Swarm-like migratory behaviour in the laboratory of a pennate diatom isolated from North Sea sediments

An unidentified Navicula species was isolated from the surface sediments in two different localities of the German North Sea shore in 1997. Cells were maintained in culture on agar plates under various environmental conditions. The diatom cells showed a previously undescribed migratory behaviour, moving synchronically from the starting point towards the periphery of the Petri dish, resulting in a swarm-like migration and giving rise to ring-like banding patterns. This mode of movement takes place horizontally on a rather small scale (cm) and usually within days; it might represent, in addition to dispersal by water, another way of colonizing new areas of its natural environment. Ring formation occurred only when diatom cells were cultured on agar plates either in continuous white light or in a 14: 10 h light: dark regime at an irradiance between 5 and 10 mu mol photons m(-2) s(-1), and not when the agar plates were kept in the dark. Salinity above or beyond 34 psu, as well as a lack of nitrate or phosphate had a negative impact on this migratory behaviour and resulted in reduced numbers of rings or lack of migration. This migratory pattern was recorded repeatedly from October to the end of March, but ceased to occur afterwards.