The Gut Microenvironment

Based on the oxygen sensitivity and the fermentative metabolism of the intestinal protozoa, the high concentrations of microbial fermentation products, the presence of anaerobic or oxygen-sensitive activities such as methanogenesis and nitrogen fixation, and the isolation of obligately anaerobic bacteria, it was initially assumed that the termite hindgut is an anoxic habitat (for literature, see Breznak 2000; Brune 2005).

However, the maintenance of anoxia in an environment of such minute dimensions is not a trivial issue (Brune 1998). Oxygen microsensor measurements in the gut of the termite Reticulitermes flavipes have demonstrated that the steep gradient in oxygen partial pressure between the oxic gut epithelium and the anoxic gut contents drives a continuous influx of oxygen into the hindgut (Brune et al. 1995; Ebert and Brune 1997). Also, in all other termites investigated, oxygen penetrates 50-200 |m into the periphery of the hindgut lumen, leaving only the central portion of the dilated compartments anoxic (for literature, see Brune 2005).

While the redox potential in the hindgut contents of Reticulitermes flavipes basically mirrors the oxygen gradients, the massive hydrogen production by the intestinal protozoa and its efficient consumption by hydrogenotrophic microorganisms gives rise to steep radial counter gradients of hydrogen and enormous dynamics of hydrogen partial pressure between the gut center and gut periphery (Ebert and Brune 1997).

The hydrogen partial pressure in the different gut regions seems to be controlled by the spatial organization of the hydrogen-producing and hydrogen-consuming populations (Tholen and Brune 2000). Radiotracer analysis of the in situ metabolism in the hindgut of Reticulitermes flavipes demonstrated that also the high oxygen fluxes significantly influence the fermentation processes in the hindgut (Tholen and Brune 2000). The oxygen-reduction potential of obligately anaerobic bacteria isolated from termite guts (see Sect. (8.)2.3) corroborates that the location of microbial populations relative to the oxygen gradient will affect intestinal carbon fluxes.

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