CONTRASTING BACTERIAL STRATEGIES TO COEXIST WITH A FLAGELLATE PREDATOR IN AN EXPERIMENTAL MICROBIAL ASSEMBLAGE

We studied predator-induced changes within a slowly growing mixed micr obial assemblage that was sustained by algal exudates in a continuous cultivation system, In situ hybridization with fluorescent monolabeled oligonucleotide probes was used for a tentative community analysis. T his method also allowed us to quantify the proportions of predators wi th ingested bacteria of different taxonomic groups, In addition, we de termined grazing rates on bacteria with fluorescently labelled prey, B acteria belonging to the alpha and beta subdivisions of the phylum Pro teobacteria (alpha- and beta-Proteobacteria, respectively) showed very different responses to the addition of a bacterivorous flagellate, Bo do saltans. Within one day, filamentous protist-inedible bacteria deve loped; these belonged to the beta-Proteobacteria and constituted betwe en 8.7 and 34% of bacteria from this subgroup. Total abundance of beta -Proteobacteria decreased from 3.05 x 10(6) to 0.23 x 10(6) cells ml(- 1), and estimated cell division rates were low. Other morphologically inconspicuous protist-edible bacteria belonging to the alpha-Proteobac teria were found to respond to predation by an increase in growth rate . Although these bacteria were heavily grazed upon, as on average >85% of flagellate cells had ingested alpha-Proteobacteria, they numerical ly dominated after the addition of B. saltans (mean, 1.35 x 10(6) cell s ml(-1)), It was thus mainly those fast-dividing strains of alpha-Pro teobacteria that supported the growth of the flagellate population. We conclude that bacteria in mixed assemblages can adopt at least two di stinct strategies as a reaction to intense flagellate predation: to ou tgrow predation pressure or to develop inedible, inactive filaments, S ince these strategies occurred within 24 h after the addition of the f lagellate, we hypothesize that chemical stimuli released by the predat or may have triggered bacterial responses.