Transformation of Microbial Complexes in Components of Soil Structures of Different Origin (Soil, Peat, Sand) during Freezing-Thawing Processes
—In a model experiment, the transformation of microbial complexes of cultivated forms of saprotrophic bacteria and yeasts during freezing-thawing was studied in various natural substrates that are used to create soil structures for urban landscaping and for growing herbaceous plants. The number of saprotrophic bacteria and yeasts depended both on the type of substrate and on temperature changes during freezing-thawing. At the stage of freezing of peat and soil (Aarable) to 0 and –5°C and at the subsequent stage of thawing to 0°C, a significant increase in yeast abundance was registered. The maximum number of yeasts in soil and peat was 5.1 lg (CFU/g). In contrast to the number of yeasts, abundance of saprotrophic bacteria in soil and peat was characterized by a sharp decrease when the substrate temperature was negative and peaked at 19–22 and 10°C, respectively. The maximum bacterial numbers in soil and peat were 7.5 and 8.0 lg (CFU/g), respectively. In sand, abundance of both saprotrophic bacteria and yeasts did not depend on the temperature and was 5.0 lg (CFU/g) for bacteria and 3.4 lg (CFU/g) for yeasts at all stages of the freezing-thawing cycles. In total, 15 species of saprotrophic bacteria and 29 species of yeasts were isolated from different components of soil structures. At the maximum temperature of freezing in the cycles (–5°C), three species of bacteria with psychrophilic properties (Flavobacterium psychrophilum) and ability to form endospores resistant to various adverse effects (Bacillus subtilis, B. megaterium) were isolated from soil and peat. Among the yeasts isolated from soil and peat at negative temperature and also having psychophilic properties were Candida sake, Rhodotorula glutinis, Rh. mucilaginosa, and Solicoccozyma terricola. Bacteria with psychophilic properties, Flavobacterium psychrophilum and Pseudomonas fluorescens, as well as two species of bacilli, Bacillus subtilis and B. megaterium were revealed in sand at negative temperatures. Only one yeast species, Debaryomyces hansenii, capable of surviving stress conditions in the form of ascospores, was isolated from sand at –5°C. The effect of short-term temperature drops on the dynamics of the number and diversity of microbial communities of the investigated components of soil structures in a model experiment showed that specialized soil structures were able to “tolerate” short-term temperature stress drops typical of the spring and autumn period, restoring the number of initial populations after cessation of the negative impacts. This also indicates development of the soil structures in the process of functioning, rather than their rapid degradation.