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Investigations of the effects of grazing by 3 species of ciliates -Pseudomicrothorax dubius Peck, Nassula citrea Kahl and Furgasonia blochmanni Faure-Fremiet -on mats of Phormidium autumnale (Agardh) Gomont showed that the cyanobacterium was capable of surviving very strong grazer pressure, thanks to both an extracellular polysaccharide (EPS) layer and behavioral defense. In the presence of ciliates, cyanobacteria trichomes were confined to a protective layer of extracellular material enclosing the mat. Trichomes attacked by a ciliate usually escaped by retreating within the EPS layer. The effectiveness of this kind of defense was reflected in dramatic changes in the condition of the ciliates: within 30 h of the start of the experiments a significant fraction was starved because they were unable to reach trichomes hidden within the shielding EPS layer.

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Experiments were done with two strain of filamentous, mat-forming Phormidium and their ciliate grazer Pseudomicrothorax dubius, to explain why the ciliates remain hungry in an apparent surplus of food, except for the first 24 hours after feeding. Under grazing pressure, both strains of cyanobacteria showed statistically significant increases in the number of filaments terminating in an empty sheath, compared to the control. Direct observations revealed that the mechanism behind this effect was active withdrawal of the trichomes inside the sheaths when disturbed by grazers. As P. dubius is unable to ingest trichomes with such endings, we conclude that cyanobacteria are not limited to chemical means of defence against grazers but can also defend themselves by means of movement and changes in filament morphology. This is apparently the first report on behavioural defence observed in cyanobacteria.

In a series of experiments we investigated the ability of a cyanobacterium of the genus Phormidium to defend itself against the ciliate grazer Furgasonia blochmanni. The experiments showed that the cyanobacterium can employ different modes of defense, depending on grazer density. Under high grazer pressure, each mat of Phormidium instantly formed a dense clump surrounded by a mucilage layer. Trichomes remaining inside such clumps were completely inaccessible to ciliates. Under medium pressure, the defense reaction was slower and consisted in enhanced sheath production by individual trichomes. The effectiveness of such a defense was also very high, as clearly reflected in the condition of the ciliates. When the pressure was low, the cyanobacteria did not exhibit any form of defense, dispersing evenly all over the bottom in a pattern similar to that observed in control mats. We conclude that the cyanobacterium is able to modify its defense reaction according to the real grazing risk.KEY WORDS: Cyanobacteria · Anti-herbivore defense · Ciliate pressure Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 27: [149][150][151][152][153][154][155][156][157] 2002 and Ph2, revealed an inducible defense capability against ciliates by means of withdrawing inside the sheaths when disturbed by grazers. Trichomes ending in empty sheaths turned out to be completely inaccessible for the ciliates (Fia8kowska & Pajdak-Stós 1997). In Phormidium autumnale, the same purpose is served by a protective layer of extracellular polysaccharides (EPS) shielding the initial piece of mat in which filaments are embedded; the defense reaction is engaged in the presence of different grazer species (Pajdak-Stós et al. 2001). However, the phenomenon was investigated only at invariable grazer density. During closer observations of the cyanobacteria mats used for feeding our ciliate cultures, we noticed interesting changes in their appearance. We suspected that those changes might be related to different ciliate densities in the culture dishes and designed a set of experiments to determine whether the cyanobacteria mode of defense depends on grazer pressure. MATERIALS AND METHODSA strain of an unidentified species of the cyanobacterium Phormidium called Ph2 in our previous paper (Fia8kowska & Pajdak-Stós 1997) and a clone of a nassulinid ciliate Furgassonia blochmanni were used for experiments. This free-living, freshwater ciliate occurs naturally in ponds and puddles along with filamentous cyanobacteria like Ph2, which form mats on the sediment surface. F. blochmanni is equipped with a cytopharyngeal apparatus, a highly specialized structure that enables the ciliate to ingest long threads of cyanobacteria (Eisler & Bardele 1986, Vigues et al. 1999.Clonal populations of experimental organisms were obtained from a single filament (Phormidium sp.) and a single cell (Furgassonia blochmanni) taken from an aquarium maintained non-axenically at the Department of Hydrobiology, Jagiellonian University, ...