Why in the News?
A recent study has revealed that the composition of microbial communities in rice paddies critically determines the buildup of arsenic compounds in rice grains.
Arsenic Toxicity in Agriculture:
- Overview: Arsenic (As) is a potent carcinogen and phytotoxin, bioaccumulating in rice and posing severe health and agronomic risks in Asian paddies.
- Mechanism in Flooded Fields: Under anaerobic conditions, microbes convert arsenic into soluble, bioavailable forms that rice roots readily absorb.
- Toxic Compounds: Organic forms like dimethylarsinic acid (DMA) and dimethylated monothioarsenate (DMMTA) cause straighthead disease, producing sterile, erect panicles and yield losses up to 70 %.
- Speciation vs. Concentration: Toxicity depends on arsenic speciation, not total soil As levels, even low-As soils may cause poisoning.
- Geographic Hotspots: Severe in West Bengal, Bihar, and Bangladesh, where arsenic-laden groundwater is used for irrigation.
About Soil Age and Microbial Composition:
- Research Insight: Study by Peng Wang (Nanjing Agricultural University) shows soil age dictates microbial dominance and arsenic behaviour.
- Young Soils (< 700 yrs): Dominated by arsenic-methylating bacteria that convert inorganic As into toxic organic forms (DMA, DMMTA).
- Old Soils (> 700 yrs): Rich in demethylating archaea that detoxify As by breaking down methylated compounds.
- Global Microbiome Survey: Across 801 paddy soils, identified 11 methylators and 6 demethylators as key toxicity predictors.
- Risk Threshold: When methylator: demethylator ratio > 1.5, probability of straighthead disease rises sharply.
How does Microbial balance govern Arsenic toxicity?
- Biological Equilibrium: Arsenic toxicity depends on balance between methylating bacteria (risk) and demethylating archaea (detoxification).
- Environmental Triggers: Flood duration, oxygen, temperature, and hydrological shifts can tilt this balance toward higher toxicity.
- Mitigation Measures: Mid-season drainage, silicon fertilisation, and microbial community management restore redox balance and reduce As uptake.
| [UPSC 2013] Which of the following can be found as pollutants in the drinking water in some parts of India?
1. Arsenic 2. Sorbitol 3. Fluoride 4. Formaldehyde 5. Uranium Select the correct answer using the codes given below. Options: (a) 1 and 3 only (b) 2, 4 and 5 only (c) 1, 3 and 5 only* (d) 1, 2, 3, 4 and 5 |
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