The rationale of antibiotic treatment is that in darkness, in the presence of (even limited amount of) organic carbon and nitrogen sources, as well as vitamins, the cellular growth of heterotrophic bacteria is promoted, these bacteria being more sensitive against different types of anitbiotics than bacteria which are not during cellular growth (as should be the case of photosynthetically-grown cyanobacteria in darkness). In light, in the absence of the antibiotic and organic resources, the growth of cyanobacteria is promoted. After one cycle of treatment with the four antibiotics, the heterotrophs still have a significant presence (results not shown).