Uptake of heavy metals (arsenic and lead) by earthworms in a contaminated soils - Literature review Example

?Uptake of Heavy Metals by Earthworms Literature Review Earthworm as an important part of the soil community According to Weeks et al (2004, p.817), earthworm plays an important role when it comes to soil formation through a continuous process that involve factors such as temperature, wind, rain and other biological forces, for example, animals and plants. Earthworms in this sense are significant in terms of shaping the soil structure, its fertility and content. On the same note, Weeks et al (2004, p.820) further explains that, earthworms inhabit various temperatures, including tropical regions that exist in different parts of the globe. Despite healthy soil formed in the absence of earthworms, their presence within the soil indicates the soil is productive. As a result of the tedious activities by earthworms their importance for the soil community includes enhancing mixing and aggregation of soil (Weeks et al 2004, p.820). While looking for food, Weeks et al. (2004, p.821) observe that, earthworms engage in a task of moving to the surface and back down, ingesting particles of soil and other organic substances in their way. Further, earthworms excrete the substances it takes in to the soil, and the excreted matter by the earthworms, are nutrient-dense casts. The result in this sense, involves a process that shift the top layer of soil down to the lower strata, and dragging the bottom soil to the upper surface. When earthworms burrow through the dust, this enhances the soil structure by loosening the soil that are compact and establish numerous tunnels under the surface. The importance of such tunnels includes enhancing soil porosity and creating pathways that allows the intake of water and air into the soil. The process further facilitates absorption and the retention of water, and minimizing run offs and soil erosion (Weeks et al 2004, p.822). Hobbelen, Koolhaas and VanGestel (2006, p.640) further concur that, the created tunnels enhance the spread of plant roots and provide stability and organic substance to the soil. Earthworms further enhance the decomposition process and add organic materials that improve the soil’s composition. As explained by Hobbelen et al (2006, p.642), earthworms often feed on dead roots and leaves that exist on the soil surface and in the process, mix the organic material as a result of tearing the organic materials and depositing these materials deep within the soil. Their cast also deposits on the soils surface thus, contributing to the soil content. In addition, they enhance bacteria and fungi activity in the soil that consumes organic materials and transforming these materials to humus. Earthworms largely, forms part of the food web within the soil, which presents a large community, composed of organism that plays a significant role in soil formation. In the same note, soil food web encompasses soil-inhabiting organisms that include algae, insects, plants, reptiles, fungi and small mammals. The organisms, impacts on soil formation through burrowing, breaking down materials (both animal and plant materials) that is significant in adding organic matter when they die. Earthworms often provide a food source to the soil community and their cast is important in facilitating the survival of other organisms within the soil community. They assist in terms of facilitation the provision of oxygen, water and nutrition for other organisms (Hobbelen et al 2006, p.644). The ecological groups of earthworms and their feeding behaviors According to ecology studies on soils by Nahmani, Hodson and Black (2007, p.402), that focuses on linkages between fauna and microbes, identifies earthworms, as a significant soil invertebrate within the ecosystem and contributes, to biomass and activity which operate as ecosystem engineers. Nahmani et al (2007, p.402) agree that, soil consists of various microorganisms and forms part of earthworm’s diet. In this regard, three ecological groups related to earthworms and their feeding behaviors, exists in literature. They include epigeic, which is earthworm species that inhabit the soil surface and within the litter layers common with forest soils and the epigeic group are not known to burrow. This species feed on plant litter and other litter that inhabits organisms; however, they ingest small quantity of litter. In relation to ecology studies, this species has the ability of inducing a higher microbial activity because of its large surface area necessary for decomposition and a minimized immobility by fungi inhabiting the surface litter. In addition, the epigeic group has the ability to modify microorganism community’s composition (Nahmani et al 2007, p.412). The anecic group, on the other