Of Terms in Biology: Zoospore
by Elio
Forget about everyday spores, by which most microbiologists mean the endospores of Bacillus, Clostridium, and a few other bacteria. Zoospores are motile asexual spores made by some protists, fungi, and bacteria that actively travel and propagate their species. They are distinctly not resting structures. In the eukaryotic microbes, zoospores come in several varieties, distinct in their numbers generated per "mother" cell and their mode of insertion into the cell surface.
Electron micrographs of the reproductive structures and motile zoospores of Actinoplanes missouriensis (courtesy of Y. Ohnishi, A. Hirata, and colleagues) (A to C) and Actinoplanes regularis (reproduced with permission from the Digital Atlas of Actinomycetes, courtesy of G. Vobis, N. Muto, and colleagues, and The Society for Actinomycetes Japan) (D to F). A, D: Sporangia arising from the vegetative mycelium. B, E: Thin sections through the sporangia showing immature zoospores. C, F: Motile zoospores with numerous flagella originating from a small patch of the cell surface. The scale bars are 2 μm (A to C) and 5 μm (D). Source
Bacterial zoospores seem to be limited to some 5 genera of actinomycetes (Actinoplanes, Catenuloplanes, Kineosporia, Planomonospora, and Spirillospora). Aficionados of this group of bacteria should not be surprised, since the actinomycetes have an extraordinary variety of reproductive structures (see the Digital Album of Actinomycetes). Bacterial zoospores are variable in shape, some embodying quite complex structures.
Zoospores are chemotactic and swim towards attractants – for example, nutrients such as sugars, amino acids, and inorganic ions – usually for 2 to 3 hours. If they find nutrients they germinate into the vegetative mycelium. Otherwise, they die. The molecular biology of at least one of the zoospore-making actinos, Actinoplanes missouriensis, has been studied in some detail (see here). This fine paper is the subject of a recent comment in the Journal of Bacteriology.
Eukaryotic zoospores enter into this discussion because those of an oomycete, Pythium aphanidermatum help mobilize bacteria, especially non-flagellate ones, so that it may help recruit them to sites that need bioremediation. It is thought that the zoospores' propulsion thrust force is what moves the bacteria along.
But back to the prokaryotic zoospores. As Mark Buttner, the author of the JBact comment says: "…for those of us who work on Streptomyces, it is fair to say that bacterial movement is not often central to our thinking. Already this year, however, the exciting discovery of explorer cells in Streptomyces (covered here in STC) and now this ground-breaking analysis of zoospore-producing Actinoplanes will force us to broaden our thinking in a very welcome way."
