PRIMARY ADHESION OF <i>ENTEROMORPHA</i> (CHLOROPHYTA, ULVALES) PROPAGULES: QUANTITATIVE SETTLEMENT STUDIES AND VIDEO MICROSCOPY¹

TLDR

Quantitative kinetic analyses were applied for the first time to study settlement and adhesion of Enteromorpha propagules. The study aimed to use high‑resolution video microscopy to delineate settlement and adhesion stages of zoospores and zygotes and to examine the mechanisms and significance of density‑dependent spore adhesion. High‑resolution video microscopy was employed to capture the sequential stages of zoospore and zygote settlement and adhesion. Rapid adhesion of quadri‑ and biflagellate zoospores and zygotes was observed, with density‑dependent cooperativity (positive at low densities, negative at high), and detailed stages including temporary apical attachment, permanent commitment with vesicle discharge and flagella adsorption, followed by amoeboid surface exploitation and raft‑forming gregarious settlement.

Abstract

ABSTRACT 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 popagule. 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.