Succession of bacterial grazing defense mechanisms against protistan predators in an experimental microbial community

We studied the effects of 2 bacterivorous protistan predators on the phenotypic and taxonomic successions of an experimental bacterial community growing in continuous culture with the cryptophyte Cryptomonas phaseolus. Two predators were inoculated to the cultivation system, the mixotrophic flagellate Ochromonas sp., an interception feeder, and the filter-feeding heterotrophic ciliate Cyclidium glaucoma. The system was sampled every second day over a period of 3 mo. The competition between algae and bacteria for limiting phosphorus (20 mu g P l(-1) d(-1)) was reflected in pronounced fluctuations of the ratio of algal versus bacterial biomass. High grazing pressure exerted on bacteria by the flagellate led to a strong decrease of bacterial abundance and biomass, with only few cells left freely dispersed. A succession of various grazing-resistant morphologies was observed, such as microcolonies, large aggregates and filaments. A population affiliated with Aquabacterium sp. formed grazing-resistant co-colonies with a Caulobacter spp., indicating that these 2 bacteria might find a common refuge from protistan grazing within aggregates or biofilms. During the course of the experiment these large colonies were succeeded by grazing-protected morphotypes related to the Flexibacter sancti-Flavobacterium ferrugineum lineage of the Sphingobacteriales. In contrast, the ciliate induced only a slight reduction of bacterial abundance, and bacteria even recovered to higher than original numbers after a few weeks. However, pronounced successions of microbial populations were also observed in the presence of the ciliate.