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 H2 microelectrodes, we determined H2 concentrations at microscale resolution in the gut of the wood-feeding lower termite Reticulitermes flavipes (Kollar). Axial H2 concentration profiles obtained from isolated intestinal tracts embedded in agarose Ringer solution clearly identified the voluminous hindgut paunch as the site of H2 production. The latter was strictly coupled with both a low redox potential (E(n) = -200 mV) and the absence of oxygen, in agreement with the growth requirements of the cellulolytic, H2-producing flagellates located in the hindgut paunch. Luminal H2 partial pressures were much higher than expected (ca. 5 kPa) and increased more than threefold when the guts were incubated under a N2 headspace. Radial H2 concentration gradients showed a steep decrease from the gut center towards the periphery, indicating the presence of H2- 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 H2, both under oxic and anoxic conditions. With O2 microelectrodes, we confirmed that the H2 sink below the gut epithelium is located within the microoxic gut periphery, but the H2- 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, O2 penetrated deeper and H2 production was suppressed; it ceased completely as soon as the gut was fully oxic. In experiments with living termites, externally supplied H2 (20 kPa) stimulated methane formation five- to sixfold to 0.93 μmol (g of termite)-1 h-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 H2 uptake rates exhibited by isolated hindguts, which would account for more than half of the CH4 formed by living termites under comparable conditions.