FIG. 3.

“Killing the winner” mechanisms on three levels. Experimental data suggest that different strains of the same “species” may have profiles of different resistance to viruses. Thus, it appears that the influence of viruses on prokaryotes is on the strain level (B_ij; j = 1, 2, …, l) and not on the “species” level (B_i; i = 1, 2, …, n) commonly detected by PCR-based fingerprinting techniques targeting the 16S rRNA gene. This is illustrated here for the case of edible bacteria B_T; however, similar arguments also hold for the case of inedible bacteria I_T. The figure suggests that the abundance of the prokaryotic community B_T is controlled by protozoan grazing and that the abundance at the strain level B_ij is controlled by viral lysis. However, the controlling mechanism for population size at the “species” level B_i is not immediately obvious from this model. The meanings of the symbols are given in Table 1.