Arsenic Contamination in the Environment

Arsenic is a ubiquitous element (atomic number 33) that occurs either as a component of many minerals, sulfur, and metals, or as a pure element such as crystal. It has several uses in industries and medical fields. However, arsenic is also well known as “the king of poisons” and has been used for political assassination. As per the European Union under directive 67/548/EEC, elemental arsenic and arsenic compounds are classified as “toxic” and “dangerous for the environment”. It is mutagenic, carcinogenic, and teratogenic. Several organizations including the International Agency for Research on Cancer (IARC) identified arsenic and arsenic compounds as group 1 carcinogens. Arsenic contamination in groundwater is a growing concern worldwide, affecting more than 70 countries on six continents. Southeast Asian countries, especially Bangladesh, India, and Nepal, are badly affected. Considerable efforts are required and detailed consolidated studies should be undertaken to provide a comprehensive basis for understanding the causes of the problem and its variations in space and time.

Exposure to arsenic is one of the most perilous public health crises. Arsenic contamination in drinking water and food sources has many harmful effects on human health. More than 200 million people in different parts of the world are exposed to arsenic concentrations in drinking water with more than the recommended limit of 10 μg L⁻¹. Several countries of the world are facing this crisis, acknowledging the adverse health implications and deterioration of quality of life due to arsenic toxicity. Properties of arsenic and its different species along with organic components that control its biogeochemical cycle are one of the most important aspects in addressing the issue. Several microorganisms play a crucial role in arsenic speciation in both anoxic and oxygen-rich environments. Furthermore, proper methodology (including appropriate field testing systems) for examination and management of arsenic is required. Thus, analysis of the arsenic content of water and other environmental samples as well as food stuffs is an important issue because it is directly correlated with key decision making regarding the maximum contaminant level. An integrated approach for understanding arsenic, its different chemical species, and their detection is required to properly mitigate the crisis.

Arsenic is a widespread element in the environment and it is highly toxic to human health. It is a Group A human carcinogen. Chronic arsenic exposure causes various dreaded ailments including cancer in skin, lung, and bladder, as well as in other organs. Arsenic induces epigenetic alterations and aberrant DNA methylation, causing inherent damage to cells. Addressing the issue at molecular level, researchers are interested to understand the distribution, metabolism, and potential modes of actions of inorganic arsenic (iAs). Paradoxically, apart from poisoning, arsenic has long been in medicinal use for treatment of various diseases like asthma, syphilis, trypanosomiasis, lichen planus, verruca plenum, tropical eosinophilia, and psoriasis. It is also a potentially important chemotherapeutic agent against acute promyelocytic leukemia and can help in activating the cytotoxic effects of DNA damaging chemotherapeutics. To extend precise therapeutic strategies against arsenic, integrated study on environmental monitoring, health surveillance, exposure data, individual risk characterization, and human biomarkers are required which will be able to provide mechanistic insight into the pathogenesis of disease processes.

The widespread toxic metalloid arsenic (As) has the ability to induce oxidative stress through generation of excess reactive oxygen species (ROS) and mediate response of disease states in animals and growth as well developmental traits in plant species through modulation of key cellular signalling molecules and pathways. In recent years, growing evidences are indicating that apart from nucleus-controlled gene expressions, various epigenetic mechanisms play major roles in response, regulations, and alterations of cellular As toxicity. Epigenetic events govern heritable changes that do not involve changes in the nuclear DNA sequence and thus, it is a potentially reversible DNA alteration and a priming mechanism for transgenerational and next generation fitness to better tolerate environmental stresses. As can modulate DNA methylation, the covalent, posttranslational modifications of histone proteins, and small noncoding RNAs or micro-RNA (miRNA) and regulate number of homeostatic and inducible gene expressions. DNA methylation is the most diverse and versatile epigenetic mechanisms of As-induced toxicity in both plants and animals and involves both hypomethylation and hypermethylation in structural gene sequence and promoter regions. On the other hand, As-induced methylation, acetylation, and phosphorylation of histone proteins are some of the prominent events during epigenetic response of As toxicity. Changes in miRNA expressions during As exposure are manifested differential expressions of their target genes and consequent changes in different growth and developmental processes in plants and animal as well as in human system. The complicated interactions among these epigenetic events and capability of As to inherit the epigenetic response mitotically and/or meiotically in next generations and even during foetal programming are some of the interesting events occurred during As-mediated epigenetic response of cell. Also, changes in epigenetic landscape during As-induced processes leading to tumorigenesis and/or carcinogenesis are important cellular events which need to be holistically explored. With development of functional epigenomics, toxicokinetics and rapid advancement of high end precision technologies, As-induced epigenetic footprint or memory may be utilized as a reliable biomarker of future risk assessment for As toxicity.

Arsenic removal from contaminated water has become a global concern, and the development of arsenic removal processes from potable water is still a major challenge. Several chemical based arsenic removal technologies are available, which are chiefly based on the combination of the processes like oxidation, coagulation, filtration, ion exchange, and adsorption. However, any technology selected for the purpose should comply standard set by WHO for drinking water with a capacity to remove arsenic below 10 ppb. Further, technology related issues like sludge disposal, operation and maintenance, etc. are very important for considering any implementation for supply of arsenic-free potable water. It is undoubtedly a challenging task to develop appropriate, efficient, and cost-effective and user friendly technology to serve the arsenic-free water to the humanity of diverse economic and ecologic locations facing dreadful situations with arsenic.