hand, as described by Nahmani et al (2007, p.415), inhabits semi-permanent and permanent burrows considered to be vertical and mainly, target mineral soil layers. This group feeds on organic matter that consist of soil particles and they burry the surface litter, in addition to forming middens. The middens include the accumulated castings on the surface that combine with organic matter, and where multiplication of microorganisms takes place. As a result, microbial degradation emanating from organic matter that is not digested is enhanced. Conversely, the quality of organic matter and the microbial composite ingested by the anecic group, may vary since, they are attracted to litter considered rich in nitrogen. The third group known as endogeic inhabits the horizons of mineral soil where they create horizontal burrows and consume higher content of soil compared to the former ecological categories. In essence, the interactions related to microorganisms are visible in earthworm’s cast or their burrow lining. The earthworms and other microorganisms within the soil community play an important role in terms of enhancing chelation related to metal ions, mineralizing and humify organic materials. On another note, microorganism facilitates the growth of earthworms. Earthworms are able to thrive in an environment considered as moist and dark, but also thrive in organic matter such as kitchen waste, humus and cattle dung. On the contrary, earthworms do not thrive in soils consisting of coarse texture; the ph is less than four or a soil with clay content. In addition, earthworms are considered to be sensitive to touch, dryness or light, and because they breathe via the skin, adequate ventilation is necessary within the soil medium (Nahmani et al., 2007, p.418). The mechanism of heavy metal uptake by earthworms According to Langdon et al (2003, p.362), the species of earthworm that includes D. rubidus and L. rubellus, are able to inhabit soil that contain highly contaminated arsenic (As). Further, according to the study conducted by Langdon et al. ( 2003, p.365), they identified the body burdens related to (As) that averaged 877mg kg-1, however, their study did not report physiological side effects on the two species of earthworm they studied in relation to the uptake of arsenic. These two species of earthworms may have developed a biological mechanism responsible for mitigating toxic effect emanating from arsenic. Langdon et al (2003, p.369) came up with a proposal for a biotransformation pathway that focuses on explaining arsenic accumulation through sequestration. On another note, a biotransformation pathway related to arsenic found in earthworms remains unclear. The observation by Langdon et al (2003, p.369), further suggests that organic species could be a product of arsenic biotransformation occurring within the earthworm. It is possible two processes occur during arsenic biotransformation and may involve, inorganic arsenic sequestration available in a form considered not biologically reactive for example, binding metallothein considered rich in sulphur and chloragogenous tissue that results in arsenic resistance. The second process as noted by Landon et al. (2003, p. 370), involves biotransformation of inorganic arsenic in a pathway that is similar to the pathways in freshwater and marine organism which, contributes to organic arsenic compounds. Andre et al (2009, p.6825) states that, within the terrestrial environment, the features of soil both chemical and physical impacts on the form of that lead exists in the soil. In addition, lead speciation in soils, plays a role in terms of determining lead’s bioavailability. This speciation and to extent, aqueous solubility related to lead is often minimized through lead’s interaction with organic matter and clays existing within soils. Biological processes further influence Pb bioavailability where, microbes play a role in altering Pb speciation through chelation, biosorption and methylation. In essence, Pb speciation within soils affects bioavailability and Pb bioaccumulation, trophic transfer and toxicity within terrestrial ecosystems. Earthworms play a significant role within the soil community and assist in indicating the quality of soil and health (Andre et al 2009, p.6829). Earthworms are often exposed to metals as a result of dermal contact with the heavy metals existing in soil solution or as a result of ingesting bulk soil (Edwards 2004, pp.3-5). Uptake related to porewater-mediated dermal, is considered as the common means of exposure in relation to metal uptake by earthworms. According to observations by Vijver et al (2003, p.128), revealed that, whether earthworms have sealed or unsealed mouth, the uptake of metal is still similar, which suggests that, uptake through the skin is considered as the