Characteristics of Actinomycete Complexes on Outcrops of Reef Limestone

The study of actinomycetes in soils of various natural complexes is necessary for a comprehensive assessment of the regional potential for their economic use as producers of biologically active substances and enzymes. The research is performed in the Beresnyatsky botanical and geological site, which is part of the Pizhemsky State Nature Reserve (Kirov oblast, Russia). Complexes of culturable actinomycetes in limestone of reef structures of the Kazanian stage of the Permian system, two Leptosols (petrozems) developed from the limestone outcrops, and associated plant substrates (algal films, mosses, and pine rhizosphere) were compared. The abundance, taxonomic composition, and position of mycelial bacteria in the prokaryotic community were determined. The population density of mycelial prokaryotes in the studied substrates varied from tens to hundreds of thousands colony forming units per gram (CFU/g). A sample of carbonate petrozem on marly limestone displayed the highest abundance (1851 ± 157) × 103 CFU/g and share (42.6%) of actinomycetes in the prokaryotic complex. The taxonomic composition of culturable actinomycetes was represented by the Streptomyces and Micromonospora genera and by oligospore forms. The functional structure of streptomycete complexes was determined in the studied soil and nonsoil substrates. Statistically significant (p = 0.05) differences were found between the complexes of streptomycetes from different limestone-related niches in terms of the presence of cellulolytics; at the same time, these differences were absent in terms of the occurrence of antagonists, auxin producers and phosphate mobilizers. The strains of interest for practical use were identified; in particular, actinomycetes with a high cellulase activity (up to 123 ± 53 units/mL) capable of producing indole-3-acetic acid (up to 30.5 μg/mL), inhibitors of gram-positive (Clavibacter michiganensis, Bacillus sp., and Curtobacterium flaccumfaciens) and gram-negative (Burkholderia cepacia, Erwinia rhapontici, and Flavobacterium saccharophilum) phytopathogenic bacteria, and strains capable of solubilizing recalcitrant phosphorus compounds. The results demonstrate that limestone outcrops are a promising natural source for isolating streptomycete cultures with high functional potential.