Carbohydrate-active enzymes (CAZymes) have significant biotechnological potential as agents for degradation or modification of polysaccharides/glycans. As marine macroalgae are known to be rich in various types of polysaccharides, seaweed-associated bacteria are likely to be a good source of these CAZymes. A genomics approach can be used to explore CAZyme abundance and diversity, but it can also provide deep insights into the biology of CAZyme producers and, in particular, into molecular mechanisms that mediate their interaction with their hosts. In this study, a Gram-negative, aerobic, rod-shaped, carrageenolytic, and culturable marine bacterium designated as AOL6 was isolated from a diseased thallus of a carrageenan-producing farmed rhodophyte, Kappaphycus alvarezii (Gigartinales, Rhodophyta). The whole genome of this bacterium was sequenced and characterized. Sequence reads were assembled producing a high-quality genome assembly. The estimated genome size of the bacterium is 4.4 Mb and a G+C content of 52%. Molecular phylogenetic analysis based on a complete sequence of 16S rRNA, rpoB, and a set of 38 single-copy genes suggests that the bacterium is an unknown species and represents a novel genus in the family Cellvibrionaceae that is most closely related to the genera Teredinibacter and Saccharophagus. Genome comparison with T. turnerae T7901 and S. degradans 2-40 reveals several features shared by the three species, including a large number of CAZymes that comprised > 5% of the total number of protein-coding genes. The high proportion of CAZymes found in the AOL6 genome exceeds that of other known carbohydrate degraders, suggesting a significant capacity to degrade a range of polysaccharides including κ-carrageenan; 34% of these CAZymes have signal peptide sequences for secretion. Three putative κ-carrageenase-encoding genes were identified from the genome of the bacterium via in silico analysis, consistent with the results of the zymography assay (with κ-carrageenan as substrate). Genome analysis also indicated that AOL6 relies exclusively on type 2 secretion system (T2SS) for secreting proteins (possibly including glycoside hydrolases). In relation to T2SS, the product of the pilZ gene was predicted to be highly expressed, suggesting specialization for cell adhesion and secretion of virulence factors. The assignment of proteins to clusters of orthologous groups (COGs) revealed a pattern characteristic of r-strategists. Majority of two-component system proteins identified in the AOL6 genome were also predicted to be involved in chemotaxis and surface colonization. These genomic features suggest that AOL6 is an opportunistic pathogen, adapted to colonizing polysaccharide-rich hosts, including carrageenophytes.
Keywords: CAZymes; Cellvibrionaceae; Kappaphycus alvarezii; Phylogenetics; Two-component system (TCS).