Trap fluid of aquatic carnivorous plants of the genus Utricularia hosts specific microbiomes consisting of commensal pro- and eukaryotes of largely unknown ecology. We examined the characteristics and dynamics of bacteria and the three dominant eukaryotes, i.e. the algae-bearing ciliate Tetrahymena utriculariae (Ciliophora), a green flagellate Euglena agilis (Euglenophyta), and the alga Scenedesmus alternans (Chlorophyta), associated with the traps of Utricularia reflexa. Our study focused on ecological traits and life strategies of the highly abundant ciliate whose biomass by far exceeds that of other eukaryotes and bacteria independent of the trap age. The ciliate was the only bacterivore in the traps, driving rapid turnover of bacterial standing stock. However, given the large size of the ciliate and the cell-specific uptake rates of bacteria we estimated that bacterivory alone would likely be insufficient to support its apparent rapid growth in traps. We suggest that mixotrophy based on algal symbionts contributes significantly to the diet and survival strategy of the ciliate in the extreme (anaerobic, low pH) trap-fluid environment. We propose a revised concept of major microbial interactions in the trap fluid where ciliate bacterivory plays a central role in regeneration of nutrients bound in rapidly growing bacterial biomass.

Biology Centre CAS v v i Institute of Hydrobiology Na Sádkách 7 České Budějovice 370 05 Czech Republic

Biology Centre CAS v v i Institute of Hydrobiology Na Sádkách 7 České Budějovice 370 05 Czech Republic Faculty of Science University of South Bohemia Branišovská 31 České Budějovice 370 05 Czech Republic

Institute of Botany CAS Section of Plant Ecology Třeboň 379 82 Czech Republic

Limnological Station Department of Plant and Microbial Biology University of Zurich Kilchberg 8802 Switzerland

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