THE TERMITE GUT MICROFLORA AS AN OXYGEN SINK - MICROELECTRODE DETERMINATION OF OXYGEN AND PH GRADIENTS IN CUTS OF LOWER AND HIGHER TERMITES

Clark-type oxygen microelectrodes and glass pH microelectrodes, each w ith a tip diameter of less than or equal to 10 mu m, were used to obta in high-resolution profiles of oxygen concentrations and pH values in isolated termite guts. Radial oxygen profiles showed that oxygen penet rated 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 vol ume) 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 t he oxygen gradients, we estimated that the entire paunches (gut tissue plus resident microbiota) of R. flavipes and N. lujae accounted for 2 1 and 13%, respectively, of the respiratory activity of the intact ani mals. Axial Oxygen profiles also confirmed that in general, only the p aunches were anoxic in their centers, whereas midguts and posterior hi ndgut 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 t he hindgut of N. lujae, which was completely anoxic despite its small diameter (approximate to 250 mu m). Axial pH profiles of the guts of N asutitermes nigriceps (Haldeman) and Microcerotermes parvus (Haviland) revealed that there were extreme shifts as we moved posteriorly from the midgut proper (pH approximate to 7) to the P1 segment of the hindg ut (pH >10) and then to the P3 segment (paunch; pH approximate to 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 junctio n was circumneutral. The oxic status of the peripheral hindgut lumen a nd its substantial oxygen consumption, together with previous reports of large numbers of aerobic and facultatively anaerobic bacteria in th e hindgut microflora, challenge the notion that termite hindguts are a purely anoxic environment and, together with the steep axial pH gradi ents in higher termites, refine our concept of this tiny microbial hab itat.