AUTHOR=Schwarzenberger Anke , Kurmayer Rainer , Martin-Creuzburg Dominik 

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

JOURNAL=Frontiers in Microbiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2020.586120

DOI=10.3389/fmicb.2020.586120

ISSN=1664-302X

ABSTRACT=<p>The harmful bloom-forming cyanobacterium <italic>Planktothrix</italic> is commonly considered to be nutritionally inadequate for zooplankton grazers, resulting in limited top-down control. However, interactions between <italic>Planktothrix</italic> and zooplankton grazers are poorly understood. The food quality of <italic>Planktothrix</italic> 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 <italic>Daphnia</italic> feeding on one <italic>Planktothrix rubescens</italic> and one <italic>P. agardhii</italic> wild-type/microcystin knock-out mutant pair. Our data suggest that the poor food quality of both <italic>Planktothrix</italic> 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 <italic>Planktothrix</italic> strain and the <italic>Daphnia</italic> clone that is used for the experiments. The toxin-responsive gene expression (transporter genes, <italic>gpx</italic>, and <italic>trypsin</italic>) and enzyme activity patterns revealed strain-specific food quality constraints and that <italic>Daphnia</italic> is capable of modulating its physiological responses according to the ingested <italic>Planktothrix</italic> strain. Future studies need to consider that <italic>Planktothrix</italic>–grazer interactions are simultaneously modulated by multiple factors to improve our understanding of top-down influences on <italic>Planktothrix</italic> bloom formation.</p>