MICROORGANISMS FOR BIOREMEDIATION OF HEAVY METALS

Introduction

Bioremediation refers to the use of microorganisms to degrade contaminants that pose environmental and human risks. Bioremedition processes typically involve the actions of many different microbes acting in parallel or sequence to complete the degradation process [1].

Trace elements or trace metals are elements that are present at low concentrations in rocks, soils, water and in the atmosphere. Some of them (for example, cobalt, copper, zinc, nickel or molybdenum) are nutrients, but a variety of them, when they are at high concentrations, are toxic to living beings. Some of these toxic elements are sufficiently volatile because their atmospheric transport is significant and can therefore be a danger to the environment. Among them are mercury, lead, arsenic, cadmium and selenium. Many of these trace elements lead to oxidoreduction reactions catalysed by microorganisms, and some can also be found forming organic compounds derived from microbial action.

Microorganisms and microbiological mechanismsb

Heavy metals

We understand heavy metal all those metallic elements that are hamful to the environment, are of high density and toxic to human beings in high quantities, such as mercury, lead and cadmium. Microorganisms have been adapted and have taken resistance to contaminants thanks to the extreme metabolic versatility.

Microorganisms generally treat heavy metals in the following way:

Figure 1: How microorganisms deal with heavy metals.

Some microorganisms involved in the bioremediation of heavy metals.

Microorganism	Metal	Metabolism	Redox metal
Staphylococcus xylosus	Cd	Facultative anaerobes	oxidation
Pseudomonas aeruginosa	Cd	Aerobic	oxidation
Pseudomonas ambigua	Cr	Aerobic	oxidation
Enterobacter cloacae	Cr(VI)	Anaerobic	oxidation
Bacillus sp.	Cd, Cr y Hg	Facultative anaerobes	oxidation
Shewanella oneidensis	Hg(II)	Anaerobic	oxidation
Rhodococcus opacus	Fe(III),Cu(II),Zn(II) Cd(II),Ni(II),Pb(II)	Aerobic	oxidation

Table 1: Microorganisms involved in the bioremediation of heavy metals [5-9].

Immobilization of heavy metals

Biolixivation: it is based on the ability of certain microorganisms to solubize heavy metals present in insoluble dies by transforming them into soluble forms that are easily extracted from the medium in which they are found and allow their subsequent recovery. This capacity mainly involves the formation of organic and inorganic acids. This solubilization mechanism is used in the mining industry.
By the action of microorganisms the metals and minerals are extracted when they are in aqueous phase.
The biolixiviate can be used within the perspective of hydrometallurgy, recovering metals from contaminated solid materials such as soils, ashes resulting from burning waste, aquatic sediments, etc.

Mobilization of heavy metals

Biosorption: it is the removal of metal or metalloid species, compounds and particulates from a solution by binding to biomass. A variety of biomaterials are known that can bind to these contaminants such as bacteria, fungi, algae and industrial and agricultural residues.

Figure 1: Photographs obtaines with an electronic microscope by Klebsiella sp. 3S1 before (left) an then (right) of the lead biosorption [10].

Bioaccumulation: increasing the concentration of a chemical in the biological food chain over time in comparison with the concentration of chemical in the environment. 
This mechanism integrates the heavy metal found on the surface of the cell through the membrane transport system.

Biomineralitzation: obtaining metals, calcium carbonates, phosphates and hydrixudes though microorganisms that make them precipitate out of the cytoplasm through a cellular pump. 

Biotransformation: a process in which microbial enzymes take place in which metal, usually toxic, suffers from a chemical change that can result in compounds that are not soluble in water or volatile compounds.

Figure 2: Scheme of the diferent microbiological mechanisms for the immobilization of heavy metals [11].

Importance of plasmids

Many of the genes transported by a plasmid are beneficial for host cells. For example for resistance to heavy metal or have specific metabolic functions that allow the bacteria to use a particular nutrient, including the ability to degrade toxic compounds.

Plasmids are important because they are transmitted from one bacterium to another through the horizontal transfer of genes. So if a bacterium has the ability to degrade a heavy metal, this ability can pass form bacterium to bacterium through plasmids.

It has been observed that isolated plasmids of Gram-positive and a Gram-negative encode some type of resistance to heavy metals. For example Staphylococcus aureus is resistant to Hg, Cd an As and arsenite, and the resistant mechanism varies for each particular metal.

Conclusions

Bioremediation can be a very effective alternative to reduce the environmental impact generated by the produced anthropic residues. Microorganisms reduce pollution pollutants to the environment by improving the diversity of ecosystems considerably. 

Some advantages of these methods are:
- These methods are less expensive in verse other processes of heavy metal removal, such as restoration.
- Bioremediation can be performed on site without moving the material that will be treated on the site.

In conclusion, the role of microorganisms is fundamental in the biogeochemical cycles of metals and their use in bioremediation processes for the care of the environment, which is why we believe that we should try to find new microorganisms carry out bioremediation of heavy metals to guarantee an improvement in this field.

Bibliography

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9. Pérez, L. Salgado, I. Larrea, C. (2018). Biosorción microbiana de metal·les pesados: características del proceso.  Revista cubana de ciències biológicas, 6 (1), 1-12.

10. Muñoz, A. Biosorción de metales pesados para el tratamiento de aguas residuales industriales. 17.01.2014. Interempresas. 8/12/2018. https://www.interempresas.net/Quimica/Articulos/118219-Biosorcion-de-metales-pesados-para-el-tratamiento-de-aguas-residuales-industriales.html

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