Dynamics of cyanobacteria-ciliate grazer activity in bitrophic and tritrophic microcosms

Abstract

Direct and indirect interactions among components of bitrophic and tritrophic communities were studied using laboratory microcosms. The filamentous cyanobacterium Phormidium sp., capable of inducible defence, was used as the primary producer. Phormidium reacts to ciliate attacks by withdrawing inside a polysaccharide envelope, overproducing exopolysaccharide material and remaining in dense and compact clumps. All of these defences are induced by the ciliate Pseudomicrothorax dubius, a specialised grazer of filamentous cyanobacteria, representing the second trophic level in our system. The gymnostomatid ciliate Homalozoon vermiculare, which preys on Pseudomicrothorax but does not directly affect cyanobacteria, was the top predator within the microcosm community. The experiment showed that Homalozoon, very effective in a simple bitrophic cascade, had little effect on its prey when Phormidium was introduced into the system. Under grazer pressure, the cyanobacterium defended itself against the grazer by creating clumps of entangled filaments that also served as refuges for Pseudomicrothorax from Homalozoon. The prey ciliate Pseudomicrothorax reacts to cyanobacterial defence by increasing its encystation rate. Gradually decreasing grazing pressure resulted in a diminished Phormidium defence reaction, which enabled Pseudomicrothorax to resume feeding on filaments. Changing the extent of induced defence in Phormidium thus resulted in the stabilisation of the microcosm community. © Inter-Research 2010.