Heavy Metal and Bacteria

  Heavy Metal and Bacteria

Bacteria are the keystone species of a large number of ecosystems including soil ecosystem.The bacteria can act as both producer and decomposer in every ecosystem. Due to their small size, they have a high surface /volume ratio and therefore provide a large contact area for interactions with the surrounding environment. Among the ecosystems, soil ecosystems are highly complex, containing a tremendous amount of organic substaces and bacterial species.Soil is simply defined as a heterogeneous,nutrient-poor and harsh environment harboring a huge diversity of bacteria (Gans etal.,2005; Urozetal.,2010). Sometimes soil ecosystem is contaminated by the heavy metal and nonmetal substances. Indigenous bacterial populations in soil are of fundamental importance for ecosystem functioning due to their capacity to determine nutrient cycles.Bacterial communities are also reknowned for their activities in elimination of heavy meatal contaminants from soil. Besides their occurrence in high numbers and their high surface to volume ratio it is the negative net charge of the cell envelope that makes these organisms prone to accumulate metal cations from the environment (Collins and Stotzky, 1992). Microbes can potentially accumulate metals either by a metabolism-independent, passive, or a metabolism dependent, active process (Gadd, 1988).

Metals with no biological function are in general tolerated only in minute concentrations, whereas the essential metals with biological functions are usually tolerated in higher concentrations. They accomplish either metabolic functions as constituents of enzymes or meet structural demands as, e.g., in supporting the cell envelope. The concentration and the speciation of the metal determine whether it is useful or harmful to the bacterial cell. Homoeostasis is therefore essential and bacteria have developed a fine-tuned regulatory system of uptake, incorporation and excretion. Adverse effects of metals on the microbial cell are decreased decomposition of soil organic matter, reduced soil respiration, lower diversity, and decreased activity of several soil enzymes (Rühling and Tyler, 1973; Tyler, 1974). Depending on the external conditions microbial cells have developed mechanisms to cope with high concentrations of metals (Silver and Misra, 1988) as well as heavy metal.

“Heavy metals” are generally defined as metals with relatively high densities, atomic weights, or atomic numbers whether metalloids are included, vary depending on the author and context. In ecological context metal and metaloloid contaminants are considered as Heavy metals (Verma et al, 2017).These elements occur naturally in rocks and in variable amounts in soils, depending on their location and the rocks that have broken down to make the soil’s components.The group “Heavy metals” for the purpose of discussing health risks or impacts generally includes: Arsenic (As), Lead (Pb), Cadmium (Cd), Chromium (Cr), Copper (Cu), Mercury (Hg), Nickel (Ni), Zinc (Zn) etc. Heavy metals occur naturally in the soil environment from the pedogenetic processes of weathering of parent materials at levels that are regarded as trace (<1000 mg kg−1) and rarely toxic (Kabata-Pendias and Pendias, 2001, Pierzynski, et al., 2000).Accumulation of such heavy metal contaminant may cause risks to human health, plants, animals or may destroy the entire ecosystems.

Fertilizer industry is considered to be a potential source of heavy metals. It contains a large majority of the heavy metals like Hg, Cd, As, Pb, Cu, Ni, and Cu; natural radionuclide like 238U, 232Th, and 210 Po (FAO, March, 2009).The application of numerous biosolids (e.g., livestock manures, composts, and municipal sewage sludge) to land inadvertently leads to the accumulation of heavy metals such as As, Cd, Cr, Cu, Pb, Hg, Ni, Se, Mo, Zn, Tl, Sb, and so forth, in the soil. Mining and milling of metal ores coupled with industries have bequeathed many countries, the legacy of wide distribution of metal contaminants in soil. During mining, tailings (heavier and larger particles settled at the bottom of the flotation cell during mining) are directly discharged into natural depressions, including onsite wetlands resulting in elevated concentrations (DeVolder, et al., 2003). Airborne sources of metals include stack or duct emissions of air, gas, or vapor streams, and fugitive emissions such as dust from storage areas or waste piles. Metals from airborne sources are generally released as particulates contained in the gas stream. Some metals such as As, Cd, and Pb can also volatilize during high-temperature processing.

Microbes have to cope with high concentrations of different heavy metals in various kinds of habitats. When the bacterial cells are exposed to the high concentrations of heavy metals, the metals react within cells with various metabolites and form toxic compounds. Thus, overall accumulation is determined by two characteristics of the cell: sorptivity of the cell envelope and capacity for uptake into the cytosol. Active uptake into the cytosol is usually slower than passive adsorption and is dependent on element-specific transport systems (Gadd, 1988). Passive adsorption is likely to be the dominant mechanism in metal accumulation, since scarcity of nutrients is the ground state for many natural environments in soils, and active uptake requires energy. Biologically essential metals, like nickel, are hard or semi-hard, i.e. they prefer oxygen ligands and usually form ionic bonds with the ligands (Hughes and Poole, 1989). On the other hand, many toxic metals, e.g., cadmium, are soft. These metals, often associated with environmental pollution, have a higher affinity for nitrogen and sulphur containing ligands and form bonds of covalent character.