Influence of gut alkalinity and oxygen status on mobilization and size-class distribution of humic acids in the hindgut of soil-feeding termites
Introduction
Termites have a keystone role in controlling carbon and nitrogen fluxes both, in semiarid and humid ecosystems such as savannas and tropical rain forests (see Wood and Johnson, 1986; Collins, 1989; Martius, 1994; Bignell et al., 1997). Also their potential impact on agriculture is receiving increasing attention (Black and Okwakol, 1997). Different species of termites consume wood and litter in different stages of decay and humification, and more than half of all termite genera are considered humivorous (Noirot, 1992; Bignell, 1994; Bignell et al., 1997). Therefore, the influences of termites on soils range from physical effects to changes in the chemical properties of soil organic matter (e.g. disturbance of soil profiles, changes in soil texture and structural stability, nature and distribution of soil organic matter, C/N ratios). [For reviews, see Wood, 1988; Lobry de Bruyn and Conacher, 1990; and Brussaard and Juma, 1996.]
The importance of the humivorous soil fauna for the degradation and stabilization of organic matter is generally acknowledged (Insam, 1996), yet extremely little is known about the processes occurring during gut passage. Soil-feeding termites consume large amounts of soil (Okwakol, 1980); a gut-content analysis showed a large proportion of mineral and humus particles, but also plant tissue fragments, fungal hyphae, and numerous microorganisms (Sleaford et al., 1996). However, the identity of the specific components used as carbon and energy sources, the mechanisms involved in their digestion, and the role of the termite gut microflora in this process are unknown (Bignell, 1994; Bignell et al., 1997). Notably, the role of the distinct gut compartmentalization and the function of the elevated pH in the anterior hindgut (Fig. 1) is still obscure. Gut alkalinity is not considered as an adaptation to soil feeding per se, since it occurs also in wood-feeding species (Bignell and Eggleton, 1995), yet the most extreme alkalinities (pH >12) were found in soil-feeding Termitinae (Brune and Kühl, 1996).
Recent microsensor studies with wood-feeding termites have shown that the anoxic status of the hindgut is maintained by rapid oxygen consumption, which is attributed to the respiratory activity of the gut microbiota (Brune et al., 1995; Brune, 1998). However, a previous study of the soil-feeding termite Cubitermes severus had attributed the `oxygen deficit' in homogenates of the alkaline hindgut compartments at least partially to a chemical reaction (Bignell and Anderson, 1980). Therefore, the oxygen status of the alkaline hindgut regions of soil feeders, and especially the importance of autoxidative processes in oxygen removal, require further investigation.
In this study, we determined the oxygen status and the nature of the oxygen uptake rates in the alkaline compartments. In addition, we investigated the influence of alkaline pH and oxygen on the extraction efficiency of humic acids from parent soil and, using high-pressure gel permeation chromatography, followed the changes in size-class distribution of humic acids during gut passage. The results help to elucidate the function of hindgut compartmentalization in soil-feeding termites and the role of different physicochemical conditions within these compartments in gaining access to the nutritive components contained in organic matter stabilized in organo-mineral soil aggregates.
Section snippets
Termites
Cubitermes umbratus Williams (collected in the Shimba Hills National Reserve, Kenya) and C. orthognathus Emerson (collected near Busia, Kenya) were identified by Julius Muli, National Museums of Kenya. C. speciosus Sjöstedt was collected in the Mayombe rainforest, Congo (Brazzaville). Nest fragments were transported to our laboratory in polypropylene containers, and experiments were performed within 2–4 weeks after collection. Although parent soil had been added to the containers, it has to be
Effects of gut passage on humic acids
Humic acids were extracted from the parent soil, from the different gut sections depicted in Fig. 1, and from the nest material, which consists mainly of feces. Fig. 2 shows two typical HP-GPC chromatograms of humic acids prepared from the parent soil and from the P1 compartment, the most alkaline region of the anterior hindgut, illustrating the effects of the gut passage on the size-class distribution:
(i) a relative decrease in the abundance of molecules with high molecular weight (HMW);
(ii) a
Discussion
The role of termites in the control of important physical and chemical soil parameters such as porosity, texture, nitrogen content, and the formation of soil aggregates and clay-mineral complexes is widely recognized (see Anderson, 1988; Wood, 1988; Garnier-Sillam and Harry, 1995; Garnier-Sillam and Toutain, 1995; Brussaard and Juma, 1996). There is, however, little information on the direct effects of gut passage on the ingested soil organic matter. In this paper, we showed for the first time
Conclusions
In soil-feeding termites, alkaline gut regions serve to sequester organic nutrients from the ingested soil by increasing the solubility of humic substances, probably breaking up stable organo-mineral complexes. The effect of the alkaline extraction in the P1 is enhanced by chemical oxidation due to the oxygen supply via the gut epithelium, whereas in the next, slightly less alkaline P3/4a region, microbial activities become increasingly important. The degradation products formed in the
Acknowledgements
The authors are grateful to Lucie Rogo, International Center for Insect Physiology and Ecology, Nairobi, and Richard Bagine, National Museums of Kenya, for their hospitality and their help in organizing termite collections, and to Nixon Onyimbo and Julius Muli for their assistance in the field. They also thank Corinne Rouland and Alain Brauman for generously collecting termites, and Gudrun Abbt-Braun for valuable advice on the analysis of humic substances. This study was funded by a research
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