Diatom adhesive trail proteins acquired by horizontal gene transfer from bacteria serve as primers for marine biofilm formation

Abstract

Summary: Biofilm-forming benthic diatoms are key primary producers in coastal habitats, where they frequently dominate sunlit submerged and intertidal substrata. The development of a unique form of gliding motility in raphid diatoms was a key molecular adaptation that contributed to their evolutionary success. Gliding motility is hypothesized to be driven by an intracellular actin-myosin motor and requires the secretion of polysaccharide- and protein-based adhesive materials. To date, the structure-function correlation between diatom adhesives utilized for gliding and their relationship to the extracellular matrix that constitutes the diatom biofilm is unknown. Proteomics analysis of the adhesive material from Craspedostauros australis revealed eight novel, diatom-specific proteins. Four of them constitute a new family of proteins, named Trailins, which contain an enigmatic domain termed Choice-of-Anchor-A (CAA). Immunostaining demonstrated that Trailins are only present in the adhesive trails required to generate traction on native substrata, but are absent from the extracellular matrix of biofilms. Phylogenetic analysis and Protein 3D structure prediction suggests that the CAA-domains in Trailins were obtained from bacteria by horizontal gene transfer, and exhibit a striking structural similarity to ice-binding proteins. Our work advances the understanding of the molecular basis for diatom underwater adhesion and biofilm formation providing evidence that there is a molecular switch between proteins required for initial surface colonization and those required for 3D biofilm matrix formation.