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.