EFFECT OF HEAVY METAL TOXICITY ON THE SOIL MICROORGANISMS

Soil is home to a large number of organisms which glows, flourish, and helps hi maintaining the structure of the whole ecosystem. Microorganisms are one of the most important parts of soil biological community and play a critical role in maintaining the physical and chemical properties of soil and also regulate the organic matter and nutrient cycles so that other organisms and plants can grow hi it. They also act as a nutrient source for other biological communities growing in soil. A part from these microorganisms also causes biotransformation and biodegradation of pollutants, whether organic or inorganic, thus cleaning and protecting the environment (Kavamura and Esposito, 2010; Chodak et al., 2013). Heavy metal contamination in the soil, whether by natural or anthropogenic sources is detrimental to the microbial diversity of soil, resulting in the imbalance of the ecological cycle (Wang et al., 2010). It has been veiy well reported that a high concentration of heavy metals in the soil leads to a decrease in microbial diversity (Gans et al., 2005; Chodak et al., 2013).

EFFECT OF HEAVY METAL TOXICITY ON HUMANS

Contamination of soil with heavy metals including chromium, cadmium, arsenic, lead have a direct impact on the health of human beings as the vegetation growing in these contaminated soils contains accumulated heavy metals in them and which will be further consumed by humans leading to different health risks. Heavy metal accumulation also takes place in animals and aquatic organisms such as fishes, which are further eaten up by humans leading to metal toxicity in them (Martin and Griswold,

  • 2009). All heavy metals are toxic to humans at high concentrations and affect cellular processes. These metals tend to accumulate inside the cells and affects or inhibits metabolic reactions by targeting different enzymes and cofactors (Singh et al., 2011). Contaminants can enter the human body from metal-polluted soil by three ways: ingestion, skin, and air.
  • 9.4.1 EFFECT OF CHROMIUM ON HUMANS

Chromium is one of the most abundant, naturally occurring elements on earth. It exists in several oxidation states between Cr2' to Cr3+ (Rodriguez et al., 2007). However, they are most stable in the environment in two oxidation states, Cr3+ and Cr6+. Both forms are veiy toxic to animals, plants, and humans (Mohanty and Patra, 2013). It enters the water and soil system mostly through industrial sources, including tannery industries, pigment- producing industries, metallurgical industries, and electroplating (Gliani, 2011; Tchounwou et al., 2012; Jaishankar et al., 2014). All these sources increase the concentration of chromium significantly in the soil and water system, which subsequently affects the vegetation and other life forms glowing in that particular environment. Plants accumulate chromium not only in their cells but also the soil particles sticks to the outer surface of the plant, which are removed by washing and from where it can enter the food chain. Humans get exposed to chromium when they consume these plants growing in chromium contaminated soil.

Cr6+ is a highly toxic form of the chromium metal. It is a strong oxidizing agent and can easily penetrate the cell membrane. On the other hand, due to its weak membrane penetration power, Cr3+ is less toxic. Cr6" is a human carcinogen, and its carcinogenic nature is attributed towards its mutagenic properties. Being a strong oxidizing agent, Cr6" gets easily reduced and generates several ROS such as OH" and superoxide ions leading to oxidative stress in cells. These ROS damages cellular DNA and proteins and impairs cell function (Stolis and Bagchi, 1995; O’Brien et al., 2003; De Mattia et al., 2004; Tchounwou et al., 2012; Jaishankar et al., 2014). The toxicity of chromium in humans is so severe that even a skin contact to chromium contaminated soil or other materials can cause skin ulcers which are very persistent.

