The effect of acquired microbial enzymes on assimilation efficiency in the common woodlouse, Tracheoniscus rathkei

Abstract

The digestive tract of the common woodlouse, Tracheoniscus rathkei Brandt (Isopoda: Oniscoidea), contains digestive enzymes active against α-1,4-glucans, which are the chief storage polysaccharides of vascular plants, algae, fungi, and animals, and β-1,3-glucans, which are present in algae and fungi. Digestive tract extracts also exhibit significant activity toward xylan and carboxymethyl-cellulose but negligible activity toward microcrystalline cellulose, substrates representative of the major structural polysaccharides of vascular plants. Low activity was detected toward pectin, and no activity was detected toward chitin. Activity toward xylan is due in part to microbial enzymes acquired from the leaf litter which was the isopod's normal food. Although ingested microbial xylanases are stable and active in the gut fluid, they do not make a quantitatively significant contribution to the isopod's ability to assimilate the hemicellulosic component of its diet. However, the assimilation of carbon from labeled plant fiber is enhanced in isopods which have acquired a cellulase by ingestion of leaf litter amended with a commercial preparation of the cellulase complex from the fungus, Penicillium funiculosum . This result demonstrates the potential contribution of acquired enzymes to the digestion of plant fiber in terrestrial detritivores. We urge caution, however, in assigning an important digestive function to ingested enzymes on the basis of evidence that only indicates that such enzymes are present in the gut fluid without additional evidence that their presence results in an enhancement of digestive efficiency.