Holdfast adhesion in the kelp <i>Alaria marginata</i> : Cell wall polysaccharides and phenolics

Adhesion is a critical process in the survival of seaweeds, especially in wave-swept environments. Kelp (Laminariales) sporophytes secrete adhesives from specialized rhizoid cells to anchor themselves to the substrate and withstand immense forces applied by waves. Although the composition of macroalgal adhesive has been described primarily in the Fucales, the current adhesive model is incomplete, and similarities between kelp and fucoid adhesives are unknown. Moreover, the cellular organization of the attachment interface of kelps is poorly understood. In this study, we examined the composition of rhizoid cells and adhesive from developing holdfasts of the kelp Alaria marginata using brightfield, fluorescence, and scanning electron microscopy. Rhizoid cells form a thick, intertwined mat cemented with adhesive. We provide the highest resolution immunofluorescent analysis of kelp adhesives to date using confocal microscopy: Kelp adhesives are composed of fucose-containing sulfated polysaccharides (FCSPs) and alginate, dominated by BAM3 and BAM4 epitopes. Holdfast adhesives are biochemical extensions of the cell wall with distinct composition from the wall proper. This work advances our basic understanding of kelp adhesion and highlights gaps in our current understanding of adhesive composition, raising questions, for example, about the roles of phenolics and proteins. Biochemical and histological methods presented here set the stage for future examination of the developmental, ecological, and evolutionary dynamics of adhesion.