Dynamics of redox potential and changes in redox state of iron and humic acids during gut passage in soil-feeding termites Cubitermes spp.) Andreas Kappler 1 , Andreas Brune * Fakultat fur Biologie, LS Mikrobielle O È kologie, Universita Èt Konstanz, Fach M 654, 78457 Konstanz, Germany Received 19 March 2001; received in revised form 27 July 2001; accepted 15 August 2001 Abstract The diet of soil-feeding termites contains large amounts of redox-active humic substances and inorganic compounds such as nitrate, sulfate, and iron minerals, which are potential mediators and/or electron acceptors for the mineralization of organic matter. We have shown previously that the intestinal tract of Cubitermes spp. Isoptera, Termitidae) is characterized by strong changes in oxygen and hydrogen partial pressures and an extreme alkalinity of the anterior hindgut. Microelectrode measurements performed in this study indicated that the intestinal redox potential is controlled not only by the presence of oxygen or hydrogen and the prevailing pH in the different gut compart- ments, but also by other electroactive components. Speciation of the acid-extractable iron showed that parent soil and nest material contained mostly ironIII), whereas the gut contained mainly ironII). Also, the humic acids in the individual gut compartments were more reduced than those in parent soil or feces. Together, these ®ndings indicate that humic acid reduction and ferric iron reduction are important processes for the mineralization of soil organic matter in the gut of soil-feeding termites. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Soil fauna; Termites; Gut; Humic substances; Iron reduction; Redox potential; Hydrogen; Microelectrodes 1. Introduction More than half of all termite genera represent soil-feeding termites Noirot, 1992). It is generally accepted that the activity of soil-feeding termites has an enormous impact on the physical and chemical properties of soils in semiarid and humid ecosystems such as savannas and tropical rain forests Wood, 1988; Jones, 1990; Lobry de Bruyn and Conacher, 1990; Brussaard and Juma, 1996; Lavelle et al., 1997; Brauman, 2000; Donovan et al., 2001; and references therein). However, only little is known about the chemical and microbial processes occurring during the gut passage Brauman et al. 2000; Brune and Friedrich, 2000). In contrast to wood-feeding termites, the diet of soil-feed- ing species consists of a heterogeneous mixture of different organic and inorganic components, where all potential substrates are present in a strongly stabilized form. Conse- quently, the mechanisms involved in gaining access to these substrates and in their subsequent digestion appear to be more complex. It has been proposed that the characteristic differentiation of the hindgut of soil-feeding Termitidae into morphologically and physicochemically distinct compart- ments Fig. 1), and in particular the extremely alkaline pH in the anterior hindgut, play a major role in digestion of the complex diet Noirot, 1992; Bignell, 1994; Brune and Ku Èhl, 1996; Brune, 1998). We have shown that the speci®c condi- tions encountered in the anterior hindgut of Cubitermes spp. alkalinity, oxic gut periphery) enhance the extraction ef®- ciency of organic matter from the inorganic matrix, cause chemical oxidation of humic substances, and lead to a decrease of the molecular weight of the organic matter Kappler and Brune, 1999). The resulting increase in solu- bility of the substrates renders them accessible for digestion in the subsequent, less-alkaline compartments Ji et al., 2000; Kappler et al., 2000; Ji and Brune, 2001). However, the role of the gut microbiota in the digestion process and the pathways of intestinal carbon ¯uxes are still not clear. The major hindgut compartments of all termites are rendered anoxic by microbial oxygen consumption and, at least in the alkaline gut compartments of Cubitermes spp., also by chemical oxidation processes in the gut periphery Brune et al., 1995; Kappler and Brune, 1999; Schmitt- Wagner and Brune, 1999). In the case of wood-feeding termites, the diet contains only small amounts of inorganic electron acceptors; therefore, fermentation processes domi- nate after O 2 is completely consumed Breznak and Brune, Soil Biology & Biochemistry 34 2002) 221±227 0038-0717/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0038-071701)00176-6 www.elsevier.com/locate/soilbio * Corresponding author. Tel.: 149-7531-883282; fax: 149-7531- 882966. E-mail address: andreas.brune@uni-konstanz.de A. Brune). 1 Present address: Swiss Federal Institute for Environmental Science and Technology EAWAG), CH-8600 Du Èbendorf, Switzerland.