**6. Conclusions**

Natural and anthropogenic activities generate large quantities of aqueous effluents containing toxic metals. Many studies have been conducted in recent decades aimed at lowering metal concentrations derived from natural resources. In addition, considerable effort has been made to develop efficient and cost-effective technologies and apply them to industrial wastewater treatment. The potential for microorganisms to remove metals from solutions through passive and active mechanisms has been shown to be an interesting approach to metal uptake in polluted waters, and the efficiency of such processes is dependent on the experimental conditions, the target pollutant and various other factors.

The application of this type of bioremediation process in large scale remains, however, a challenge, and a preventive approach to metal pollution problems is therefore encouraged. Further investigations aimed at the identification of the mechanisms involved the characteri‐ zation of biosorbents, and advances in genetic engineering are required.

Many microorganisms can break down metals naturally, but this is not a sufficient solution on a global scale. Therefore, as a means to resolve this problem, engineered microorganisms can be developed with the help of genetic engineering. A better understanding of the way in which both eukaryotes and prokaryotes metabolize heavy metals and the detoxification pathways will help future researchers to deal with this type of environmental problem with maximum efficiency. The choice of the most promising type of biomass must be made, taking into account its cost and availability, and this is necessary on an industrial scale. The micro‐ organisms should be easy to obtain and to cultivate. For example, industrial-scale application would not be of interest if the microorganism is difficult to cultivate, a rare species or a species in danger of extinction.

Although some progress has been made in the recognition of the importance of microorgan‐ isms for the decontamination of polluted waters, some important points still need to be addressed. However, a new challenge has emerged for science. Thus, further studies need to focus on the development of new clean environmentally acceptable technologies with commercial feasibility.

