MECHANISMS OF WOOD DIGESTION IN THE SHIPWORM BANKIA GOULDI BARTSCH: ENZYME DEGRADATION OF CELLULOSES, HEMICELLULOSES, AND WOOD CELL WALLS

¹ Department of Biology, Kenyon College, Gambier Ohio, 43022, and Duke University Marine Laboratory, Beaufort, North Carolina, 28516

Tissue extracts from the digestive tract of the wood-boring bivalve Bankia gouldi were tested for their ability to degrade celluloses and wood to soluble sugars. Tissue extracts showed high levels of glucose production from cellobiose, demonstrating the presence of -glucosidase.

The crystal structure of cellulose determines its lability to enzymes. Microcrystalline cellulose (MC) was used as an analogue to native crystalline cellulose in wood cell walls. Swollen microcrystalline cellulose (SMC) and carboxymethyl cellulose (CMC) are more labile celluloses. Bankia goudi extracts showed maximum activity on SMC and minimum activity on MC, when assayed as glucose production.

When assayed over time, glucose production from SMC gradually declined as a function of enzyme inactivation and decreasing substrate lability. These results are interpreted as showing the removal of a cellulose fraction of altered crystal structure, leaving the more inert native crystalline fraction.

Activity of Bankia gouldi extracts over time was compared to commercial cellulose-digesting enzyme preparations. The pattern of activity closely resembles that of Aspergillus niger, a fungus unable to grow on native crystalline cellulose. The pattern is very different from that seen with Trichoderma viride and Helix pomatia enzymes, characterized as true cellulose decomposing systems.

The production of glucose from ground wood by Bankia gouldi extracts is consistent with the crystallinity of the substrate and the results for purified cellulose. A higher level of production of reducing sugars other than glucose indicates the digestion of wood hemicelluloses.

Digestibility of wood hemicelluloses is limited by the lignin fraction in the wood cell wall.

The experimental results were interpreted in terms of a model for the molecular architecture of the wood cell wall. The cellulose is present as crystalline fibrils coated with hydrophilic hemicellulose polymers. These coated fibrils are embedded in a hydrophobic lignin matrix. The digestibility of wood is a function of polymer accessibility and lability to digestive enzymes.

The enzymes demonstrated in vitro in Bankia gouldi tissue extracts are -1,4-glucanases with limited activity on native crystalline cellulose, which they cannot digest at a nutritionally significant rate. The mechanism of cellulose digestion by shipworms remains an unsolved problem.