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

Abstract

Quantitative methods and associated kinetic analyses have been used for the$rst time to study detailed aspects of the settkment and adhesion of various types of Enteromorpha popagule. Time course expm'ments showed that quadri-and biJlagellate zoospores and zygotes adhered rapidly, but a proportion within any one population appeared to be incompetent at adhm'ng to the substratum. Kinetic (Scatchard) analysis of adhesion expm'ments pe$ormed 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 microscq@ was used fw thejrst time to reveal details Ofthe various stages in the settlement and adhesion of zoospores and zygotes. Novel obseruations were made of a n initial, temporary phase of attachment via the apical papilla, followed by a permanent phase of commitment, charactm'ed by discharge of adhesive-containing cytoplasmic vesicles, as the cell contracted against the suqace, and adsm-ption of p agella. ?'he phase of commitment was followed by exploitation of the surface through amoeboid-like movements at the interj4ace. Gregarious settlement behavior was frequently o b serued leading to the formation of rafts of cells. The possible mechanisms and signifcance of density-dependent spore adhesion are discussed.

Settlement Behavior of Zoospores of Ulva linza During Surface Selection Studied by Digital Holographic Microscopy

“…This spinning motion could take up to several minutes, but most spores left the surface soon after spinning was initiated and continued exploration. A minority of spores spun for a longer time and finally settled permanently, which involves secretion of adhesive and retraction of the flagella [2,3]. Figure 4c-e shows the occurrence of the different motion patterns depending on the surface chemistry as they occurred in the volume extending 200 lm away from the surface.…”

“…The analysis of spore motility by DHIM, coupled with the information gleaned by video microscopy in earlier investigations [2], leads us to the model (Fig. 7), which illustrates the course of events as spores select a suitable surface for settlement.…”

“…Soon after this temporary 'sticking' event, spores initiate a spinning motion (d) that varies in length, depending on the surface chemistry. Video evidence suggests that the spinning event may involve the secretion of a small amount of temporary adhesive as an elastic pad [2], although there is no direct biochemical evidence for the existence of this adhesive. Spinning spores may then either initiate permanent adhesion (e), but the majority of spores leave the surface to continue exploration (f).…”

“…In order to complete the life cycle, zoospores must locate a surface and settle on it (i.e., permanently attach to it). Previous investigations by video microscopy (2) revealed that spores show complex swimming behaviors as they approach and make contact with surfaces. The attachment process appears to involve an initial, temporary, and reversible phase of varying duration, and is characterized by a rapid ''top-like spinning'', although in certain circumstances spores may also settle without spinning.…”

“…Final commitment to irreversible and permanent adhesion involves the discharge of a preformed glycoprotein adhesive from cytoplasmic vesicles, as the cell contracts against the surface. The membrane of the sheaths surrounding the flagella are shed into the water, the axonemes are retracted into the settling spore [2,3]. The attached spore secretes a cell wall and can then germinate to grow into the macroscopic, easily visible green seaweed.…”