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.

Fungal enzymes transferred by leaf-cutting ants in their fungus gardens

“…The presence of active acquired enzymes in the gut does not necessarily prove that these enzymes increase the digestive efficiency of the animal partner (Kukor & Martin 1986) and similarly, the faecal enzymes of leaf-cutting ants could just be traces of fungal enzymes that accidentally have escaped proteolysis in the gut and that do not contribute to the degradation of leaf material in the fungus garden. However, we found that the activity of a specific fungal enzyme was on average five times higher in the faecal droplets as in the mature garden material.…”

Genetic diversity of bacteria associated with the hindgut of the terrestrial crustacean Porcellio scaber (Crustacea: Isopoda)

“…Since it is assumed that terrestrial isopods do not possess endogenous cellulases, these crucial depolymerising enzymes must originate from microorganisms that are either indigenous to the ecosystem or ingested with food. The majority of authors support the latter [6,17] and some have even suggested that the gut of the woodlouse is actually not a favourable environment for microbial growth, as demonstrated by studies on the typical soil bacterium Pseudomonas fluorescens [19,20,39,40]. However, a process of intensified bacterial growth was observed in the faeces of P. scaber , and this is assumed to be the net effect of lysis during food transit through the gut and growth in the faeces after voiding.…”

“…Microbes in the gut of terrestrial isopods have been observed and studied by several authors [11–20]. Their presence indicates that the isopod gut represents a suitable environment for microbial colonisation, although it lacks specialised structures or modifications of the gut, which would facilitate colonisation [11,12,14].…”

Bacterial endosymbionts in Asellus aquaticus (Isopoda) and Gammarus pulex (Amphipoda) and their contribution to digestion

“…Terrestrial isopods (Isopoda: Oniscidea), which feed on the leaf litter of terrestrial plants, are capable of hydrolytically digesting cellulose (Hartenstein 1964(Hartenstein , 1982Hassall and Jennings 1975;Kozlovskaja and Striganova 1977;Kukor and Martin 1986;Zimmer and Topp 1998a) and oxidatively degrading phenolic leaf litter compounds (Neuhauser and Hartenstein 1976;Zimmer and Topp 1998b;Zimmer 1999). It has been proposed that these digestive capabilities, being adaptive to terrestrial food sources, are brought about by bacterial symbionts of the isopods' midgut glands (hepatopancreas) (Zimmer and Topp 1998a,b;Zimmer 1999;Zimmer et al 2001Zimmer et al , 2002.…”