Ultrastructure of tetraspore germination in the agar-producing seaweed Gelidium floridanum (Gelidiales, Rhodophyta)

Spore germination is a crucial step in the dispersion and establishment of algae. Here we describe for the first time the ultrastructure of the initial stages of the tetraspore germination of Gelidium floridanum, a species considered to be of potential economical value in Brazil. The stages of the germination process of G. floridanum tetraspores were studied under light and transmission electron microscopy. When released, the spores had no cell walls and were surrounded by a mucilage envelope that enabled primary attachment of the spore to the substratum. Following spore release, numerous tubular invaginations of the plasma membrane became apparent. These structures appeared to be connected to the endoplasmic reticulum and may play a role in the transport of cell wall material. Small vesicles with fibrillar content, although abundant in the tetraspores, were no longer seen in the sporelings, suggesting that these vesicles contain the precursors of the cell wall material that accumulates during sporogenesis. We observed that, soon after attachment, cellulose deposition began at one pole of the cell and gradually extended over the spore Surface in a thin layer. As germination proceeded, a germ tube formed, to which most of the cytoplasmic content of the spore migrated. A cell division and the production of a cross wall separating the tube from the original spore followed. Subsequently. the germ tube elongated and underwent a series of transverse and diagonal septations, producing cells of various sizes. We can conclude that germination of tetraspores in G. floridanum depends on a basic process consisting of: attachment. cell wall deposition. activation of dictyosomes, mobilisation of starch accumulated during sporogenesis, polarised vacuolation, germ tube formation, and cell division.