Potential toxicity of chrysophytes affiliated with Poterioochromonas and related 'Spumella-like' flagellates

The chrysophyte genera Poterioochromonas and Ochromonas andtheir heterotrophic analogons, i.e. the ‘Spumella-like’flagellates, account for a significant and often dominatingfraction of the pelagic nanoplankton. Even though several osmotrophicallyand autotrophically grown strains of Ochromonas and Poterioochromonasare assumed to produce toxins, the potential toxicity has beeninvestigated neither for its association with bacterivorousnutrition nor within the related exclusively heterotrophic ‘Spumella-like’flagellates. We investigated the toxic potential of severalflagellate strains using cultures of flagellates, cell extractsand filtrate of flagellate cultures. The effect on potentialpredators was exemplarily tested for the cladoceran Daphniamagna and the rotifer Platyias sp. All tested heterotrophicand mixotrophic flagellate strains were toxic to zooplanktonat abundances exceeding 10⁴ flagellates mL^–1. For therotifers, survival on any of the flagellate strains was significantlylower than that in the control treatment (P < 0.001) alreadyafter 24 h. We conclude that (i) ‘Spumella-like’flagellates can be toxic to zooplankton, (ii) all tested flagellates,i.e. heterotrophic and mixotrophic flagellates, feeding phagotrophicallycan be toxic to zooplankton and (iii) sublethal effects maybe observed at typical field abundances, even though acute toxicityseems to be restricted to flagellate abundances observed onlyat peak events.