Effects of bacteriophages on the population dynamics of four strains of pelagic marine bacteria

Viral lysis of specific bacterial populations has been suggested to be an i mportant factor for structuring marine bacterioplankton communities. In the present study, the influence of bacteriophages on the diversity and popula tion dynamics of four marine bacterial phage-host systems was studied exper imentally in continuous cultures and theoretically by a mathematical model. By use of whole genome DNA hybridization toward community DNA, we analyzed the dynamics of individual bacterial host populations in response to the a ddition of their specific phage in continuous cultures of mixed bacterial a ssemblages. In these experiments, viral lysis had only temporary effects on the dynamics and diversity of the individual bacterial host species. Follo wing the initial lysis of sensitive host cells, growth of phage-resistant c lones of the added bacteria resulted in a distribution of bacterial strains in the phage-enriched culture that was similar to that in the control cult ure without phages after about 50-60 h incubation. Consequently, after a ti me frame of 5-10 generations after lysis, it was the interspecies competiti on rather than viral lysis of specific bacterial strains that was the drivi ng force in the regulation of bacterial species composition in these experi ments. The clonal diversity, on the other hand, was strongly influenced by viral activity, since the clonal composition of the four species in the pha ge-enriched culture changed completely from phage-sensitive to phage-resist ant clones. The model simulation predicted that viral lysis had a strong im pact on the population dynamics, the species composition, and the clonal co mposition of the bacterial community over longer time scales (weeks). Howev er, according to the model, the overall density of bacteria in the system w as not affected by phages, since resistant clones complemented the fluctuat ions caused by viral lysis. Based on the model analysis, we therefore sugge st that viral lysis can have a strong influence on the dynamics of bacteria l populations in planktonic marine systems.