Abstract

         All aspects of life on earth are directly or indirectly dependent on soil, its hydrodynamics, elemental composition and microflora. In last few decades, as a result of extensive unsustainable developmental activities a great percentage of global soil have been degraded due to heavy metals, metalloids, persistent organic pollutants etc. The necessity of soil revitalization is inevitable in ensuring biodiversity, sustainable agriculture and a healthy environment for the generations to come. Heavy metal contamination of soil is highly concerned as biggest threat to the homeostasis of soil ecology and the productivity of plants. Heavy metals are persistent and bio accumulate through food chain and possibly implicate potential health hazards in humans and organisms. Even though chemical and physical methods are widely pursued for the remediation of contaminated soil, bio-remediation techniques are clean, effective solution, with ecological and cost-associated benefits yet often out weighed due to various constrains. Several indigenous microbes from heavy metal contaminated ecosystem can withstand the effect of toxic metals, contribute to revival of tarnished soil and help other organisms to overcome many of the adverse. A better and effective understanding about possibilities of bioremediation is a bright ray of hope in recovery of our soils. The present review deals with measures for toxic heavy metals contamination on agricultural soil and other effects on this biome.

Keywords: Bioremediation, Bacteria, Toxic heavy metals, Soil, diseases.

Introduction

         Groundbreaking and intricate development of our civilization in this century has raised pollution as our greatest problem. The future of our planet depends on how wise we decide to redefine our relationship with the environment. The complex ecological succession whether land, water, atmosphere or any other environment, is a function of time and inevitable efforts from an assorted group of organisms, which in most cases does not include a lot of microbes as itself. But later on, they obviously display unacquitted roles in the community. Environments today are passing through an alarming situation of serious threats to existence of various ecosystems due to drastic anthropogenic actions as well as natural disasters and demands to pursue towards a sustainable and feasible. One of those threats, the heavy metal contamination of soil is a dire condition that evoke worldwide concern. They stance the biggest threat to the homeostasis of soil ecology as well as the productivity of plants. Also, when they are present in soil for a long time, they get accumulated and when taken up by plants, they result in potential health hazards to the consumers in food chain (Cooper et al., 2010).

         Many at times, us humans have explored the microbial world for its enigmatic potential, as such as our sunlight-driven hotspot for revitalizing the life-sustaining conditions of our biome. Despite from chemical and physical methods, bioremediation using microbes has proven to be sustainable, clean and comprehensive solution for heavy metal toxicity; it has also overweighed the ecological and economical concerns of the chemical methods. Several indigenous microbes have been found naturally, that understand and help in heavy metal biodegradation which not only withstand but also help ultimately in renewal of the environment (Gupta, 2016).Plants associated with these microbes have been found to accumulate and remove these extremely toxic substances from the soil, leading to the recovery of the productive farming lands (Dixit, 2015). On a global scale, these steps are essential in the field of agriculture for ensuring sustainable agriculture and thereby ensuring productivity in term securing a healthy environment.

Effects of heavy metals on the environment

         Increasing presence of heavy metals in environment has been showing many deadly effects in plants and animals. Non-threshold toxic heavy metals like arsenic (As), cadmium (Cd), chromium(Cr)(VI), mercury (Hg),and lead (Pb) etc., has received more attention than any other due to their higher concentration in environment. In plants they result in oxidative stress and destruct the cell structure and lead to abnormal growth and metabolism due to inhibition of cytoplasmic enzymes. Exposure to these pollutants is a major cause of deadly degenerative diseases in humans (Fig. 1).

Figure 1: Schematic summarizing terms and concepts involved in various human diseases of toxic heavy metals.

