Introduction
Heavy metal contamination, without doubt, represents a major environmental threat due to toxic effect of metals and their invasion, and subsequent proliferation into the food chain, which results in serious ecological and health issues (Hrynkiewicz and Barum 2014). Contamination due to chemicals like heavy metals is a problem that may have a debilitating influence on the biosphere. The concentration of metals in our environments namely; air, water and soil are increasing at an exponential rate, with contributions mainly from different variety of industrial and domestic sources. The existing heavy metals in the industrial wastewater is a major global concern due to toxicity as well as persistence within the environment, thus necessitating physical extraction (Jina Tanzadeh, 2017).
Currently, attention is turned to the biological removal of these recalcitrant heavy metals from the environment via the process known as an bioremediation. Bioremediation is a branch of environmental biotechnology used as alternative process for the removal of toxic contaminants from the environment (Maria et al., 2015). It is regarded as a cleaner, greener, cost-effective and environmentally friendly approach for removing pollutants and converting them into less toxic forms using microbes to degrade and detoxify hazardous substances (Maria et al., 2015). Bioremediation uses biological agents, mainly the conventional microbes to tidy up contaminated environments (Breisha et al., 2010). However and more recently, attention is shifted towards the use of halophilic microorganisms to accomplish this task.
What are halophilic microbes?
Halophilic microbes otherwise known as salt-loving or salt-tolerant microorganisms are organisms that thrive in an environment with extreme salinity. They are salt-loving organisms that live and thrive in environment with extreme salt concentrations that will destroy most other microbes. They can grow in a hypersaline environment and require a minimum of 0.2 M salt concentration for their growth and metabolism (Bhuva et al., 2013). These organisms can be isolated anywhere with a salt concentration ofsea water. Halophiles have been able to adapt to extreme physico-chemical situations because they have adopted certain molecular mechanisms that will enable them to thrive in such environments (Delgado-Garcia et al., 2012). They are either non-sporing rods or cocci, red to orange in colour. Generally, the salt concentrations in the surrounding environment greatly influence the morphology of the growing cells. Halophiles that can grow in various high concentrations of salt and at the same time survive in the absence of high salts are termed as halotolerant (Margesin and Schinner, 2001). Halophiles may be grouped into 5 main categories based on their salt requirement as; non-halophiles, slight halophiles, moderate halophiles, borderline extreme halophiles, and extreme halophiles (Arora et al., 2014).
Bioremediation application of halophilic microbes
Different traditional methods namely; membrane filtration, chemical precipitation, oxidation and/or reduction, evaporation, ion exchange, and electrochemical treatment process have been used to remove heavy metals from industrial wastewaters. However, these techniques are no longer favorable because of their high cost and ineffectiveness. Bioremediation which involves the use of biological materials to sequester heavy metals using a variety of strategies like bioaccumulation and biosorption is now considered inexpensive, considerably efficient and environmentally friendly alternative (Zmorrod et al., 2021). Microorganisms adopt different mechanisms such as biosorption, biotransformation of heavy metals into less toxic form, extrusion, exopolysaccharide production, and degradative enzymes for heavy metals bioremediation (Igiri et al., 2018 and Orji et al., 2021).
Heavy metals are frequently found in saline and extreme saline environments because of evaporation in such environment, and also as a result of several industrial activities. Hence, some halophilic bacteria and archaea have developed tolerance to heavy metals. Several works (Sowmya et al., 2014, Amoozegar et al., 2012; Zmorrod et al., 2021; Orji et al., 2021) have reported the ability of halophilic microbes to tolerate and remove heavy metals from industrial effluents. These studies revealed the current trend andprospects of salt-tolerant microbes in bioremediation field.
Conclusively, this short but informative research update provides valuable information for exploration and possible application of halophilic microorganisms in bioremediation technology for potential restoration of heavy metals contaminated saline environments.
References
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Bhuva, C. G., Arora, S. and Rao, G. G. (2013). Efficacy of halophilic microbes for salt removal
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Dr. Haliru Musa is a Lecturer II in the Department of Biological Science in Skyline University Nigeria. He holds a PhD in Bioprocess Engineering from Universiti Malaysia Perlis, Malaysia.
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