The effect of attached vorticellids on the buoyancy of the colonial cyanobacteriumAnabaena lemmermannii

Abstract

Colonies of the cyanobacterium Anabaena lemmermannii, from the southern basin of Windermere in the English Lake district, had heavy infestations of attached vorticellids. The carnivorous ciliates Trachelius ovum and Acaryphrya sp. were grazing on the vorticellids, and an unidentified parasite was also present. Many of the Anabaena colonies appeared to swim, propelled by the action of the attached vorticellids' cilia. In samples left to stand, the majority of the colonies aggregated at the surface. This net upward movement was caused by the buoyancy provided by gas vesicles in the Anabaena cells; the colonies sank if the gas vesicles were collapsed by pressure. From measurements of the buoyant density of separated Anabaena filaments and detached vorticellids it was calculated that an average colony with a volume of 1·3 × 106 μm3 would buoy up 103 vorticellids. It was observed that single vorticellids could be buoyed up by filaments with as few as 19 cells.