Influence 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 sourc e of soil-feeding termites, and also the role of the extreme alkalinity in their anterior hindguts are still unknown. Using soil-feeding Cubitermes sp ecies and high-pressure gel-permeation chromatography, we found a significa nt 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 t he highly alkaline (pH 12) P1 compartment, where the apparent molecular wei ght calculated for the peak maximum of the most abundant medium-molecular-w eight fraction decreased by approximate to 30%. Microelectrode profiles dem onstrated steep oxygen gradients into the alkaline hindgut compartments, ca used by intestinal oxygen consumption in the gut periphery. In the less alk aline P3 region, oxygen consumption was at least partially attributable to biological processes, whereas in the highly alkaline P1 compartment, it see med to be largely due to a chemical process. In vitro extraction of parent soil with NaOH solutions of increasing concentration confirmed that extract ion efficiency 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 conc entration in the P1 region. However, in these experiments the pronounced mo lecular-weight shift found in the alkaline hindgut compartments was never o bserved, which indicates that additional factors must be present within the gut. We conclude that the extreme alkalinity in the anterior hindgut, supp orted by autoxidative processes, facilitates not only desorption of humic s ubstances from the mineral matrix, but also decreases their molecular weigh t 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. (C) 1999 Elsevier Science B.V. All righ ts reserved.