Bioremediation

         Bioremediation utilizes biological mechanisms of microbes to degrade hazardous environmental contaminants like heavy metals, metalloids, and persistent organic pollutants etc. (Fig. 2). It depends on the nature and degree of contamination and the metabolic potential of microorganisms. Microbes in general are capable of developing certain resistances phenotypically or genotypically; in needs of survival threats. They adopt various mechanisms to overcome threats to themselves and to the vegetative community they are associated with. Certain microorganisms inhabited in a highly heavy metal polluted soil are found capable of heavy metal degradation or detoxification and can be exploited for environmental remediation purposes (Dixit et al., 2015). Agrobacterium, Azospirillum, Pseudomonasetc., have shown great potential of oxidation of Arsenic (Bachate et al., 2012). Penicilliumchrysogenumis known to mediate bio-mineralization of Lead (Qian et al., 2017), Shewanellaputrefaciens can reduce Chromium (Myers et al., 2000), Klebsiellapneumoniae and Bacillus sp. precipitates Mercury-Sulhate complexes in soil (Essa et al.,2005; Gutknecht, 1981), Acidophilum and Arthrobacter helps in recovery of some heavy metals through bioleaching by producing some organic acids and chelating agents (Pathak et al., 2009). These are only a few microbes to mention while there are even many more, although conventional chemical and physiochemical methods for remediation is already a prevailing field, it is always better to focus on a cost effective and sustainable solution while possible.

         Phytoremediation is another segment of microbe-mediated bioremediation, which includes plants and associated rhizospheric microorganisms. Metal tolerant-hyper accumulator plants growing in highly contaminated areas bind, extract and remediate pollutants from the soil.

         Bioaugmentation is an application-level technique of bioremediation by inoculating the microbes into a polluted environment, like agriculture lands. Different aspects of bioremediation can be considered as a cost effective and sustainable solution for our present situation.

Figure 2: Sources of heavy metals pollution in agricultural soil.

         Major anthropogenic sources can be classified intothree categories: agricultural, industrial and mining. Heavy metals can enter agricultural soil through atmospheric deposition, following release into the atmosphere from fossil-fuel burning, waste incineration or cement manufacture. Heavy-metals-contaminated runoff from mining and industry can enter waterways and reach agricultural land. The use of manure or sewage contaminated with heavy metals to fertilize crops can also contaminate agricultural land.

Pollutant- microbe interactions

         Even though most of the microbes such as protozoa, bacteria, fungi and algae have some trivial level of heavy metal remediation capacity, metalscan prove to be hostile when in higher values than threshold. This causes toxic effects on cells by manipulating essential element availability, inhibiting metabolic reactions, and destabilizing DNA, RNA and thereby proteins (Yin et al., 2019). Bioremediation of heavy metals with help of micro-organisms is influenced by heavy metal resistant microorganisms with different phenotypic expressions, which is a more effective interaction (Ojuederie and Babalola, 2017). These microorganisms are capable of degrading toxic heavy metals by binding them to the functional groups of proteins/lipids or polysaccharides present in their cell wall and altering the chemistry to block their uptake by plants, or converting them to a less toxic state (Jan, 2014). Bacteria and plants, in association stimulate different signal molecules as a specific receptor between their interactions to react with various metals from the environment (Arul Kumar Murugesan, 2020). Their chemical coordination relies on an array of some fundamental chemical reactions such as redox reactions, transformation, mineralization, leaching, precipitation, volatilization, biosorption, accumulation etc. These interactions depend upon various physical and chemical factors such as pH, motility, bio-availability and possess a great environmental significance in the circulation of heavy metals in the biogeochemical cycles (Hashem et al., 2017). Phytoremediation deals with the cleanup of organic pollutants and heavy metal contaminants using plants and rhizospheric microorganisms (Fig. 3). It is inexpensive, eco-friendly and an efficient means of restoration of polluted environments especially those that of heavy metals (Ali et al., 2013; Jan and Parray, 2016).

Figure 3: Processes used in phytoremediation of heavy metals.

Future of microbe mediated heavy metal bioremediation

         While microbe mediated bioremediation is still a developing area on its own and in combination with plant assisted bioremediation (phytoremediation), we cannot stop exploring every single aspect we can. In the era of emerging transgenics and systems biology, environmental biotechnologists are surfing around new possibilities for better results. Genetically modified microorganisms can be considered as a more effective possibility in this subject (Wu et al., 2010). But we sure need more detailed understanding about the metabolic activities and capacities of transgenic microbes to be certain of the effects and side-effects in the environment.

Conclusion

         Global soil pollution by toxic heavy metals represents one of the biggest challenges for sustainable development and developing countries are particularly vulnerable to this threat for their food, health and livelihoods. The accumulation of toxic heavy metals in agricultural soils is an obstacle to achieving global food safety and security. Bioremediation is a promising nature-based solution for treating heavy met

als contamination; however, several issues must be addressed before it can be more broadly implemented.

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