Faunal Feedbacks On Microbial Community Composition And Diversity

Since the time of Darwin's epochal book on soil biology (1881), there has been considerable interest in the effects of fauna on microbial communities. Satchell (1983), in a centenary celebration of Darwin's book, stated that, by the culture methods existing up until then, there was little or no indication that earthworms have a qualitatively different flora

Faunal Feedbacks on Microbial Community Composition and Diversity 183

TABLE 4.13. Properties of Soil Fauna for Use as Indicators of Soil Quality

1. Organisms and populations

Individuals

Behavior, morphology, physiology

Populations

Numbers and biomass

Rates of growth, mortality, and reproduction Age distribution

2. Communities

Functional groups

Guilds (e.g., burrowers vs. nonburrowers, litter vs. soil dwellers, etc.)

Trophic groups

Food chains and food webs (microbivores, predators, etc.)

Biodiversity

Species richness, dominance, evenness Keystone species

3. Biological processes

Bioaccumulation Heavy metals and organic pollutants

Decomposition Fragmentation of organic matter Mineralization of C and nutrients

Soil structure modification Burrowing and biopore formation Fecal deposition and soil aggregation Mixing and redistribution of organic matter

From Curry and Good, 1992 and Linden et al., 1994.

in their guts or in their castings. Yet there are numerous studies that have indicated an increase in microbial numbers or activity during or after passage through the gut and in the drilosphere (Barois, 1992; Daniel and Anderson, 1992; Kristufek et al., 1992; Schoenholzer et al., 1999). Recent studies using both molecular and culture-based analyses of agricultural soil and burrows and casts of the epigeic earthworm Lumbricus rubellus have revealed interesting differences between earthworm- and non-earthworm-influenced soils (Furlong et al., 2002). Clone libraries of the 16S rRNAgenes were prepared from DNAisolated directly from the soil and earthworm casts. Representatives of the Pseudomonas genus as well as the Actinobacteria and Firmicutes increased in number, and one group of unclassified organisms found in the soil library was absent in that of the cast. In fact, Singleton et al. (2002) isolated a new species of bacterium, Solirubrobacter paulii, from the intestinal wall of Lumbricus rubellus. This was not found in the soil library, and may represent the first known instance of a bacterium unique to the gut wall of an earthworm.

Studies of microbial community similarity have been conducted comparing termite mounds and nearby tropical soils. Harry et al. (2001) used RAPD (random amplified polymorphic DNA) molecular markers to estimate the similarity of microbial communities in the mounds of several termite species and surrounding soils. They studied four species of soil-feeding termites and one species of fungal-feeding termite in a tropical rain forest area of the Nyong River basin in Southern Cameroon. They found that microbial communities of the mounds of the soil-feeding termite species were clustered in the same clade, whereas those of the mounds of the fungus-growing species were distinct like those of the control soils. The microbial changes were dependent upon the species' behavior, with the soil feeding species including feces in their mound building and the fungal-feeding species using saliva as particle cement in its mounds.

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