Delphinidin

Years ago, prospectors looking for promising sites to mine silver or gold often noticed that certain plants grew on old mine tailings. They reasoned that these plants might be indicators of the precious metals entrapped in the soil. It turns out that these prospectors had it right.

Certain plants not only grow in heavy, metal-laden soils but are able to extract these metals through their root systems and accumulate them in their tissues without being damaged. Plants absorb metals because they require certain ones such as zinc and copper as components of their proteins and enzymes for normal growth and development. In certain soils, plant mineral uptake makes no distinction between heavy metals such as cadmium or selenium and these required elements.

In either case, these metals are absorbed by the plant’s extensive root system, which may extend a meter or more in depth. Scientists have coined a new term to describe this process—phytoremediation—the use of plants to facilitate the removal of toxic compounds from ground water and soil. The plants that “munch metal” so well are called “hyperaccumulating” plants.

What makes a good hyperaccumulator? Researchers are attempting to answer this question, but so far, nobody knows for sure. One clue may be metal-binding polypeptides called phytochelatins that sequester and detoxify heavy metals in plant tissue. A survey of numerous plants has shown that phytochelatins are produced when these plants are exposed to heavy metals in soil. Interestingly, a wide range of plants—from the most advanced flowering plants (even orchids) to red, green, and brown algae—produce these detoxifying polypeptides.One possible method of accumulating heavy metals is for the plant to transport them into the cell’s vacuoles, a sort of waste disposal dump.

Phytoremediation is attracting the increased attention not only of scientists, but also of regulators who see it as a low-cost alternative for cleaning up contaminated sites across the country. The conventional process of soil excavation and reburial in a landfill is very expensive, typically costing about $1.5 million per acre, depending on the pollutant. The price tag opens the door for many alternative ideas. These plants can be harvested and disposed of—and in certain instances, the metals can even be recovered by sending the plants to a smelter. But can hyperaccumulators effectively get the job done?

Alpine pennycress (Thlaspi caerulescens) is a remarkable hyperaccumulator, able to accumulate 4% of its dry body weight in zinc. This translates into 10 metric tons of zinc per hectare. The problem, however, is that Thlaspi is a small and slow-growing plant. For phytoremediation to become practical, plants with metal uptake rates comparable to those of Thlaspi, but with faster growth rates and larger tissue mass, must be found. Screening of other plants is being done in several laboratories around the world. One plant that has been identified so far is Indian mustard, Brassica juncea, a relative of some highly nutritious vegetables—cabbage, cauliflower, broccoli, collard, kale, and mustard greens. Indian mustard can accumulate 3.5% of its dry body weight in lead. It also can absorb cadmium, chromium, nickel, selenium, zinc, and copper. Cattails (Typha spp.), which also are known to accumulate heavy metals, are already in use in some areas in the final stages of the treament of human sewage.

Despite the advantages of phytoremediation, one drawback is that multiple crops must be planted over several years to reduce contamination to acceptable levels, while removal of the soil provides an immediate resolution. Additionally, there is concern about increasing the accumulation of these metals in the food chain as wildlife and insects eat the plants, accumulating toxicity in their bodies. In this way, toxic metals could work their way up the food chain and pose a new set of problems.

Phytoremediation was put to a big test at Chernobyl, the site of the largest environmental disaster in modern history. In 1986, the meltdown of the nuclear reactor left radioactive wastes scattered over the Ukrainian countryside. Indian mustard is being used to remove radioactive strontium from the soil. Scientists continue to test this promising natural process, and environmental cleanups of the future could be as simple as letting the flowers grow.