Microbes Are Essential Components of Arsenic Cycling in the Environment: Implications for the Use of Microbes in Arsenic Remediation

Arsenic (As) is a ubiquitously distributed toxic element, and it has been present in the environment since the very beginning of evolution. Hence, microbes to higher organisms possess mechanisms to tackle arsenic that include conversion of arsenic from one form to another including inorganic to organic and vice versa. Microbes present in different environments possess a number of pathways for arsenic conversion and therefore play a crucial role in arsenic cycling in the environment. Arsenic contamination has emerged as a serious problem in some parts of the world in the past few decades. These include Bangladesh, India, China, Vietnam, Pakistan, etc. The presence of arsenic in soil and groundwater in affected areas also leads to the entry of arsenic in plants. The level of arsenic accumulation in plants including edible portions depends on the arsenic species. In this scenario, microbes, which affect arsenic speciation, can play a role in regulating arsenic accumulation and consequently arsenic stress in plants. The microbes can therefore be utilized effectively to safeguard crop plants from arsenic. If the microbes also possess plant growth-promoting ability, this strategy can impart further benefits. A number of plant growth-promoting microbes (PGPMs) have been identified, characterized and utilized for the improvement of growth of plants in arsenic-contaminated environment as well as for the reduction of arsenic levels in plants. This review presents the role of microbes in arsenic cycling in the environment and discusses efforts for their utilization in the amelioration of arsenic stress in plants.