9.4.2 EFFECT OF CADMIUM ON HUMANS

As discussed earlier, cadmium is a toxic metal and adversely affects human beings. Cadmium toxicity in humans affects major organs like the kidney, lungs, skeletal system, and brain (Sobha et al., 2007; Johri et al., 2010; Singh et al., 2011). Long-term exposure to cadmium leads to several chronic disorders of kidneys, lungs, bones, and brain. As cadmium is a severe pulmonary irritant inhaling cadmium polluted dust particles or working in areas having cadmium contaminated soil causes lung disease including cadmium pneumonitis. Apart from this, cadmium also leads to bone disorders like osteomalacia (Duruibe et al., 2007). Cadmium metabolism in the human body is veiy slow; therefore, it can accumulate for a very long time. More harmful effects of cadmium toxicity on bones include hypercalciuria and dysfunctional vitamin D metabolism (Johri et al., 2010). Cadmium toxicity in the human body results in the synthesis of ROS, which directly damages the DNA in the cell and inhibits normal cellular processes such as replication, transcription, and translation (Mitra, 1984; Stolis and Bagchi, 1995; Tchounwou et al., 2012). Heavy metal concentration has a very significant effect on the level of toxicity in the organism. It has been reported that cadmium at a concentration of 0.1 to 10 mM leads to DNA damage. Apart from these, cadmium induces several other effects; for example, it activates mechanism of protein degradation and affects cell signaling pathways (Dumam and Palmiter, 1981; Tchounwou et al., 2012).

9.4.3 EFFECT OF LEAD ON HUMANS

Lead is potentially one of the most toxic heavy metals to humans even at very low concentrations. It affects the major organs in the human body like kidney, liver, gastrointestinal tract. Humans can be exposed towards lead through ingesting contaminated food, soil, inhaling contaminated soil particles or dust and also through contaminated water (Fenter, 2001; Dumibe et al., 2007). Inside the body lead can cause serious damage to the human system. For example, lead toxicity can cause inhibition of haemoglobin synthesis. This can cause serious deficiency in the transport of oxygen in the blood, putting an extra amount of load on the cardiovascular system and such condition also affects the nervous system both short term and long term (Ogwuebgu and Muhangha, 2005). Lead can also damage the gastrointestinal tract and cause serious ailments related to digestive system, hi addition to this, Lead toxicity also affects children of a young age, causing developmental disorders, the most prominent being the development of brain. According to Morgan (2012), 10 pg/dl of lead in the blood can cause a serious reduction in the intelligence quotient of the children.

Lead inhibits the functioning of several important enzymes by incorporating itself into different functional groups resulting in the change in configuration of enzymes and thus affecting their activity (Tchounwou et al., 2012). It can also act as a competitive binder for other metals acting as a cofactor such as calcium and disrupts major metabolic processes going in the cell (Flora et al., 2007). Affecting calcium function in the body lead can cause cell signaling disruption (Goldstein et al., 1993). Chronic exposure of lead to the people working in lead-contaminated areas results in several other effects, including damage to DNA and oxidative stress in the body.

9.4.4 EFFECT OF ARSENIC ON HUMAN HEAL TH

Arsenic is a toxic and carcinogenic heavy metal found in nature. Arsenic, in its inorganic form of arsenite (As3+) and arsenate (As5+), is most harmful to humans. It can have both chronic and acute effects on the human being. Exposure of humans to arsenic can take place by natural sources such as by dissolving in groundwater or consumption of plants gl owing in arsenic contaminated soils. Arsenic is used to manufacture many kinds of herbicides, insecticides, and fungicides, which are one of the major sources of increasing the arsenic concentration in soil and contaminating it and also the vegetation and other life forms glowing in it (Tchounwou et al., 2012). Arsenic affects most of the organs of the human body, but mostly it affects the kidney. It also affects ATP synthesis in the cell, thus damaging cellular respiration and other important processes (Duruibe et al., 2007). In addition to this, arsenic toxicity is a major cause of skin diseases such as the development of lesions on the skin, which later on induces skin cancer (Lage et al., 2006; McCarty et al., 2007; Jomova et al., 2011). In humans, inorganic arsenic gets methylated and is biotransformed into monometh- ylarsonic acid and dimethylarsinic acid. Monomethylarsonic acid is not excreted through mine and gets accumulated inside the body leading to be a potential reason for arsenic-induced cancer (Singh et al., 2007; Jais- hankar, 2014). Arsenic also inhibits enzymes responsible for important processes like oxidative phosphorylation, beta-oxidation of fatty acids (Belton et al., 1985). Furthermore, arsenic toxicity damages cellular DNA and also inhibits cellular division in mammalian cells (Hartmann and Speit, 1994; Banu et al., 2001).

 
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