7 cells mL−1 for the four replicates. Determination of intrinsic growth rates was as in Koch & Ekelund (2005). To evaluate the overall food quality of the seven bacteria tested,

we calculated, for each bacterial strain, the average growth rate for the nine protozoa. Likewise, to evaluate the individual protozoa’s ability to cope with metabolite-producing bacteria, we calculated, for each protozoan strain, the ratio between the selleck average growth rate on the four metabolite-producing bacteria and the three well-suited food bacteria. We calculated each of these compound parameters separately for the four individual replicates as to allow the application of statistics. We used a two-way glm (sas program package, Statistical Analysis System

Institute, version Selleck AZD6244 9.1) with protozoan and bacterial strains as factors for preliminary analysis of the data set (Table 1). For each flagellate strain, differences in growth rate on the different bacterial strains were tested using a one-way anova, followed by a Tukey pair-wise comparison (α=0.05). Similarly, the resulting average growth rate for each bacterial strain when fed to the nine different protozoa (Fig. 1), and the ratio between the average growth rates for the nine different protozoa, on the four metabolite-producing bacteria and the three nonproducers (Fig. 2), were tested using a one-way anova followed by Tukey’s pair-wise comparison (α=0.05). When needed, data were log transformed before analyses. Bodo D-malate dehydrogenase designis UJ illustrates in an exemplarily manner the different possible outcomes of the protozoan–bacterial combinations (Fig. 3). Protozoa fed with suitable food bacteria generally followed a regular pattern with an exponential phase that gradually levelled out into a stationary phase (Fig. 3: P. fluorescens DSM50090) and displayed a positive growth rate (Table 1). Protozoa exposed to bacteria that did not support growth, or to phosphate buffer without bacteria, either lysed

(Fig. 3: P. fluorescens CHA0) and were thus assigned the growth rate 0 or remained at an almost constant level with little or no growth (Fig. 3: no bacteria added). In some cases, protozoa transferred to a medium without bacteria performed a few reductive cell divisions before entering a constant cell level (Fig. 3: no bacteria added). In order to follow a consistent procedure, we assigned such outcomes a positive growth rate, even though the initial cell divisions yielded no extra biomass, but just more, smaller bacteria. The protozoan and bacterial strain as well as their interaction significantly affected protozoan growth rate (P<0.0001). Pseudomonas fluorescens DSM50090T yielded the highest average growth rates (Fig. 1). For all tested protozoan strains, except B. caudatus, the growth rates for this strain were similar to, or higher than, on E. aerogenes (Table 1). The two Pseudomonas strains without any known production of secondary metabolites, i.e.