The multifunctional roles of the extracellular matrix in the sessile life of the zygnematophyte Penium margaritaceum: stick, glide and cluster

Adhesion and consequent adoption of a sessile habit is a common feature of many green algae and was likely a key mechanism in terrestrialization by an ancient zygnematophyte (i.e., the Zygnematophyceae, the group of algae ancestral to land plants). Penium margaritaceum is a unicellular zygnematophyte that exhibits a multistep adhesion mechanism, which leads to the establishment of the sessile habit. Based on microscopic and immunological data, a dense aggregate of fibrils containing arabinogalactan‐protein (AGP)‐like components covers the cell surface and is responsible for initial adhesion. The AGP‐like fibrils are 20 μm in diameter and possess chemical profiles similar to land plant AGPs. The fibrils attach to the inner cell wall layers and are very likely connected to the plasma membrane as glycophosphatidylinositol (GPI) lipid‐anchored proteins, as they are susceptible to phospholipase C treatment. The presence of GPI‐anchored AGPs in Penium is further supported by the identification of putative Penium homologs of land plant AGP genes responsible for GPI‐anchor synthesis. After adhesion, cells secrete a complex heteropolysaccharide‐containing extracellular polymeric substance (EPS) that facilitates gliding motility and the formation of cell aggregates. Fucoidan‐like polymers, major components of brown algal CWs, are a major constituent of both the EPS and the adhesive layer of the CW and their role in the adhesion process is still to be examined.