Membrane recycling and calcium dynamics during settlement and adhesion of zoospores of the green alga Ulva linza

Abstract

Recruitment of individuals of the marine alga Ulva linza on to a suitable habitat involves the settlement of motile zoospores on to a substratum during which a preformed adhesive is secreted by vesicular exocytosis. The fluorescent styryl dye FM 1-43 and fluorescent Ca2+ indicators were used to follow membrane cycling and changes in cytosolic Ca2+ ([Ca 2+]cyt) associated with settlement. When swimming zoospores were exposed continuously to FM 1-43, the plasma membrane was preferentially labelled. During settlement, FM 1-43-labelled plasma membrane was rapidly internalized reflecting high membrane turnover. The internalized membrane was focused into a discrete region indicating targeting of membrane to an endosome-like compartment. Acetoxymethyl (AM)-ester derivatives were found to be unsuitable for monitoring [Ca2+]cyt because the dyes were rapidly sequestered from the cytoplasm into sub-cellular compartments. [Ca2+]cyt was, however, reliably measured using dextran-conjugated calcium indicators delivered into cells using a biolistic technique. Cells loaded with Oregon Green BAPTA-1 dextran (Invitrogen, Paisley, UK) showed diffuse cytosolic loading and reliably responded to imposed changes in [Ca2+]cyt. During settlement, zoospores exhibited both localized and diffuse increases in [Ca2+]cyt implying a role for [Ca2+]cyt in exocytosis of the adhesive. © 2007 The Authors.