Bacterial Degradation of Archaeological Wood in Anoxic Waterlogged Environments
Anoxic ecosystems offer extreme environmental conditions and under these conditions wood can be preserved for thousands of years. The recalcitrant nature of wood is not only controlled by the anoxic environment but also by the structure and organisation of the cell wall components. The decay mechanisms acting in anoxic ecosystems are though not fully understood. Preserved archaeological wood objects are due to their age, archaeological, and cultural record well suited for studying the degradation of biogenic carbon in anoxic environments. Erosion bacteria are the main degraders of waterlogged archaeological wood but it has so far not been possible to isolate or explicitly identify the bacteria species. The typical erosion bacteria decay pattern is a mixture of sound and decayed cell wall areas. The cellulose rich secondary cell wall is decayed and an amorphous residual material is left behind. The lignin rich compound middle lamella and often also parts of the S1 and S3 cell wall is preserved. The ultrastructural decay pattern corresponds well with the general observation that cellulose and hemicellulose is lost in preference to lignin. Hemicellulose is decayed in preference to cellulose. The degree of polymerisation is reduced for the remaining carbohydrates in waterlogged wood. Decayed cell wall areas have lost birefringence due to loss of crystalline cellulose, but the crystallinity of the remaining cellulose microfibrils is not significantly changed compared to sound reference wood. The lignin degradation is minor and incomplete.