In ̄uence of gut alkalinity and oxygen status on mobilization and size- class distribution of humic acids in the hindgut of soil-feeding termites

The majority of termite species do not feed on wood, but have a humivorous mode of nutrition. However, the exact nature of the carbon and energy source of soil-feeding termites, and also the role of the extreme alkalinity in their anterior hindguts are still unknown. Using soil-feeding Cubitermes species and high-pressure gel-permeation chromatography, we found a signi®cant shift toward lower molecular weight in the humic acid samples extracted from different hindgut regions, compared to the parent soil ingested by the termites. The changes in size-class distribution were most pronounced in the highly alkaline (pH 12) P1 compartment, where the apparent molecular weight calculated for the peak maximum of the most abundant medium-molecular-weight fraction decreased by 30%. Microelectrode pro®les demonstrated steep oxygen gradients into the alkaline hindgut compartments, caused by intestinal oxygen consumption in the gut periphery. In the less alkaline P3 region, oxygen consumption was at least partially attributable to biological processes, whereas in the highly alkaline P1 compartment, it seemed to be largely due to a chemical process. In vitro extraction of parent soil with NaOH solutions of increasing concentration con®rmed that extraction ef®ciency of humic acids was not only enhanced by a high pH, but also by the simultaneous presence of oxygen. Similar results were obtained with 200 mM potassium carbonate solution, which mimics the pH and potassium concentration in the P1 region. However, in these experiments the pronounced molecular-weight shift found in the alkaline hindgut compartments was never observed, which indicates that additional factors must be present within the gut. We conclude that the extreme alkalinity in the anterior hindgut, supported by autoxidative processes, facilitates not only desorption of humic substances from the mineral matrix, but also decreases their molecular weight and increases their solubility. This renders so far unknown constituents of the humic matter accessible to microbial degradation in the subsequent, less alkaline hindgut compartments. # 1999 Elsevier Science B.V. All rights reserved.