Chemotaxis and appetence of Paulsenella sp. (Dinophyta), an ectoparasite of the marine diatom Streptotheca thamesis Shrubsole.

The parasitic dinophyte, Paulsenella sp., is attracted chemotactically by its host, the centric diatom Streptotheca thamesis. Seemingly rather short-lived components of the diatom mucilage are involved in the process. These components are presumably secreted by the Golgi apparatus and pass the thecal slit between epi-and hypocingulum. Experiments with concanavalin A indicate that glucose and-or mannose are constituents of the effective component, which is obviously not produced by egg cells, sperm cells and auxozygotes, since these stages do not attract the parasite. Plasmolysis inhibits secretion of the effective component. The readiness of the parasite to attack a host cell (its appetence) is light dependent: it is low in the dark period and increases after the beginning of the light period in cultures with a 14 h light/10 h dark regime. Endogenous rhythms of the parasite do not seem to be involved. In contrast, the attractiveness of the host is not influenced by the illumination.