Detection of Chitinolytic Capabilities in the Freshwater Planctomycete Planctomicrobium piriforme

Members of the phylum Planctomycetes colonize a wide range of aquatic and terrestrial habitats, but their functional role in the environment remains poorly understood. One of the suggested roles of these bacteria is their participation in degradation of biopolymers of plant and fungal cell walls. The first experimental evidence for the presence of chitin-degrading ability was obtained for Fimbriiglobus ruber SP5^T. The genome of this planctomycete encodes the complete set of enzymes required for utilization of chitin, including the chitinase affiliated with the glycoside hydrolase family GH18. Phylogenetic analysis of the chitinase from Fimbriiglobus revealed occurrence of its close homologue in the genome of another planctomycete, Planctomicrobium piriforme Р3^Т. Experimental tests performed with the type strain Р3^Т and the newly obtained isolate of this species, strain РХ70, confirmed their ability to grow on amorphous chitin as a source of carbon and nitrogen. Activities of the enzymes involved in chitin degradation, which were determined in tests with fluorochrome-labeled chito-oligosaccharides, were exclusively associated with planctomycete cells and were absent from the culture liquid. Further search in the protein database revealed a number of close homologues of these chitinases in other planctomycetes affiliated with different taxonomic groups. Moreover, phylogenetic analysis pointed to planctomycetes as a possible source of multiple horizontal transfers of chitinase genes to other bacterial phyla, as well as to some eukaryotes. Obtaining proofs for chitin-degrading capabilities in Planctomicrobiumpiriforme suggests emendation of the earlier published description of this species and extends the list of planctomycetes with experimentally confirmed chitinolytic potential.