Impact of oxygen on metabolic fluxes and in situ rates of reductive acetogenesis in the hindgut of the wood-feeding termite Reticulitermes flavipes

The symbiotic digestion of lignocellulose in the hindgut of the wood‐feeding termite Reticulitermes flavipes is characterized by two major metabolic pathways: (i) the oxidation of polysaccharides to acetate by anaerobic hydrogen‐producing protozoa; and (ii) the reduction of CO2 by hydrogenotrophic acetogenic bacteria. Both reactions together would render the hindgut largely homoacetogenic. However, the results of this study show that the situation is more complex. By microinjection of radiolabelled metabolites into intact agarose‐embedded hindguts, we showed that the in situ rates of reductive acetogenesis (3.3 nmol termite−1 h−1) represent only 10% of the total carbon flux in the living termite, whereas 30% of the carbon flux proceeds via lactate. The rapid turnover of the lactate pool (7.2 nmol termite−1 h−1) consolidates the previously reported presence of lactic acid bacteria in the R. flavipes hindgut and the low lactate concentrations in the hindgut fluid. However, the immediate precursor of lactate remains unknown; the low turnover rates of injected glucose (