This article was written for NoCamels by ZAVIT – Science and environment in Israel.
Anyone who has seen the classic 1986 movie, Little shop of horrors, remembers Audrey II, the talking carnivorous plant hungry for human blood. As the film takes creative liberties with the plant’s abilities, Audrey II’s character draws inspiration from natural flora like Venus fly traps (Dionaea muscipula) and pitchers from the Nepenthaceae, Sarraceniaceae, and Cephalotaceae families that devour foraging insects. A parallel can even be drawn from Audrey II’s speech (although exaggerated for dramatic effect) to a natural phenomenon in which certain plants are able to communicate with each other via chemical signaling.
The list of unconventional abilities exhibited by plants does not end there. A new Israeli study presented at 49e Annual conference for science and the environment in Tel Aviv last July discovered another surprising ecosystem service that certain plants can render – the ability to absorb and remove unusually large amounts of polluting metals from contaminated soils. Cadmium, a common metal component in fertilizers, is a common example. Oddly enough, the researchers found that these plants can purify the soil even faster if the plants are tricked into thinking they are being attacked or eaten.
Both in Israel and around the world, contamination of human-made soil has become an all-too-common consequence of modern industrial and infrastructural activity. Due to the refining of fuel, the use of pesticides and agricultural fertilizers, military activities and the presence of municipal sewers and landfills, heavy metals eventually seep into groundwater and soil layers. ground. This not only toxic the environment, but also puts us at risk of direct exposure through drinking water or the consumption of agricultural products.
“A few years ago, while visiting India, I saw first-hand the pollution of the environment and water sources,” says Eyal Grossman, graduate student and senior researcher at the School. of Environmental and Earth Sciences from Tel Aviv University. Seeing the horror it was and the hardships it caused, he understood how important pollution treatment is. “I dreamed of treating metal pollution by natural means,” he adds.
So how much soil is contaminated with heavy metals? With the rise of industry, agriculture and urbanization, it is difficult to assign an exact and definitive value to it, as reports of soil pollution are actively filed. In fact there is more than 10 million soil pollution sites reported worldwide to date, and more than half of these sites are contaminated with heavy metals and / or metalloids. According to Grossman, around 28 percent of the soil on the European continent is currently contaminated with one or more heavy metals at a concentration above the threshold values set by the Finnish Ministry of Environmental Protection (MoEP).
This appears to be a common environmental dilemma that extends to all semi-highly industrialized countries, including Israel. Following a 2014 soil pollution survey, Israel’s environment ministry identified 23,100 localized areas throughout Israel who are or have the potential to be contaminated by currently or formerly active operations.
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Israel’s coastal aquifer is a prime example. Between gas stations, industrial factories, military bases and landfills, the coastal plain area has become heavily polluted by heavy metals over the years; so much so that in 2018, 8 percent drinking water available in the coastal plain had been contaminated and nearly 200 groundwater drilling sites (about a quarter of Israel’s drilling sites) had been closed.
And repairing the earth is no small task; it’s not cheap either. Between the excavation, transport and treatment of polluted soil, the cost of land remediation in Israel alone is about 9.8 billion shekels ($ 3.035 billion), an amount that polluters themselves pay the vast majority. Versions of this financial burden are occurring in all industrialized countries, resulting in a cumulative global economic impact exceeding $ 10 billion per year.
“The problem with polluting metals in soil is that they don’t break down,” says Eyal Grossman.
Without proper soil treatment and treatment of pollution at its source, the concentration of heavy metals in the soil will remain and is likely to increase. However, metals don’t stay in one place in the ground, which is where the problem begins. Metals are taken up by plants and inevitably move up the food chain. In agricultural areas, for example, hazardous metals from fertilizers, pesticides, irrigated sewage, biosolids and manure can build up in crops and farm animals before they reach our plates. In other words, if the metals first avoid infiltrating our groundwater to later contaminate the drinking water in our glasses.
Either way, the environment suffers and public health could be seriously threatened, especially with regard to lead, arsenic or cadmium, all common metal pollutants that are particularly toxic.
Let nature do its job
Defined by IARC (International Agency for Research on Cancer) as a definite carcinogen in humans, exposure to cadmium in particular can lead to health complications such as kidney damage and bone malformations. From an environmental standpoint, the high levels of cadmium in sassafras (Sassafras tzumu), for example, alter the photosynthetic capacity of the tree.
For the purposes of the new Israeli study, the researchers tinkered with phytoremediation, a plant strategy aimed at reducing the bioavailability of pollutants in the soil using certain plants qualified as hyperaccumulators: plants that are hundreds or thousands of times more efficient at storing certain metals and metalloids in their body tissues than what is considered normal for most plants. Why do these plants do this?
“The most common assumption today is that hyperaccumulators take up metals in their bodies and transfer them to the leaves to ward off metal-avoiding parasites,” says Grossman.
To date, approximately 721 hyperaccumulators have been identified which include commonly known plants such as sunflower, corn and rapeseed (from which canola oil is extracted).
“Usually contaminated soil is bulldozed and sent to a treatment facility, which costs a lot of money, or it is simply buried elsewhere, in which case the pollution is simply transported and the problem remains unresolved,” Grossman explains.
But with the hundreds of hyperaccumulative plants identified and the well understood process of phytoremediation, we can instead sit back and let nature “do its job”. Based on suspected or known metal contaminants embedded in a particular site, some hyperaccumulators could be planted in that area, and these plants could ideally take up metals in their tissues and thus improve soil fertility over time. Then, after the plants were harvested, the metals would be removed from the environment without incurring additional expense or damaging the earth itself.
Also, these metal-filled factories could become an important raw material rather than ending up as discarded organic waste and defeating their purpose.
“Often the biofuel can be made from contaminated plants,” says Grossman.
However, to date, the pollutant removal method has hardly been used outside the confines of laboratories and greenhouses. Grossman points out in his study that the main reason this method is overlooked is that the growth rate of plants is relatively slow. This means that it can take several years before all pollutants are completely eradicated from the soil, so the method is often not seen as economically viable, especially in Israel where rapid land restoration is usually undertaken due to pressure from real estate and infrastructure development. , both of which require immediate solutions.
Speed up the process?
Due to the time it takes to revive an area of land via phytoremediation, the researchers wondered what could possibly speed up the process to perhaps make the strategy more economically viable. The solution the researchers found was unusual: damage the plant.
The researchers injected cadmium into the potted soil of 80 common sunflower plants, then made a group of plants “think” that they were being attacked by insects. To do this, they made small perforations in the leaves with a toothpick and sprayed them with jasmonic acid, a plant hormone. “It’s produced by almost all plants when pests attack them, and it triggers the plant’s defense mechanisms,” says Grossman.
The results of the study were more than encouraging. The amount of cadmium absorbed by the group of acid sprayed and perforated sunflowers was 40% higher than that of the group of sunflowers which did not receive any special treatment. According to Grossman, this is equivalent to cutting the time required for effective phytoremediation by almost half. In addition, it should be noted that the sunflowers which received the spray and puncture treatment suffered no inhibitory damage. Instead, this group of treated sunflowers grew identically to the untreated group.
“We are already working on the next experiment, in which we will examine whether cadmium is also stored in the body of the plant, and not just in its leaves. That is, if the plant absorbs even more cadmium than we think, ”says Grossman.
“I call on municipalities and local councils that own contaminated land that will soon be abandoned to cooperate with universities,” says Grossman. “It is a shame that contaminated sites continue to damage the health of the land and the public when you instead apply viable green remediation methods and technologies, which will cleanse the land for the benefit of all. “