Arsenic, a naturally occurring toxic element, is found throughout the Earth’s crust in rocks, soil, air, and water. Both geological processes and human activities, such as mining and the historical use of arsenic-containing pesticides, contribute to its presence. Understanding how plants absorb arsenic is important for assessing its impact on human health and developing environmental management strategies. This interaction can lead to arsenic entering the food chain, raising concerns for consumers globally.
How Plants Take Up Arsenic
Plants primarily absorb arsenic from the soil and water through their root systems. The form of arsenic in the soil influences how readily it is taken up by plants. In aerobic (oxygen-rich) soils, arsenate [As(V)] is the dominant form, which plants can mistakenly absorb because its chemical structure is similar to phosphate, a necessary nutrient for plant growth. Phosphate transporters facilitate the entry of arsenate into plant cells.
Under anaerobic (oxygen-poor) conditions, such as those found in flooded rice paddies, arsenite [As(III)] becomes the predominant form of arsenic. Plants absorb arsenite through aquaporin channels, which also transport silicic acid. Once inside the plant cell, arsenate is rapidly converted to arsenite by arsenate reductase (ACR2). Arsenic can then move within the plant, with xylem tissues facilitating its translocation from roots to shoots, and phloem transportation moving it to edible parts like grains.
Plants Known to Accumulate Arsenic
Certain plant species are good at accumulating arsenic, making them important for environmental risk and remediation. Rice (Oryza sativa) is a prominent example, known to take up significant amounts of arsenic, especially when grown in flooded conditions. This is partly due to rice’s efficient silicon uptake pathway, which arsenite can exploit for entry into root cells. Different rice varieties grown in the same soil can accumulate varying amounts and forms of arsenic, with some accumulating more inorganic arsenic, which is generally more toxic.
Ferns, like the Chinese brake fern (Pteris vittata), are recognized as hyperaccumulators of arsenic. These ferns can absorb high concentrations of arsenic from the soil and store it in their fronds without experiencing severe toxicity. While most plants tend to keep arsenic in their roots, these hyperaccumulators efficiently translocate it to their above-ground tissues. Other plants like horsetail (Equisetum spp.) and common bird’s-foot trefoil (L. corniculatus) have also shown strong accumulation in shoots.
Health Concerns from Plant-Accumulated Arsenic
Consuming plants that have accumulated arsenic poses various health risks to humans, primarily through dietary exposure. Both inorganic and organic forms of arsenic are toxic, with inorganic forms being more acutely hazardous. Chronic exposure to even low levels of arsenic can lead to a range of adverse health effects over time.
Long-term consumption of arsenic-contaminated food can result in skin lesions, including pigmentation changes and hyperkeratosis. Beyond skin manifestations, chronic arsenic exposure is linked to more severe internal health issues. These include an increased risk of developing various cancers, such as those of the bladder, liver, lung, and kidney. Additionally, arsenic exposure has been associated with cardiovascular diseases like stroke and ischemic heart disease, as well as conditions such as diabetes and peripheral vascular disease.
Phytoremediation: Plants for Arsenic Cleanup
Phytoremediation is an environmentally conscious approach that uses plants to clean up arsenic-contaminated sites. This method leverages the natural ability of certain plants to absorb, accumulate, or stabilize contaminants from soil and water. The goal is to remove harmful elements from the environment or to render them less mobile and thus less hazardous.
One technique, phytoextraction, involves plants absorbing arsenic into their harvestable tissues, effectively removing it from the soil. The Chinese brake fern (Pteris vittata) is a prime example used in this process, capable of concentrating arsenic in its fronds. After the plants have accumulated the arsenic, they are harvested, and the arsenic-rich biomass can be disposed of safely or even processed for arsenic recovery. Another technique, phytostabilization, uses plants to immobilize arsenic in the soil, reducing its bioavailability and preventing its spread. This can be achieved by plants sequestering arsenic in their roots or by altering soil conditions to reduce arsenic mobility.
Minimizing Arsenic Exposure from Plant-Based Foods
To reduce dietary exposure to arsenic from plant-based foods, especially rice, several practical steps can be taken. Rinsing raw rice thoroughly before cooking can help remove some of the arsenic. Cooking rice with excess water, similar to how pasta is prepared, can further diminish arsenic levels. Draining the leftover water after cooking can also reduce arsenic.
Selecting certain rice varieties can also help lower exposure. Brown rice generally contains more arsenic than white rice because arsenic tends to accumulate in the bran layer, which is removed during the processing of white rice. Basmati rice often has lower arsenic levels compared to other types. Diversifying one’s diet by including other whole grains such as quinoa, barley, or millet, which do not accumulate arsenic, can also help minimize overall exposure.