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The harmful bloom-forming cyanobacterium  Planktothrix  is commonly considered to be nutritionally inadequate for zooplankton grazers, resulting in limited top-down control. However, interactions between  Planktothrix  and zooplankton grazers are poorly understood. The food quality of  Planktothrix  is potentially constrained by morphological properties (i.e., filament formation), the production of harmful secondary metabolites, and a deficiency in essential lipids (i.e., primarily sterols). Here, we investigated the relative significance of toxin production (microcystins, carboxypeptidase A inhibitors, protease inhibitors) and sterol limitation for the performance of  Daphnia  feeding on one  Planktothrix rubescens  and one  P. agardhii  wild-type/microcystin knock-out mutant pair. Our data suggest that the poor food quality of both  Planktothrix  spp. is due to deleterious effects mediated by various harmful secondary metabolites and that the impact of sterol limitation is partially or completely superimposed by toxicity. The significance of the different factors seems to depend on the metabolite profile of the considered  Planktothrix  strain and the  Daphnia  clone that is used for the experiments. The toxin-responsive gene expression (transporter genes,  gpx , and  trypsin ) and enzyme activity patterns revealed strain-specific food quality constraints and that  Daphnia  is capable of modulating its physiological responses according to the ingested  Planktothrix  strain. Future studies need to consider that  Planktothrix –grazer interactions are simultaneously modulated by multiple factors to improve our understanding of top-down influences on  Planktothrix  bloom formation.

frontiers in microbiology

Toward Disentangling the Multiple Nutritional Constraints Imposed by Planktothrix: The Significance of Harmful Secondary Metabolites and Sterol Limitation