Abstract
Aphanizomenon flos-aquae can form extensive blooms from freshwater to the brackish environment and, being a diazotrophic species, contribute significantly to the nitrogen and carbon cycle. It occurs as single filaments or aggregates and could be used as an alternative nutrients source for bacteria and ciliates. Ciliates are a group of organisms playing a crucial role in the transfer of nitrogen from primary producers to higher trophic levels in aquatic food webs. The aim of the experiment was to study the effects of the cyanobacterium A. flos-aquae on the community of five ciliate species (Spirostomum minus, Euplotes aediculatus, Strobilidium sp., Vorticella sp. and Paramecium tetraurelia). The response of each species to the presence of a low/high cyanobacterial biomass and to the different morphological forms of A. flos-aquae (single filaments or aggregates) was demonstrated. The results of the experiment showed the variability of interactions between the cyanobacterium A. flos-aquae and ciliates and pointed out the possible benefits that A. flos-aquae provides to the ciliates (e.g., a substrate for the development of bacteria as food for ciliates or as a source of nitrogen and carbon).