Composition of the bacterial community degrading Phaeocystis mucopolysaccharides in enrichment cultures

As described recently (Janse et al. 1999; Limnol Oceanogr 44(6):1447-1457), mucopolysaccharides of the marine microalga Phaeocystis can be degraded in enrichment cultures. In this paper we report on the characterization of th e microbial community in such enrichments. Denaturing gradient gel electrop horesis (DGGE) profiles that were obtained during mucopolysaccharide degrad ation showed a substantial number of sequence types, suggesting the occurre nce of multiple bacterial species in the enrichments. Only after the rate o f mucopolysaccharide degradation had slowed down to less than 5 % of its in itial value could a significant change in the relative abundance of certain bacterial species in the enrichments be detected. Therefore, degradation o f this complex substrate does not seem to require a succession of bacterial populations. Several mucopolysaccharide-degrading enrichments obtained by inoculation from different sources (colony mucus, the water column and sedi ments), and grown under either oxic or anoxic conditions, appeared to conta in very different microbial communities with only a few overlapping species . Therefore, the selection pressure imposed by mucopolysaccharides as growt h substrates is only one of the factors shaping the species composition in the enrichments. Attempts to isolate pure cultures of bacteria capable of m ucopolysaccharide degradation using plating arid dilution techniques failed . However, following a new approach which couples community analysis (using DGGE) and the physiological capability of the enrichment to degrade mucopo lysaccharides, bacteria involved in the degradation process could be identi fied. This was based on the correlation between inhibition of mucopolysacch aride degradation and absence of certain bands from DGGE profiles when enri chment cultures were incubated at an elevated temperature. Phylogenetic ana lysis on clones of DNA fragments that were excised from DGGE gels, placed t he putative mucopolysaccharide degraders in the alpha and gamma subdivision s of the Proteobacteria, the Cytophaga-Flexibacter cluster, and the Plancto myces and Verrucomicrobiales clade. These findings directly Link representa tives of these abundant bacterial clusters with the degradation of complex algal polymers in the sea.