Relative assimilations of 14C-labelled microbial tissues and 14C-plant fibre ingested with leaf litter by the millipede Glomeris marginata under experimental conditions

Abstract

Reference strains of four bacteria ( Escherichia coli, Erwinia herbicola, Pseudomonas syringae and Bacillus subtilis) and the fungus Mucor hiemalis were grown in pure liquid cultures containing 14C-thymidine, 14C-glucose or a 14C-amino acid mixture and harvested for feeding to the millipede Glomeris marginata after topical application to oak or beech leaf litter discs. After 24 h the assimilation of ingested microbial tissue by the animals was determined from the partitioning of radiolabel between carcass, haemolymph, evolved CO 2, faecal pellets and the residual gut contents. The 14C-amino acid mixture was found to give the greatest absolute level of labelling of microbial cells and assimilation efficiencies with this source of radiolabel were: Escherichia coli, 72.2 ± 0.9%; Erwinia herbicola, 95.9 + 2.3%; Pseudomonas syringae, 93.2 ± 2.2%; Bacillus subtilis, 82.6 ± 5.0%; and Mucor hiemalis, 73.5 ± 1.5%. The assimilation of 14C-leaf fibres (labelled from 14CO 2), determined by a similar method, was 18.8 ± 3.3% on beech discs and 29.6 ± 4.8% on oak. Previous incubation of 14C-leaf fibre with various bacteria, including the cellulase-secreting Cytophaga hutchinsonia, failed to change the calculated assimilation by millipedes significantly. The results support a proposition that microorganisms colonizing leaf litter are a valuable nutritional resource to detritivorous animals such as millipedes and are assimilated with high efficiency. Scanning electron microscopy of the food, gut contents and faeces of Glomeris marginata confirmed that colonizers were stripped from leaf litter on ingestion, but also indicated that recolonization of faecal pellets by bacteria occurred during or immediately after transit of the hindgut.