Hydrogen Concentration Profiles at the Oxic-Anoxic Interface: a Microsensor Study of the Hindgut of the Wood-Feeding Lower Termite Reticulitermes flavipes (Kollar)

Abstract

Molecular hydrogen is a key intermediate in lignocellulose degradation by the microbial community of termite hindguts. With polarographic, Clark-type H(inf2) microelectrodes, we determined H(inf2) concentrations at microscale resolution in the gut of the wood-feeding lower termite Reticulitermes flavipes (Kollar). Axial H(inf2) concentration profiles obtained from isolated intestinal tracts embedded in agarose Ringer solution clearly identified the voluminous hindgut paunch as the site of H(inf2) production. The latter was strictly coupled with both a low redox potential (E(infh) = -200 mV) and the absence of oxygen, in agreement with the growth requirements of the cellulolytic, H(inf2)-producing flagellates located in the hindgut paunch. Luminal H(inf2) partial pressures were much higher than expected (ca. 5 kPa) and increased more than threefold when the guts were incubated under a N(inf2) headspace. Radial H(inf2) concentration gradients showed a steep decrease from the gut center towards the periphery, indicating the presence of H(inf2)-consuming activities both within the lumen and at the gut epithelium. Measurements under controlled gas headspace showed that the gut wall was also a sink for externally supplied H(inf2), both under oxic and anoxic conditions. With O(inf2) microelectrodes, we confirmed that the H(inf2) sink below the gut epithelium is located within the microoxic gut periphery, but the H(inf2)-consuming activity itself, at least a substantial part of it, was clearly due to an anaerobic process. These results are in accordance with the recently reported presence of methanogens attached in large numbers to the luminal side of the hindgut epithelium of R. flavipes. If the oxygen partial pressure was increased, O(inf2) penetrated deeper and H(inf2) production was suppressed; it ceased completely as soon as the gut was fully oxic. In experiments with living termites, externally supplied H(inf2) (20 kPa) stimulated methane formation five- to sixfold to 0.93 (mu)mol (g of termite)(sup-1) h(sup-1), indicating that the methanogenic activity in R. flavipes hindguts is not saturated for hydrogen under in situ conditions. This rate was in good agreement with the H(inf2) uptake rates exhibited by isolated hindguts, which would account for more than half of the CH(inf4) formed by living termites under comparable conditions.