T. G. Dobrovol'skaya

Analysis of Soil Bacterial Diversity: Methods, Potentiality, and Prospects

The paper presents a comparative description of the modern molecular genetic and routine culture techniques for assessing bacterial diversity in soils and gives analysis for the different results obtained by these two groups of methods. The necessity of the collaboration of soil scientists, microbiologists, and molecular biologists in integrating different research methods for a proper assessment of soil microbial diversity is discussed. The paramount importance of soil as the source and reserve of biodiversity on the Earth is emphasized.

Microbial Complexes Occurring on the Apogeotropic Roots and in the Rhizosphere of Cycad Plants

The microbial complexes of soil, the rhizosphere, and the rhizoplane of the apogeotropic (coralloid) roots of cycad plants were comparatively studied. The aseptically prepared homogenates of the surface-sterilized coralloid roots did not contain bacterial microsymbiont, indicating that in the root tissues the symbiosis is a two-component one (plant–cyanobacteria). At the same time, associated bacteria belonging to different taxonomic groups were detected in increasing amounts in the cycad rhizoplane, rhizosphere, and the surrounding soil. The bacterial communities found in the cycad rhizoplane and the surrounding soil were dominated by bacteria from the genus Bacillus. The saprotrophic bacteria and fungi colonizing the cycad rhizosphere and rhizoplane were dominated by microorganisms capable of degrading the plant cell walls. The local degradation of the cell wall was actually observed on the micrographs of the thin sections of cycad roots in the form of channels through which symbiotic cyanobacterial filaments can penetrate into the cortical parenchyma.

Evaluation of bacterial diversity in soil microcosms at different moisture contents

The succession analysis of bacterial diversity in the A horizons (rich in organic matter) of three contrasting types of soil—burozem, soddy gley soil, and chernozem—showed that the bacterial diversity of soil microcosms in humid regions can be adequately evaluated only if soil samples are incubated at different soil moisture contents. A complete account of actinobacteria and proteobacteria requires the levels of soil moisture corresponding to the maximum capillary–sorption moisture and capillary moisture, respectively. The bacterial diversity, whose value was maximum on the 40th day of succession, was higher in soddy gley soil than in burozem. The taxonomic structures of the bacterial communities of these two types of soil were different. After wetting chernozem samples from arid regions, the soil bacterial community changed insignificantly with time and drastically differed from that of soils from humid regions. The difference in the bacterial diversity of soils was the most distinct when it was evaluated by measuring the proportion between proteobacteria and actinobacteria.

Taxonomic composition of denitrifying bacteria in soddy podzolic soil

The taxonomic composition of denitrifying bacteria in soddy podzolic soil was studied by the succession analysis method. This method revealed a significant variation in the taxonomic composition of denitrifying microorganisms in the course of succession. In contrast to succession analysis, the single microbiological analysis of soil samples reflected only the late stage of succession and thus led to an underestimation of the major members of succession. Myxobacteria were found to be the most active denitrifiers at the early stages of succession, whereas bacilli dominated at its late stages. The bacilli were represented by three facultatively anaerobic species:Bacillus cereus, Bac. circulons, andBac. polymyxa.

The Structure of Bacterial Complexes in the Protva River Floodplain

The synecological analysis of bacterial communities from the Protva River floodplain biogeocenosis showed that all of their horizons contain spirilla, which are typical hydrobionts, and pigmented coryneform bacteria associated with the herbaceous plants of the floodplain meadows. The alluvial meadow soils of the inundated regions of the floodplain differ from the unflooded regions of the floodplain in that they have a more diverse bacterial population that is continuously distributed over the soil profile.

The Structure of Cyanobacterial Communities Formed during the Degradation of Apogeotropic Roots of Cycads

1 Cycads are contemporary relic plants, which, in number of species, rank second place to gymnosperms; their evolutionary conservation is possibly associated with the ability to form symbioses with various groups of microorganisms (cyanobacteria, bacteria, and fungi) in specialized apogeotropic roots (coralloids). Compared to other plant syncyanoses, the symbiosis of cycads and cyanobacteria is characterized by the following features: (1) cycads are perennial plants; (2) cycads are adapted to environments with low humidity; they are generally xerophytes belonging to the group of sclerophytes; (3) among plant communities inhabiting near-ocean savannas, sclerophyll forests, steppe-veldts, and sand-dunes, cycads occupy niches with weak interspecies competition such as abrupt carbonate hillsides, precipices, and rocks, i.e., zones with thin soil layers [1, 2]. The infection of apogeotropic roots, both in ontogenesis and phylogenesis of cycads, occurs in vivo permanently; there is no pool of cyanobionts in the tissues of apogeotropic roots to ensure the invasion of newly formed coralloids in the course of the plant’s development. Branches of a single coralloid often contain different species of cyanobacteria [2], i.e., cyanobacteria, which are obligatory components of soil microflora and are involved in soil formation [3], infect coralloids as they are formed.