Variations in bacterial decay between cell types and between cell wall regions in waterlogged archaeological wood excavated in the intertidal zone

Abstract

The bacterial decay of waterlogged archeological wood (WAW, hard pine spp.) taken from Daebudo shipwreck No. 2, which was buried in the intertidal zone in the mid-west coast (Yellow sea) of South Korea approximately 800 years ago, was investigated. The maximum moisture content of the outer parts (approx. 3 cm of depth) of WAW was approximately 4.2 times higher than that of undegraded reference pine wood. ATR-FTIR and solid-state 13C-NMR analysis indicated a relative increase of the lignin concentration in WAW caused by the degradation of cellulose and hemicelluloses across the board studied (31-cm-wide and 14.5-cm-thick board). Micromorphological studies also revealed that bacterial degradation was progressed to a depth of 15 cm (vertically 7.3 cm) from the surface, which is the innermost part of the board. Erosion bacteria (EB) were identified as the main degraders of WAW. Degradation by tunneling bacteria (TB) was occasionally detected. Decay resistance to bacterial attacks in WAW varied between cell types and between cell wall regions. Axial tracheids showed less resistance than ray tracheids, ray parenchyma cells, and axial intercellular canal cells, including strand tracheids, subsidiary parenchyma cells, and epithelial cells. Decay resistance was higher in ray tracheids and strand tracheids than in ray parenchyma cells and subsidiary parenchyma-/epithelial cells, respectively. Bordered- and cross-field pit membranes and the initial pit borders showed higher decay resistance than the tracheid cell walls. Overall, the S2 layer of the axial tracheids showed the weakest resistance to bacterial attacks.

Microstructural and Chemical Characteristics of Archaeological White Elm (Ulmus laevis P.) and Poplar (Populus spp.)

“…This clearly indicates that these fungi are not ligninolytic or are less so. Some brown-rot fungi can only change the lignin structure [55,56] and could therefore also fall under this category. Unfortunately, no clear evidence for a brown-rot infestation could be found.…”

“…The ray parenchyma contains a higher quantity of sugars in comparison with fibers and vessels, as they are part of the nutrient reserves of the trees [21], and it is expected that those cells are susceptible to various degrading agents. Very recently, Cha et al [56] revealed that the degradation of the parenchymal cell wall is delayed relative to its adjacent tracheas when they are exposed to erosion bacteria. However, it did not exhibit entire resistance to bacterial decay.…”

Advances in Understanding Microbial Deterioration of Buried and Waterlogged Archaeological Woods: A Review

“…Wood-degrading microorganisms (soft-rot fungi and bacteria) that are present in water-saturated and waterlogged woods can readily depolymerise cellulosic and hemicellulosic components but differ in their ability to modify lignin, a recalcitrant component of the cell wall. Therefore, it is important to have knowledge of the concentration and distribution of lignin in various cell wall regions and tissues in order to understand the degradability of different cell wall layers/structures (e.g., vestures, warts), wood types (hardwood, softwood, normal wood, reaction wood) and cell types [18,29].…”

“…Partial waterlogging can support the activity of soft-rot fungi and wood degrading bacteria [6]. However, when exposure conditions become anoxic due to complete waterlogging, for example, in deep ocean waters, ocean, river and lake sediments, and mud, wood is mainly degraded by erosion bacteria [7][8][9][10][11][12][13][14][15][16][17][18]. Soft-rot fungi and tunnelling bacteria may also be present but are much less frequent [13,14,16,[18][19][20].…”

“…However, when exposure conditions become anoxic due to complete waterlogging, for example, in deep ocean waters, ocean, river and lake sediments, and mud, wood is mainly degraded by erosion bacteria [7][8][9][10][11][12][13][14][15][16][17][18]. Soft-rot fungi and tunnelling bacteria may also be present but are much less frequent [13,14,16,[18][19][20]. Erosion bacteria are regarded as the wood degrading microorganisms most tolerant to depleted oxygen concentration.…”