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Zul, Delita (2008): Determinants of the Bacterial Diversity in Manipulated and Natural Soils. Dissertation, LMU München: Faculty of Biology
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Abstract

Soils harbor highly diverse bacterial communities. It is still poorly understood whether functional redundancy or a multitude of ecological niche modify the abundance and community composition of bacteria in soil. Understanding why soil microorganisms are so diverse and which factors control their community composition is of importance because they are essential for maintaining ecosystem processes and functions. Alterations of biotic or abiotic factors as results of natural or anthropogenic disturbances are known to influence soil bacterial diversity. However, the relation of those factors on microbial diversity is not well understood. This work examined effects of several environmental factors, specifically the presence of higher plant species, water content, land use, and soil properties, on bacterial diversity by employing two different soil sources. The reproducibility of bacterial community composition in manipulated soil was analyzed by use of group-specific phylogenetic PCR-DGGE fingerprinting. Soils were taken from lysimeters that had been planted with four different types of plant communities and the water content was adjusted. The composition of Alphaproteobacteria, Betaproteobacteria, Bacteroidetes, Chloroflexi, Plancto-mycetes, and Verrucomicrobia populations were clearly different from soils without plants compared to that of populations in planted soils. In contrast, the composition of Acidobacteria, Actinobacteria, Archaea, and Firmicutes populations did not influenced by the environmental factors tested. No clear influence of plant diversity and water content could be observed. The reproducibility of bacterial composition associated with the absence or presence of plants was true, even for the low-abundance phylotypes as shown by phylotype beta10 representing up to 0.18% of all bacterial cells in planted soils compared to 0.017% in those unplanted. A high throughput cultivation approach was performed by employing the MicroDrop and the soil slurry dilution techniques. Soil-solution-equivalent medium (pH 7.0) supplemented with artificial root exudates, yeast extract, and inducers was utilized. From 217 cultures obtained, isolate byr23-80 showing the same sequence with phylotype beta10 was recovered and studied in detail as this phylotype displayed a distinct response towards the presence of higher plant species and its sequence affiliated with uncultured bacteria, so far. The strain exhibited high physiological flexibility and was capable of utilizing major constituents of root exudates. A polyphasic taxonomic analysis and DNA-DNA hybridization data supported an assignment of strain byr23-80 as a novel species to the genus Massilia within the family Oxalobacteraceae of the subphylum Betaproteobacteria, for which the name Massilia brevitalea is proposed. Effects of land use and soil properties on the bacterial diversity and activity were determined by employing natural soil from the Kavango region, Namibia. Soil properties in fact controlled the soil respiration rates rather than land use as pristine dark loam soil had remarkably higher respiration rate than pristine sand soil. Exoenzyme activities greatly varied among sites, but did not show a clear correlation to one of the two factors. The quantitative PCR identified Acidobacteria and Actinobacteria as the most abundant phyla about of 30 and 20% of all Bacteria, respectively. Alphaproteobacteria, Bacteroidetes, and Planctomycetes accounted for below 10%, whereas Betaproteobacteria, Chloroflexi, and Firmicutes represented less than 1%. Clone library of 16S rRNA genes from pristine dark loam soil revealed a high bacterial diversity with an estimated number of about 5600 phylotypes. The PCR-DGGE fingerprinting of Acidobacteria and Actinobacteria did only show minor differences in composition of the bacterial communities among sampling sites. This study suggests that the bacterial species compositions in soil are determined to a significant extent by abiotic and biotic factors, rather than by mere chance, thereby reflecting a multitude of distinct ecological niches.