Host-driven evolution shapes the polysaccharide utilization profiles of alga-associated Flavobacteriaceae.
BACKGROUND: Marine algae represent major producers of complex polysaccharides and serve as hosts for diverse microbial communities in the phycosphere. Flavobacteriaceae are among the key bacterial taxa involved in polysaccharide degradation and carbon remineralization in this environment. However, the extent to which algal hosts drive the divergence of polysaccharide utilization profiles in these bacteria remains unclear. RESULTS: We conducted a genome-resolved analysis of 103 cultured Flavobacteriaceae strains isolated from red, green, and brown macroalgae, as well as from diatoms and dinoflagellates. We found that macroalga-associated strains generally harbored more abundant and diverse CAZyme-encoding genes than their microalga-associated counterparts. Moreover, strains associated with different algal phyla showed distinct metabolic specializations that aligned with the typical polysaccharides of their respective hosts, strongly supporting host-specific adaptation. In four widely distributed genera (Maribacter, Flagellimonas, Polaribacter, Winogradskyella), CAZyme profile dissimilarity and key glycoside hydrolase gene divergence exhibited phylogenetic congruence with algal host phylogeny (Mantel r up to 0.76 and 0.85, respectively), indicative of host-associated functional adaptation. Using Maribacter as a model, cultivation experiments and transcriptome characterization demonstrated that polysaccharide utilization efficiency is not solely linked to the organization of genes into polysaccharide utilization loci (PULs), but also associated with the expression dynamics of key transcription factors (TFs), particularly those from AraC and DeoR families, whose expression patterns were coordinated with laminarin degradation. Notably, these two TF families also exhibited host-associated divergence patterns similar to those of CAZyme-encoding genes. Furthermore, analysis of the Tara Oceans metagenomic data indicated that, within the AraC and DeoR families, a higher proportion of genes were positively correlated with chlorophyll a content compared to other TF families, reinforcing their specialized roles in alga-associated bacterial lifestyles. CONCLUSIONS: Our integrative genomic and transcriptomic analyses reveal evolutionary and regulatory adaptation of marine Flavobacteriaceae to distinct algal hosts. These findings highlight algae-derived habitats as specialized niches that shape microbial metabolic potential, and suggest that carbohydrate metabolism plays a key role in host-driven bacterial evolution across global oceans. Video Abstract.