Digestion System of the Earthworm

The digestion of organic compounds in the earthworm's body depends on a facultative symbiotic interaction between the earthworm and microorganisms. While feeding, earthworms absorb microorganisms that grow on plant litter. These microorganisms proliferate in the intestine of the earthworm and produce microbial metabolites (Barois and Lavelle 1986). Soil absorbed by the earthworm is mixed with these metabolites and other excretions (i.e., simple sugars, amino acids, and low molecular weight compounds) from the intestinal system. This material, which is rich in different organic compounds and microflora, is then discharged into the soil. The intestinal microflora of the earthworm also synthesize various enzymes (Lavelle and Spain 2001). Due to the microflora in the intestinal system, earthworm excrement usually has higher enzymatic activity than that of soil. However, this situation depends on the ecological category of the earthworm. For example, it is not yet clear whether epigeic earthworm species synthesize enzymes. However, some epigeic species have been found to include cellulose in their intestines and to digest organic compounds without requiring the microfloral symbiosis (Parle 1963; Neuhauser et al. 1978). Additionally, various members of this category are capable of using and digesting microflora just as they use other organic compounds in the soil. For example, Lumbricus rubellus digests microorganisms that colonize clay and polysaccharide particles (Kristufek et al. 1994). Anecic and endogeic species have been shown to increase enzyme activity through their symbioses with microflora (Nielsen and Hole 1964; Wright 1972; Hamilton and Sillman 1989; Cortez et al. 1989). However, the enzyme activities of the excrement from epigeic, endogeic, and anecic species have been found to be higher than those of soil and plant litter (Kizilkaya and Hep en 2004, 2007; Kizilkaya 2008). The efficacy of the earthworm digestion system is also linked to the temperature. The optimal temperature for earthworm intestinal microflora is approximately 20°C, but these microorganisms are more active in the soil environment at 27°C.

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