by the reduction in volume and mass of initial feed mixture. Many reports also
stated that the heavy metal concentrations were enhanced in final mixture, but
the levels were below the internationally permissible levels of metals for compost. They further analyzed that the variation in heavy metal concentration in the
vermicompost considerably depends on various factors, including environmental conditions, nature of raw feed mixture, species of earthworms used, etc.
Heavy metals were reported high in sewage soils, cultivated lands, winery
wastes, paper-pulp mill sludge, and even in agricultural wastes because of the
carbon loss by mineralization, and the overall biomass reduction due to the
breakdown of organic matter during vermiconversion may be the ultimate reason
for the enrichment of heavy metals in the composts (Yadav and Garg 2011a).
Even the increase in the levels of humic fractions by earthworm mediated
biodegradation process can contribute to the heavy metals content, which
strongly immobilizes metals by forming stable metal–humus complexes (Kang
et al. 2011). During vermicomposting process, there is an improvement in degree
of humification that indicates the formation of highly stable aromatic compounds
having great potential for sequestration of metals (Romero et al. 2007).
(b) Heavy metal concentration in the intestine of Eisenia fetida: Many species of
earthworms are known to accumulate metals in their intestine by synthesizing
cysteine-rich metal-binding metallothioneins (Stürzenbaum et al. 2012).
Metallothionein is a metal-inducible protein with low molecular weight that
regulates the bioavailability-detoxification dynamics of nonessential and essential metals in the gut of earthworms (Maity et al. 2011). The physiological
tolerance of heavy metals in many organisms is attributed to induction of
metal-chelating proteins known as metallothioneins (Stürzenbaum et al. 1998).
Heavy metal content in the intestine of earthworms was increased by
P. hysterophorus treatment. The heavy metal content accumulated in the tissues
of earthworm is directly related to the amount of metals present in organic
materials consumed by them. The earthworms inhabiting metal soils of industrial
areas contain more metals as their enzymes help to mineralize the organic waste
to form simpler or short-chain organic acids, and these simpler acids bind to
metals to form stable metal complexes during vermicomposting (Wang et al.
2013). Previous studies revealed two different metal-binding mechanisms in
earthworms.
• First, the metals are retained in chloragosomes or insoluble calcium phosphate
granules (Morgan et al. 2012), and these insoluble forms of metals cannot
affect the normal biochemical processes in the cytoplasm.
• The sulfur-donating ligands of metallothionein chelate these insoluble metals,
and the metals are then transported to chloragogenous tissues of the intestine of
earthworms, where they get neutralized (Asensio et al. 2007). The
chloragogenous tissue is a sheath of modified peritoneal cells that surround
the outer wall of the gut (Fischer and Molnar 1993).
Earthworms consume organic matter to achieve sufficient nutrition, and during
this process due to enzymatic actions in the gut, metals are liberated in free forms.
15 Waste Management Practices and Their Impact on Earthworms
253
stated that the heavy metal concentrations were enhanced in final mixture, but
the levels were below the internationally permissible levels of metals for compost. They further analyzed that the variation in heavy metal concentration in the
vermicompost considerably depends on various factors, including environmental conditions, nature of raw feed mixture, species of earthworms used, etc.
Heavy metals were reported high in sewage soils, cultivated lands, winery
wastes, paper-pulp mill sludge, and even in agricultural wastes because of the
carbon loss by mineralization, and the overall biomass reduction due to the
breakdown of organic matter during vermiconversion may be the ultimate reason
for the enrichment of heavy metals in the composts (Yadav and Garg 2011a).
Even the increase in the levels of humic fractions by earthworm mediated
biodegradation process can contribute to the heavy metals content, which
strongly immobilizes metals by forming stable metal–humus complexes (Kang
et al. 2011). During vermicomposting process, there is an improvement in degree
of humification that indicates the formation of highly stable aromatic compounds
having great potential for sequestration of metals (Romero et al. 2007).
(b) Heavy metal concentration in the intestine of Eisenia fetida: Many species of
earthworms are known to accumulate metals in their intestine by synthesizing
cysteine-rich metal-binding metallothioneins (Stürzenbaum et al. 2012).
Metallothionein is a metal-inducible protein with low molecular weight that
regulates the bioavailability-detoxification dynamics of nonessential and essential metals in the gut of earthworms (Maity et al. 2011). The physiological
tolerance of heavy metals in many organisms is attributed to induction of
metal-chelating proteins known as metallothioneins (Stürzenbaum et al. 1998).
Heavy metal content in the intestine of earthworms was increased by
P. hysterophorus treatment. The heavy metal content accumulated in the tissues
of earthworm is directly related to the amount of metals present in organic
materials consumed by them. The earthworms inhabiting metal soils of industrial
areas contain more metals as their enzymes help to mineralize the organic waste
to form simpler or short-chain organic acids, and these simpler acids bind to
metals to form stable metal complexes during vermicomposting (Wang et al.
2013). Previous studies revealed two different metal-binding mechanisms in
earthworms.
• First, the metals are retained in chloragosomes or insoluble calcium phosphate
granules (Morgan et al. 2012), and these insoluble forms of metals cannot
affect the normal biochemical processes in the cytoplasm.
• The sulfur-donating ligands of metallothionein chelate these insoluble metals,
and the metals are then transported to chloragogenous tissues of the intestine of
earthworms, where they get neutralized (Asensio et al. 2007). The
chloragogenous tissue is a sheath of modified peritoneal cells that surround
the outer wall of the gut (Fischer and Molnar 1993).
Earthworms consume organic matter to achieve sufficient nutrition, and during
this process due to enzymatic actions in the gut, metals are liberated in free forms.
15 Waste Management Practices and Their Impact on Earthworms
253