Tolerance of Microorganisms to Heavy Metals

Introduction

Heavy metals are a variety of elements whose distribution in the environment is ubiquitous. Their distinguishing characteristics are a high atomic weight, density and assorted applications in different areas. Many heavy metals occur naturally in the earth’s crust and have essential functions in living organisms. For instance, copper (Cu) is a cofactor in many metabolic reactions and proteins containing this metal enhance iron transport, free radical protection and respiration in living organisms (Singh, Parihar, Singh, Singh & Prasad, 2015). Other metals such as sodium (Na), nickel (Ni), zinc (Zn), manganese (Mn), magnesium (Mg), iron (Fe) and potassium (K) are essential minerals required for growth and functioning of plants and animals. These heavy metals are only required in trace concentrations. Other heavy metals such as cadmium (Cd) and mercury (Hg) do not have biological roles and are harmful even in minute concentrations. The heavy metals however can occur in excess levels in the environment because of anthropogenic activities as illustrated in Figure 1. In such a case, they induce harmful effects to plants, animals, humans and the ecosystems. Heavy metal pollution of the environment through human activities is an ecological concern worldwide because metals persist and bio-accumulate in the environment causing disruptions in food chains and ecosystems (De Silva et al. 2012). Additionally, heavy metals do not decompose and cannot be degraded. Once in food chains, they enter in human and animal tissues, which increases the vulnerability of these organisms to diseases and gene transformations/mutations, which are carcinogenic in the long term.

Figure 1.

Anthropogenic sources of named heavy metals

The focus of this book chapter is heavy metal toxicity on microorganisms and approached to bioremediation. Some of the toxic effects of heavy metals on microorganisms documented in literature are summarised in Table 1. To alleviate this pollution problem, a number of biotechnological advances including geno-remediation, cyano-remediation, myco-remediation, rhizo-remediation, bio-stimulation, hyper-accumulation, phyto-stabilization, bio-sorption and bio-mineralization among others have been applied to remove heavy metals from the environment (Samantaray et al., 2014). The toxicity and ubiquitous nature of heavy metals has forced organisms to adapt to their presence. This is a natural process where microbes develop recycling and degradation potential to accumulated heavy metal contaminant resulting to the decrease of their toxicity. Singh et al. (2015) for instance, reported that plants are developing tolerance to heavy metals. Yang, Agouri, Tyrrell and Walsh (2018) associated enterobacteriaceae species to heavy metal resistance genes in humans. These bioremediation measures are growing to be effective and economic strategies to detoxify and accumulate metals in microorganisms so that they are no longer harmful to the environment. Apart from intrinsic bioremediation, some chemicals can be added to stimulate microbial bioavailability in the process of bio-stimulation.