Scientists have discovered that the tiny microbes living in rice fields play a crucial role in how much arsenic, a harmful substance, ends up in our rice. A new study shows that a balance between certain types of bacteria and archaea can either increase or decrease the risk of arsenic building up in rice grains. This buildup can lead to serious crop losses and health problems.
The research, published in the journal Proceedings of the National Academy of Sciences, highlights the importance of a microbial balance. Arsenic-methylating bacteria convert inorganic arsenic into toxic organic forms, while demethylating archaea can reverse this process. When the methylating bacteria are more active, rice plants absorb harmful compounds like dimethylarsinic acid (DMA) and dimethylated monothioarsenate (DMMTA). These compounds are not only dangerous for human health but also cause a problem known as straighthead disease in rice.
Straighthead disease causes rice plants to have upright, empty grains. “Straighthead needs to be considered as a physiological disorder rather than a disease in the absence of any infectious agent,” explains rice pathologist Sridhar Ranganathan. He notes that affected plants have erect panicles with unfilled grains that stay green, unlike healthy plants where the panicles droop with mature, filled grains.
While once seen as a minor local issue, straighthead disease is now a global concern. Farmers in parts of the US and China have reported significant outbreaks, especially in newly established or rotated rice paddies. Areas like West Bengal in India and Bangladesh have also faced this problem, which can lead to yield losses of up to 70% in severe cases. The issue isn’t just the total amount of arsenic in the soil, but its chemical form, known as speciation.
The study found that the types of microbes in the paddies determine this speciation. Researchers led by Peng Wang at Nanjing Agricultural University in China examined rice fields of different ages. They observed that soils younger than 700 years were dominated by methylating bacteria, leading to more arsenic accumulation and a higher risk of straighthead disease. In contrast, older soils (over 700 years) had more demethylating archaea, which helped break down the harmful arsenic compounds.
By combining field data with lab tests and genetic analysis, the team identified specific microbes that can predict arsenic risk. They found that newer paddy regions, including parts of the US, southern Europe, and northeast China, have a higher ratio of methylating to demethylating microbes, making them more vulnerable. Older rice-growing areas in South and Southeast Asia, however, have stronger demethylating communities.
The study noted that a ratio of methylating to demethylating microbes exceeding 1.5 significantly increases the risk of straighthead disease. India, a major rice producer, has many old rice fields with balanced microbial communities. However, some states, especially in East and South India, have newer fields that could be at greater risk. This is compounded by existing arsenic contamination in groundwater in states like West Bengal, Bihar, and Assam.
Experts also point out that climate change could worsen the situation. Higher temperatures and changing flood patterns might increase arsenic levels in soils and shift the microbial balance towards the more harmful types. With rice being a staple food for a large part of India’s population, ensuring its safety and productivity is vital.
Dr. Ranganathan suggests that agronomic practices can help manage these risks. Draining rice fields during the growing season can reduce methylating microbes by introducing oxygen into the soil. Applying silicon fertiliser can also decrease arsenic uptake by rice plants. Crop rotation can also be adjusted to maintain healthy microbial communities.
At a policy level, the findings emphasize the need to monitor arsenic speciation, not just total arsenic levels, in food safety regulations. Current international standards, like the UN Food and Agriculture Organisation’s ‘Codex Alimentarius’, often focus on inorganic arsenic, leaving gaps for potentially harmful methylated species.