Predator-specific enrichment of actinobacteria from a cosmopolitan freshwater clade in mixed continuous culture

We investigated whether individual populations of freshwater bacteria in mi xed experimental communities may exhibit specific responses to the presence of different bacterivorous protists. In two successive experiments, a two- stage continuous cultivation system was inoculated,vith nonaxenic batch cul tures of the cryptophyte Cryptomonas sp, Algal exudates provided the sole s ource of organic carbon for growth of the accompanying microflora. The dyna mics of several 16S rRNA-defined bacterial populations were followed in the experimental communities. Although the composition and stability of the tw o microbial communities differed, numerous members of the first assemblage could again be detected during the second experiment. The introduction of a size-selectively feeding mixotrophic nanoflagellate (Ochromonas sp.) alway s resulted in an immediate bloom of a single phylotype population of member s of the class Actinobacteria (Acl). These bacteria were phylogenetically a ffiliated with an uncultured lineage of gram-positive bacteria that have be en found in freshwater habitats only. The Ac1 cells were close to the avera ge size of freshwater bacterioplankton and significantly smaller than any o f the other experimental community members. In contrast, no increase of the Acl population was observed in vessels ex-posed to the bacterivorous cilia te Cyclidium glaucoma. However, when the Ochromonas sp. was added after the establishment of C. glaucoma, the proportion of population Acl within the microbial community rapidly increased. Populations of a beta proteobacteria l phylotype related to an Aquabacterium sp, decreased relative to the total bacterial communities following the addition of either predator, albeit to different extents, The community structure of pelagic microbial assemblage s can therefore be influenced by the taxonomic composition of the predator community.