Scent of Danger: Floc Formation by a Freshwater Bacterium Is Induced by Supernatants from a Predator-Prey Coculture

Abstract

We investigated predator-prey interactions in a model system consisting of the bacterivorous flagellate Poterioochromonas sp. strain DS and the freshwater bacterium Sphingobium sp. strain Z007. This bacterial strain tends to form a subpopulation of grazing-resistant microscopic flocs, presumably by aggregation. Enhanced formation of such flocs could be demonstrated in static batch culture experiments in the presence of the predator. The ratio of aggregates to single cells reached >0.1 after 120 h of incubation in an oligotrophic growth medium. The inoculation of bacteria into supernatants from cocultures of bacteria and flagellates (grown in oligotrophic or in rich media) also resulted in a substantially higher level of floc formation than that in supernatants from bacterial monocultures only. After separation of supernatants on a C 18 cartridge, the aggregate-inducing activity could be assigned to the 50% aqueous methanolic fraction, and further separation of this bioactive fraction could be achieved by high-pressure liquid chromatography. These results strongly suggest the involvement of one or several chemical factors in the induction of floc formation by Sphingobium sp. strain Z007 that are possibly released into the surrounding medium by flagellate grazing.Interactions between free-living aquatic bacteria and predatory flagellates are determined by the balance between bacterial cell growth and mortality rates (1,8,23). High levels of grazing mortality have favored the evolution of various bacterial counterstrategies, such as small cell sizes, high-speed motility, and the production of toxins and other growth inhibitors (for a review, see reference 25). The particle uptake abilities of predators set tight constraints on the size of the prey that is preferentially ingested. As a consequence, filamentous cells inedible by protists may accumulate in heavily grazed freshwater bacterial communities, as may cells with other complex morphologies, such as aggregates and microcolonies, that are beyond the prey size spectrum of the predators (14,18,27). The formation of such grazing-resistant flocs at high protistan foraging levels is known both from static and continuous culture (13, 26) and from enrichments of natural waters (17).A shift toward cell aggregates or microcolonies might simply be a result of the elimination of single-celled morphotypes (13) but could also reflect an active response of bacteria to the presence of predators. Two nonexclusive causes can be envisaged for the enhanced growth of cells in aggregated form under such conditions. For one, higher levels of floc formation might be a consequence of higher bacterial growth rates due to the release of additional substrates and nutrients by the predator (5,8,14,36). Second, such growth behavior might be induced by a chemical factor. Inducible morphological defense due to compounds released by the predators is common in other planktonic organisms, e.g., the spine induction in rotifers (12) and daphnids (20, 34) and the formation of grazing-resistant col...

Higher-order interaction inhibits bacterial invasion of a phototroph-predator microbial community

“…Previous measurements of ciliate feeding rates 29 , however, suggest that at these densities, C should be able consume most of the B present (see SI). We propose that the ability of B to sustain comparatively high densities in the presence of C is driven by B aggregation 30 . Bacterial aggregation is a common defense against predation due to the fact that the oral apparatus of the ciliates has a limited range of prey sizes it can accommodate 31, 32 .…”

Protistan predation interferes with bacterial long‐term adaptation to substrate restriction by selecting for defence morphotypes

“…These isolates were also preserved at −80 °C in 50% glycerol and regrown in DSMZ 7 medium as described above for further experiments. Axenic cultures of the bacterivorous mixotrophic flagellate predator Poterioochromonas strain DS were used for the experiments (Blom et al ., ). Stock cultures of the flagellates were maintained in nutrient‐rich ‘ Ochromonas medium’ (yeast extract 1 g L −1 ; meat extract 1 g L −1 ; glucose 1 g L −1 ; peptone 1 g L −1 ; Culture Collection of Algae at the University of Göttingen, Germany) at 18 °C in the dark, and flagellates were fed weekly with heat killed bacteria from a different species ( Flectobacillus major DSMZ 103, 1 × 10 7 cells mL −1 ; pre‐incubated at 70 °C for 2 h).…”

“…Biofilm‐associated Streptococcus mutants have been shown to grow more slowly than planktonic cells (Welch et al ., ). To avoid these costs, bacteria may resort to phenotypic plasticity for the expression of defence morphotypes: both, the formation of filaments and aggregated cells can be conditionally induced by a signal from the predator (Corno & Jürgens, ; Blom et al ., ). Theoretical analyses suggest that long‐term evolutionary pressure imposed by intraspecific substrate competition should either lead to the loss or the constitutive fixation of such physiological trait variability if predators are permanently absent or present, respectively (Yamamichi et al ., ).…”

“…Z007 increases the formation of aggregates in the presence of a bacterivorous nanoflagellate (Blom et al ., ). Supernatants of a predator–prey coculture also induce this behaviour, indicating the involvement of an infochemical (Blom et al ., ). While the strain has been isolated from central Lake Zurich waters, its closest relatives are from habitats such as soil or the rhizosphere (Pal et al ., ).…”

Competition and niche separation of pelagic bacteria in freshwater habitats

“…Moreover, the classifying of genotypes as being ‘functionally redundant’ is an operational procedure that is only valid within a particular environmental scenario; a change in growth conditions may well lead to a role change of so‐called functionally redundant (competing) taxa into functionally complementary (cooperating) ones (Fetzer et al ., ). In addition, bacterial communities as a whole, but also individual genotypes (even at the level of individual strains) may greatly differ in their metabolic or phenotypic plasticity, that is, they physiologically acclimate to particular scenarios such as increasing substrate levels (Hahn et al ., ; Comte et al ., ) or the presence of predators (Corno and Jürgens, ; Blom et al ., ). This trait is regarded to be particularly advantageous at instable environmental conditions, while more stable environments tend to select for genotypic specialisation (Yamamichi et al ., ).…”