groups of delignified wood increased with the degree of delignification [29].…”

An in situ Raman spectroscopic method for quantification of polyethylene glycol (PEG) in waterlogged archaeological wood

“…A further drawback of these studies is the assumption that the content of the wood component used as an internal standard is constant. This assumption does not hold true for most AW, as one of the major constituents of wood, cellulose, may, as a consequence of microbial decay, vary as much as to make up one third of the dry weight to approach almost complete depletion (Rowell et al 2013;Pedersen et al 2015).…”