Protists — A Dominant Component of the Ballast-Transported Biota

Abstract

Ship’s ballast water and sediments serve as a main vector in the transportation and spreading of protists: toxic dinoflagellates, parasitic labyrinthulids and other potentially harmful and harmless unicellular beings. The omnipresence of protists in the sea means that photo- and heterotrophic protists invariably occur in ballast water and sediments. Ballast tanks — especially in container vessels with a high turnover of ballast water — serve as mesocosms supporting rich assemblages of heterotrophic protists. We estimate that more than 250 protist taxa are commonly present in a ballast tank, since their size and diet favour survival in there. As protists form an important part of the marine food web, otherwise harmless protists may be indirectly involved in the successful transport of bioinvaders by increasing the chances of survival of entrained filter-feeding and biofilm-grazing metazoans. Furthermore, we suggest that ballast-transported protists, harmless in their native habit ts, may modify or trigger changes in native assemblages, or may affect them by wholly outcompeting or partially displacing native taxa. Of momentous concern is whether protists, that are harmless in their native habitats, may become toxic after ballast-mediated spreading into the microbial associations of a recipient area, or vice versa, that the introduction of bacteria may start toxin production in formerly harmless eukaryotes.