Struktur und räumliche Verteilung mikrobieller Gemeinschaften im Verdauungstrakt ausgewählter Boden-Invertebraten

Soil macroinvertebrates play an important role in the transformation of soil organic matter, which is responsible for many key soil functions. During gut passage of ingested soil organic matter, intestinal microorganisms participate in the transformation processes, which are also of great importance for the nutrition of their hosts. However, little is known about the structure of microbial communities in the intestinal tract of most soil macroinvertebrates and their spatial distribution within different gut sections. In fact, particularly the topology of microbial communities is regarded as a prerequisite for a deeper understanding of the functionality of invertebrate intestinal tracts, which are characterized by pronounced axial and radial gradients of important physicochemical parameters. Therefore, in the course of this thesis structure and topology of microbial communities in the intestinal tract of the larva of two scarabaeid beetles (Pachnoda ephippiata and Melolontha melolontha) and in the midgut of the earthworm Lumbricus terrestris were analyzed applying techniques of molecular microbial ecology. Moreover, a novel PCR-artifact, so called pseudo-T-RFs, was detected during this thesis, markedly affecting T-RFLP analysis of microbial communities. Protocols allowing the identification and prevention of pseudo-T-RFs formation were developed and applied. In a T-RFLP-based study with earthworms it could be shown, that the intestinal microbiota of these animals is largely soil-derived, i.e. that they lack a gut-specific flora typical for many other soil invertebrates. It could be shown that the ingested microbial community undergoes significant changes during gut passage and that the differences in microbial community composition between food soil, gut and casts strongly depend on the diet of the earthworms. The studies conducted with scarabaeid beetle larvae were the first comprehensive molecular studies about the gut microbiota of beetle larvae and among the first studies about the gut microbiota of other soil arthropods than termites. It could be shown that the pronounced differences in physicochemical parameters existing between the two major gut sections (midgut and hindgut) of scarabaeid beetle larvae are paralleled by marked differences in the colonization with microorganisms. In contrast to the investigated earthworms, the gut microbiota of the scarabaeid beetle larvae could be regarded as a special gut flora, because its composition was considerably different to the microbial community of the ingested food. In both larval species the midgut section was less densely colonized than the hindgut, which possibly functions as a fermentation chamber. Methanogenesis was always restricted to the hindgut section; in the larva of M. melolontha, Methanobrevibacter-species were identified to account for methane production, in P. ephippiata larvae additionally Methanomicrococcus-species occurred. However, in comparison to Bacteria, diversity and relative abundance of Archaea was low. Phylogenetic analysis of the intestinal bacterial communities revealed a high diversity, obviously comprising many so far uncultivated species. The majority of sequences was affiliated with Actinobacteria, Bacillales, Bacteroidetes, Clostridiales, Lactobacillales, and Proteobacteria; many clones were grouping with clones and isolates from other intestinal systems, which underlined the gut specificity of the intestinal microbiota of the scarabaeid beetle larvae. The affiliation of many clones with hydrolytic, cellulolytic and fermenting isolates corroborated the intestinal fatty acid spectra (dominated by acetate and lactate) and suggests the existence of an anaerobic food chain, at least for the hindgut sections. It is not clear if this also holds true for the midgut, since in case of the Melolontha larvae no stable bacterial community could be proven for this gut section. The Melolontha larvae investigated during this thesis were the first arthropods with which a comprehensive analysis of the microbial community structure of different subcompartmental fractions of major gut sections was performed. The gut wall and lumen fractions of the Melolontha hindgut were found to be differentially colonized by microorganisms, which may reflect adaptations to morphological (occurrence of chitin trees at the hindgut wall) and/or physicochemical (penetration of oxygen into the hindgut periphery) features of the investigated subcompartments. The most noticeable difference between the microbial communities of hindgut wall and lumen was a high abundance (10 - 15% of all bacteria) of Desulfovibrio-related bacteria at the hindgut wall, proven with both PCR-dependent and ?independent methods. Regarding its clearness, this finding is so far unique for arthropods.