Physicochemical Conditions and Microbial Activities in the Highly Alkaline Gut of the Humus-Feeding Larva of Pachnoda ephippiata (Coleoptera: Scarabaeidae)

The soil macrofauna plays an important role in the carbon and nitrogen cycle of terrestrial ecosystems (18, 41, 62). The intestinal tracts of litter-feeding and humivorous soil macroinvertebrates are favorable habitats for microorganisms and typically harbor a dense and active gut microbiota. The major function commonly attributed to the microorganisms in the guts of such animals is the depolymerization and fermentative breakdown of the cellulosic or lignocellulosic component of their diet, which leads to degradation products that can be resorbed by the host. This is supported by the high concentrations of microbial fermentation products and by the presence of fermentative bacteria and protozoa, accompanied by obligately anaerobic homoacetogenic and methanogenic microorganisms, in the guts of such animals. However, the extent and importance of such processes and their specific function in the nutrition of the host are scarcely understood (for reviews, see references 8, 9, 11, 12, 14, 36, and 44). In the case of soil-feeding termites, host factors such as the extreme alkalinity of the anterior hindgut and the influx of oxygen seem to play a key role in sequestering organic matter from the inorganic soil matrix (37). The decrease of molecular weight and increase in solubility resulting from alkaline extraction and chemical oxidation render the organic matter accessible for digestion in subsequent, less-alkaline compartments (34, 35). Alkaline gut regions are encountered also in many representatives of other insect orders and seem to be connected with the dietary preferences of the respective taxa (Coleoptera, Diptera, and Lepidoptera; see references 15 and 32). Comparing several beetle larvae feeding on a lignocellulosic diet, Grayson (30) already had pointed out an apparent correlation between the degree of humification of the diet and the alkalinity of the intestinal tract. The highest pH values among beetle larvae were encountered among the Scarabaeidae (50, 52, 58, 60), which comprise species from humivorous, detritivorous, and coprophagous feeding guilds. Although scarab beetle larvae are among those few arthropods that have received at least a minimum of attention from a microbiological perspective (3, 4, 21, 23) and although their importance for the transformation of soil organic matter is undisputed, little is known about the composition of the gut microbial community and its role in the digestive process. Using the humivorous larva of the cetoniid beetle Pachnoda ephippiata (Coleoptera: Scarabaeidae) as a model organism, we investigated the physicochemical conditions and microbial activities in the gut in order to gain more insight into the role of the intestinal microbiota in transformation and mineralization of organic matter during gut passage. Since a representative analysis of the microbial community structure requires covering also those populations that resist cultivation (14), the project also included a cultivation-independent approach, involving molecular cloning and fingerprinting techniques. These results are reported and discussed in the companion paper (28).