Chemical and mechanical signals in inducing Phormidium ( Cyanobacteria ) defence against their grazers

The nature of stimuli evoking cyanobacterium defence was investigated in experiments on Phormidium sp. strain able to defend itself against ciliate grazers. Limited dispersion of trichomes in reaction to Pseudomicrothorax dubius separated from cyanobacterium with a mesh insert indicates the existence of a chemical cue originating from the ciliates. Grazers released into the wells where trichomes' dispersion was already limited by the cue initially had no difficulty finding food, but started to starve 24 h later. Similar situation was observed in control wells. Direct observations of trichomes attacked by the ciliates showed a distinct difference between the trichomes previously subjected to mesh‐separated ciliate and the control ones. The former withdrew more frequently into a rigid sheath, whereas the latter usually withdrew into elastic tubes. This suggests that both chemical and mechanical stimuli are necessary to express cyanobacterium defence to the fullest extent. Further investigations showed that ciliates specialised in ingesting filamentous Cyanobacteria limit trichomes' dispersion, whereas filter‐feeding Euplotes and Cyanobacteria ‐feeding rotifer do not. The cyanobacterium can detect grazer presence even without direct contact and modify its morphology in a way enabling full expression of defence reaction. This is the first report on ciliate–cyanobacterium chemical mediation.