Role of Prokaryotes for Arsenic Reduction and Detoxification in Rice
By: 1Afroz Ali and 2U.N. Rai
Arsenic toxicity is a well debated topic among the science fraternity and has become a global concern due to its ever-increasing contamination of soil, water and crops.
Arsenic is a class one carcinogen present ubiquitously in the environment. Mining activities, use of Arsenical herbicide &insecticides and irrigation with Arsenic contaminated ground water results in arsenic accumulation in paddy soil, especially in south and South East Asia.
Arsenic posses a dietary risk for human health. Crops and vegetables grown on the arsenic contaminated soils are responsible for human exposure to arsenic. Accumulation of arsenic in various food composites (potato skin, leaves of vegetables, rice, wheat, cumin, turmeric powder and cereals ranged between <0.0004and0.693mgKg
Rice is particularly efficient in Arsenic accumulation compared to other cereal crops because of anaerobic conditions of the paddy soil and due to sharing the highly efficient silicon pathway..A number of studies have shown that consumption of the rice contributes a large proportion of the dietary intake of inorganic arsenic, particularly for those subsisting on rice. In addition, arsenic causes significant yield losses due to arsenic phytotoxicity. Moreover much higher percentage is present in rice straw which is used as cattle feed and provides another entry route into the food chain.
Along with the adverse health effects caused due to the accumulation of arsenic in rice grain our country is also facing a challenge on economic front, India has a great export market of Basmati rice but due to the increasing levels of arsenic in rice the exports are getting affected.
Iran is biggest Basmati rice importer, contributing about 1 million tons which is 32% of total basmati export of around 3.4 million tons in2012-2013.but in 2014, Iran has modified the accepted level of arsenic content in Basmati rice (150 ppm to 120ppm).
Long term exposure to Arsenic has been associated with skin, lung, bladder, liver, kidney and prostate cancer. Low levels can cause skin lesions, diarrhea and other symptoms.
Bioremediation of Arsenic toxicity in rice is one of the major challenges of the scientific community. In the developing countries of south and south-East Asia there is a dire need of development of strategies which are cost effective and at the same time are environment friendly. Development of potential mitigation strategies of arsenic accumulation in grain through manipulating microbial communities is highly promising.
Arsenic biotransformation, mainly reduction, oxidation and methylation in soil rice system have fundamental impacts on its mobility and toxicity. In addition to soil chemical properties (Ph, Eh, metallic oxides, organic matter) micro organism play critical roles in arsenic transformation and mobility in paddy soil. Arsenic species in paddy soil directly influences arsenic speciation in the rice grain because the methylated arsenic species are mainly derived from the microbial methylation in paddy soil. Arsenic speciation and mobility is also highly affected by the micro flora present in the paddy field soil.
A recent study has identified bacterial strains capable of oxidizing toxic Arsenic into less toxic forms, offering a feasible and affordable solution to the problem of Arsenic in soil and water. Bacterial isolates of Geobacillus stearothermophilous could oxidize 30 millimolars of toxic inorganic Arsenic 3 into less toxic arsenate within 24 hrs.
These strains have been found to be more effective than any other bioengineered strains so far. The bioremediation method is cheaper than the metal decontamination method used world wide.
1 International Society of Environmental Botanists, National Botanical Research Institute, Lucknow (India)
2Ecotoxicology & Bioremediation, National Botanical Research Institute, Lucknow (India)