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 the microbial community in such enrichments. Denaturing gradient gel electrophoresis (DGGE) profiles that were obtained during mucopolysaccharide degradation showed a substantial number of sequence types, suggesting the occurrence of multiple bacterial species in the enrichments. Only after the rate of mucopolysaccharide degradation had slowed down to less than 5% of its initial value could a significant change in the relative abundance of certain bacterial species in the enrichments be detected. Therefore, degradation of 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 sediments), and grown under either oxic or anoxic conditions, appeared to contain very different microbial communities with only a few overlapping species. Therefore, the selection pressure imposed by mucopolysaccharides as growth substrates is only one of the factors shaping the species composition in the enrichments. Attempts to isolate pure cultures of bacteria capable of mucopolysaccharide degradation using plating and dilution techniques failed. However, following a new approach which couples community analysis (using DGGE) and the physiological capability of the enrichment to degrade mucopolysaccharides, bacteria involved in the degradation process could be identified. This was based on the correlation between inhibition of mucopolysaccharide degradation and absence of certain bands from DGGE profiles when enrichment cultures were incubated at an elevated temperature. Phylogenetic analysis on clones of DNA fragments that were excised from DGGE gels, placed the putative mucopolysaccharide degraders in the α and γ subdivisions of the Proteobacteria, the Cytophaga-Flexibacter cluster, and the Planctomyces and Verrucomicrobiales clade. These findings directly link representatives of these abundant bacterial clusters with the degradation of complex algal polymers in the sea.