Primary adhesion of Enteromorpha (Chlorophyta, Ulvales) propagules: Quantitative settlement studies and video microscopy

Quantitative methods and associated kinetic analyses have been used for the first time to study detailed aspects of the settlement and adhesion of various types of Enteromorpha propagule. Time course experiments showed that quadri- and biflagellate zoospores and zygotes adhered rapidly, but a proportion within any one population appeared to be incompetent at adhering to the substratum. Kinetic (Scatchard) analysis of adhesion experiments performed at a range of zoospore concentrations revealed density-dependent effects not previously reported, with positive cooperativity at low spore densities and negative cooperativity at high spore densities. High-resolution video microscopy was used for the first time to reveal details of the various stages in the settlement and adhesion of zoospores and zygotes. Novel observations were made of an, initial, temporary phase of attachment via the apical papilla, followed by a permanent phase of commitment, characterized by discharge of adhesive-containing cytoplasmic vesicles, as the cell contracted against the surface, and adsorption of flagella. The Phase of commitment was followed by exploitation of the surface through amoeboid-like movements at the interface. Gregarious settlement behavior was frequently observed leading to the formation of rafts of cells. The possible mechanisms and significance of density-dependent spore adhesion are discussed.