Bacterial degradation of alder (Alnus glutinosa) leaves in a freshwater stream

Abstract

A sequential colonisation by bacteria of decaying alder leaves in the River Bourne and of sterile alder leaf discs in vitro was demonstrated by SEM. Enumeration of colonising organisms by fluorescence microscopy confirmed SEM observations that colonisation commenced within 24h and that numbers of bacteria then increased slowly over the first week of incubation, then declined and finally stabilised for the remainder of the decay period. Colonisation was divided into 4 phases: initial colonisation, establishment, instability, stabilisation.Organisms colonising alder leaves were isolated by direct plating and enrichment culture and characterised by standard morphological and biochemical testing, API 20NE strips where appropriate, and fatty acid profiling. Thirteen different genera were isolated and these were Xanthomonas, Pseudomonas, Aeromonas, Alcaligenes, Erwinia, Bacillus, Rhodococcus, Escherichia, Cytophaga, Proteus, Janthinobacterium, Streptomyces and Klebsiella. Morphologies observed under SEM of pure cultures of these genera on sterile alder leaf discs were similar to those observed in vivo and a known mixture of bacteria colonised sterile alder leaf discs in vitro in a pattern similar to that observed in vivo. A survey of production of pectinases, cellulases and xylanases by representatives of each genus showed thatrepresentatives of all class of enzymes were produced although not all isolates were shown to produce the appropriate enzymes. Bacterial activity in pure cultures and in mixed culture with the aquatic Hyphomycete Tricladium splendens, however, showed that with the exception of Aeromonas significant reductions in dry weight of alder leaves were affected by all isolates. The largest weight losses were caused by Streptomyces sp, the mixture of bacteria and splendens and Cytophaga all of which possess cellulolytic enzymes. This suggests tht cellulolytic activity accounted for the greatest reduction in dry weight.