Across the globe, millions of people are exposed to toxic effects of arsenic (As) due to contaminated water intake and involvement in different activities, including agriculture. As is present in various oxidative forms in environment and enters into food chain through soil and water. As-contaminated underground water used for irrigating the crops affects crop production and creates a human health risk and even death. Further, a couple of important crop plants require considerable amount of water. Many countries are depending on Rice as staple food counting India. Rice (Oryza sativa) being a cereal crop and a staple food for many countries around the world potentially accumulates As particularly inorganic arsenic (iAs) in its different parts including in grains. Rice absorbs arsenate (As^V) through the phosphate transporters, and arsenite (As^III) through the nodulin 26-like intrinsic (NIP) by silicon transport pathway and plasma membrane intrinsic protein aquaporins. Recently, studies have been carried out on arsenic reductase (HAC1) for arsenic elimination from crops. Considerable work has been done in rice to elucidate As transportation and uptake processes in rhizosphere and metabolism in rice plant, which can further help in developing better strategies. Common agronomic practices like rain water harvesting for crop irrigation, use of natural chelators, hyperaccumulator plants, and genetic modification may be explored to reduce As uptake by food crops.

Arsenic (As) pollution is a significant environmental problem. In nature, As exists as inorganic or organic species but is normally not encountered in its elemental state. As is a nonessential metalloid and does not play any biological role in algae, plants and causes toxic response after gaining entry into the cell. Upon translocation to the shoots As can severely impede growth of the plants by slowing or arresting accumulation of biomass, as well as induce loss of fertility, yield, and fruit production. Several reports are there indicating that an elevated concentration of As in soil causes a significant reduction in crop yield. Algae and plants have developed a range of strategies to combat As toxicity including chelation and sub-sequestration of complexes in vacuole. As contamination in human occurs through consumption of cereals, vegetables, and fruits irrigated with As-contaminated water. The consequence is a global epidemic of As poisoning, leading to skin lesion, cancer of bladder, lung, and kidney and other symptoms. Remediation of As-contaminated soil and groundwater, therefore, is an urgent need for providing safe drinking water and food. Among the various bioremediation processes, phytoremediation by algae and plants is quite effective. Phytoremediation strategy involves suitable plants including arsenic hyperaccumulating ferns and some aquatic or terrestrial angiosperms that efficiently remove the metalloid from highly contaminated soil/water. Utilization of transgenic plants is becoming a new promising tool to enhance phytoremediation potential. There is an urgency to have extensive knowledge on arsenic uptake, transport, metabolism, and detoxification in algae as well as plants for improving phytoremediation. The objective of this review is, therefore, to provide an overview about the uptake of the inorganic and organic species of arsenic, their translocation and biochemical fate in algae and plants and to explore the current concepts of phytoremediation along with their limitations and challenges associated with the developed processes.

Arsenic (As) is one the extremely toxic metalloids that adversely affects health and hence it is categorized under group A human carcinogen. Generally, As-contaminated sites are not remediated due to high cost. Phytoremediation is the process of using plants to treat or clean up contaminated sites and it relies on natural ability of plants to extract, accumulate, or detoxify chemicals from water, soil, and air using energy from sunlight. Over the past several years, significant progress has been made to improve the effectiveness and efficiency of phytoremediation for removal of many hazardous metals from environment. Recent progress in understanding and identification of several genes involved in As uptake, transport, and metabolism in plants led to use of transgenic plants for remediation. Initial experiments of using transgenic plants as a tool to remove As were not promising; however the last decade witnessed a dramatic increase in the reports on the ability of plants to remove/detoxify As. Transgenic plants exploit the natural ability of plants, which rely on uptake of As by roots, transport through vascular system and leaf as a sink to concentrate. An array of genes from different sources including microbes, plants, and animals were successfully used to improve the ability of plants to tolerate, detoxify, and accumulate As. Transgenic plants containing specific genes converted toxic As to other forms that are less harmful. This review examines the recent advances in enhancing phytoremediation through transgenic approach for phytoremediation of As.

Arsenic (As) is an extremely toxic metalloid that naturally occurs in the environment from geochemical weathering of rocks, volcanic emission, and anthropogenic activities. The detrimental effects of arsenicals on humans is an increasing menace chiefly due to contaminated drinking water and foods as the levels of As have been elevated in soil and groundwater across the globe. Remediation of arsenic-contaminated soil and groundwater therefore, is an urgent need for providing safe drinking water and food. Bioremediation became an emerging alternative to conventional energy intensive, instrument and chemical based expensive restoration technologies of heavy metal or metalloid contaminated areas of land and water. Bioremediation by microbes (bacteria, fungi, yeast) are quite effective and relies on deliberate action of natural or engineered microbial activity to reduce, mobilize, or immobilize, volatilize As through sorption, bio-methylation, complexation and redox reactions.

To improve the As bioremediation, extensive idea about uptake and the biochemical pathway for metabolism and detoxification of this metalloid by the microbes is necessary. In this review, uptake and metabolism of As in bacteria and fungi and their potential utility on environmental arsenic remediation has been focused.