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It has been established that the algae in the algobacterial cenoses (ABC) on carbonate rock outcrops make up 96-98% of the total biomass of the cenoses, and are represented by the orders Chlorococcales, Chlamydomonadales, Nostocales, Oscillatoriales. Chroococcales, and Rhaphinales. In the heterotrophic component of the ABC, actinomycetes are dominant in terms of both number and biomass; these are represented mainly by streptomycetes of the sections Helvolo-Flavus and Roseus. Distinctive features of the bacterial component of the ABC are the high percentage contents of actinomycetes (more than 60%) and Gram-negative bacteria (more than 30%) and the low content of coryneform bacteria (10-15%) in comparison with the surface of algae-free rocks. The composition of the bacterial complex of the ABC does not depend on the geographic location at which the samples are taken, but rather is determined by the combination of two factors - the presence of carbonate rock and the presence of algae. There may be a specific ecological niche in which the streptomycetes, together with the algae, play the role of cenosis-formers.