The Termite Gut Microflora as an Oxygen Sink: Microelectrode Determination of Oxygen and pH Gradients in Guts of Lower and Higher Termites.

Clark-type oxygen microelectrodes and glass pH microelectrodes, each with a tip diameter of <=10 (mu)m, were used to obtain high-resolution profiles of oxygen concentrations and pH values in isolated termite guts. Radial oxygen profiles showed that oxygen penetrated into the peripheral hindgut contents up to about 150 to 200 (mu)m below the epithelial surface in both the lower termite Reticulitermes flavipes (Kollar) and the higher termite Nasutitermes lujae (Wasmann). Only the central portions (comprising less than 40% of the total volume) of the microbe-packed, enlarged hindgut compartments ("paunches") were completely anoxic, indicating that some members of the hindgut microbiota constitute a significant oxygen sink. From the slopes of the oxygen gradients, we estimated that the entire paunches (gut tissue plus resident microbiota) of R. flavipes and N. lujae accounted for 21 and 13%, respectively, of the respiratory activity of the intact animals. Axial oxygen profiles also confirmed that in general, only the paunches were anoxic in their centers, whereas midguts and posterior hindgut regions contained significant amounts of oxygen (up to about 50 and 30% air saturation, respectively). A remarkable exception to this was the posterior portion of an anterior segment (the P1 segment) of the hindgut of N. lujae, which was completely anoxic despite its small diameter ((apprx=)250 (mu)m). Axial pH profiles of the guts of Nasutitermes nigriceps (Haldeman) and Microcerotermes parvus (Haviland) revealed that there were extreme shifts as we moved posteriorly from the midgut proper (pH (apprx=)7) to the P1 segment of the hindgut (pH >10) and then to the P3 segment (paunch; pH (apprx=)7). The latter transition occurred at the short enteric valve (P2 segment) and within a distance of less than 500 (mu)m. In contrast, R. flavipes, which lacks a readily distinguishable P1 segment, did not possess a markedly alkaline region, and the pH around the midgut-hindgut junction was circumneutral. The oxic status of the peripheral hindgut lumen and its substantial oxygen consumption, together with previous reports of large numbers of aerobic and facultatively anaerobic bacteria in the hindgut microflora, challenge the notion that termite hindguts are a purely anoxic environment and, together with the steep axial pH gradients in higher termites, refine our concept of this tiny microbial habitat.