ABSTRACTBacterial adhesion is affected by environmental factors, such as ionic strength, pH, temperature, and shear forces. Therefore, marine bacteria must have developed adhesins with different compositions and structures than those of their freshwater counterparts to adapt to their natural environment. The dimorphic alphaproteobacteriumHirschia balticais a marine budding bacterium in the cladeCaulobacterales.H. balticauses a polar adhesin, the holdfast, located at the cell pole opposite the reproductive stalk, for surface attachment and cell-cell adhesion. The holdfast adhesin has been best characterized inCaulobacter crescentus, a freshwater member of theCaulobacterales, and little is known about holdfast compositions and properties in marineCaulobacterales. Here, we useH. balticaas a model to characterize holdfast properties in marineCaulobacterales. We show that freshwater and marineCaulobacteralesuse similar genes in holdfast biogenesis and that these genes are highly conserved among the species in the two genera. We determine thatH. balticaproduces a larger holdfast thanC. crescentusand that the holdfasts have different chemical compositions, as they containN-acetylglucosamine and galactose monosaccharide residues and proteins but lack DNA. Finally, we show thatH. balticaholdfasts tolerate higher ionic strength than those ofC. crescentus. We conclude that marineCaulobacteralesholdfasts have physicochemical properties that maximize binding in high-ionic-strength environments.IMPORTANCEMost bacteria spend a large part of their life spans attached to surfaces, forming complex multicellular communities called biofilms. Bacteria can colonize virtually any surface, and therefore, they have adapted to bind efficiently in very different environments. In this study, we compare the adhesive holdfasts produced by the freshwater bacteriumC. crescentusand a relative, the marine bacteriumH. baltica. We show thatH. balticaholdfasts have a different morphology and chemical composition and tolerate high ionic strength. Our results show that theH. balticaholdfast is an excellent model to study the effect of ionic strength on adhesion and provides insights into the physicochemical properties required for adhesion in the marine environment.