TOXICITY OF THE MIXOTROPHIC CHRYSOPHYTE POTERIOOCHROMONAS-MALHAMENSISTO THE CLADOCERAN DAPHNIA-AMBIGUA

The toxicity of the mixotrophic chrysophyte Poterioochromonas malhamen sis to the common cladoceran Daphnia ambigua was investigated with lab oratory survivorship experiments and observations of feeding behavior. When cultured autotrophically in an inorganic medium, P. malhamensis provided sufficient nutrition for survival and reproduction by D. ambi gua. However, P. malhamensis grown heterotrophically on an organic med ium (osmotrophically) in the dark or light was toxic to D. ambigua, ki lling all experimental animals within five days. Toxicity was not caus ed by the organic medium since daphnids fed the autotroph Chlamydomona s reinhardi that was grown in the organic medium had high survivorship . Changes in feeding behavior supported the results from the survival studies. Daphnids feeding in suspensions of heterotrophically cultured P. malhamensis exhibited significantly greater food rejection rates t han animals fed autotrophically cultured P. malhamensis or C. reinhard i. Differences in the growth rate or biochemical pathways of heterotro phically versus autotrophically grown P. malhamensis likely influenced toxin production. Poterioochromonas may affect zooplankton feeding an d behavior in nature if heterotrophic toxin production is comparable b etween phagotrophic acid osmotrophic nutritional modes.