main exposure route for earthworms. Monitoring the potential impact of Pb is realisable by studying the uptake of metals by earthworms because of their limited mobility. As stated by Wallace and Lopez (1997, p.149), two processes exist in terms of metal detoxification and which alter metal speciation in earthworms. This includes metallothioneins induction and metal rich granules formation that reduces the toxicity of metals. Further, since lead does not play a major role as a metallothioneins inducer, detoxification that occurs in earthworms is as result of MRG (metal rich granule) formation. On the same note, the existence of structures containing sulphate or phosphate in earthworms, play a crucial role in metal-sequestering (Wallace and Lopez, 1997, p.150). Earthworms respond to a high metal exposure by increasing debris vesicles for purposes of sequestering lead from reaching highly toxic levels. However, as reiterated by Wallace and Lopez (1997, p.151), the biological importance related to accumulation of metal concentration is influenced by how an organism metabolizes the metals, after exposure. The factors that affect the uptake of metal by earthworms According to a study conducted by Reinecke and Reinecke (2004, p.299),rapid development related to urbanization and mushrooming of industries, in addition to population explosion, results in an increased solid waste generation. During vermicompositing, earthworms engage in converting solid waste to compost that has nutrients such as nitrogen. However, there are physiochemical factors that impacts on the process of vermicompositing and include the existence of heavy metals which are toxic (Reinecke and Reinecke 2004, p.305). Steenbergen et al (2005, p.5695) states that, research regarding the importance of earthworms within the solid waste and the utilization of vermicasts deposited on land, raises concerns in regard to the concentrated heavy metals that exists in the solid waste. The metals, adversely affect the activities of earthworms and the overall process related to the vermicompositing process (Steenbergen et al., 2005, p.5702). Honsi, Stubberud, Andersen and Stensern (2003, p.30), concur that, earthworms play a significant role in terms of accumulating heavy metals that emanate from different sources such as insecticides, industrial discharge, lead foils and paint chips. Impellitteri et al (2003, p.1383), further state that the relevance of heavy metals in vermicompositing exists in two forms which involves their lethality to worms and affecting negatively on vermiconversion. The second form in which metals exists involves their ability of entering the food chain as a result of the application of municipal solid waste compost on the soil (Impellitteri et al 2003, p.1385). When in small amounts, metal elements, are important in enhancing animals and plant’s growth. Conversely, arsenic and lead pose a threat as contaminants due to their potential in affecting soil organisms. A major determinant of metal toxicity is solubility and further, reactivity depends on the quantity of toxic agent an organism’s body may tolerate. The uptake of metals by earthworms often occurs in the free form and metals such as lead and arsenic tend to be more toxic for the earthworms. When these metals combine, they produce a pronounced toxicity compared to a single metal (Bleeker and VanGestel 2007, p.826). According to Zorn, VanGestel and Eijsackers (2005, p.193), their study observed a reduced metal bioavailability for earthworms, contributed by a high content of organic matter in the investigated sludge. In addition, Zorn et al (2005, p.195) note that, the impact of metals such as lead or arsenic on earthworms, is more of antagonistic and the magnitude of the antagonism is influenced by the concentration of a mixture related to heavy metals. Further investigations from Zorn et al (2005, p.197) suggest that mixtures of chemicals in the soil may increase toxicity in organisms. Investigations related to earthworms and metals have observed heavy metals as affecting the physiological state of earthworms during bioaccumulation. However, earthworms are able to compartmentalize heavy metals using the metal binding proteins and in this context, metallothioneins that mediates a trafficking pathway (Zorn et al 2005, p.198). 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Weeks, J.M., Spurgeon, D.J., Svendsen, C., Hankard, P.K., Kammenga, J.E.,Dallinger, R., Kohler, H.R., Simonsen, V., Scott-Fordsmand, J., 2004, Criticalanalysis of soil biomarkers: a ?eld case study in Avonmouth, UK. Ecotoxicology,Vol. 13, 817–822. Zorn, M.I., Van Gestel, C.A.M., Eijsackers, H., 2005. Species-speci?c earthworm population responses in relation to ?ooding dynamics in a Dutch ?ood plain soil. Pedobiologia, Vol. 49, 189–198. Read More