Abstract
Gene clusters rich in carbohydrate-active enzymes within Flavobacteriia genera provide a competitiveness for their hosts to degrade diatom-derived polysaccharides. One such widely distributed polysaccharide is glucuronomannan, a main cell wall component of diatoms. A conserved gene cluster putatively degrading glucuronomannan was found previously among various flavobacterial taxa in marine metagenomes. Here, we aimed to visualize two glycoside hydrolase family 92 genes coding for α-mannosidases with fluorescently-labeled polynucleotide probes using direct-geneFISH. Reliable in situ localization of single-copy genes was achieved with an efficiency up to 74% not only in the flavobacterial strains Polaribacter Hel1_33_49 and Formosa Hel1_33_131 but also in planktonic samples from the North Sea. In combination with high-resolution microscopy, direct-geneFISH gave visual evidence of the contrasting lifestyles of closely related Polaribacter species in those samples and allowed for the determination of gene distribution among attached and free-living cells. We also detected highly similar GH92 genes in yet unidentified taxa by broadening probe specificities, enabling a visualization of the functional trait in subpopulations across the borders of species and genera. Such a quantitative insight into the niche separation of flavobacterial taxa complements our understanding of the ecology of polysaccharide-degrading bacteria beyond omics-based techniques on a single-cell level.