Undoing pollution with plants

In abandoned explosives testing areas and mining sites now reduced to rubble, one notices very little life except for some surprisingly vigorous vegetation growing in the midst of the poor soil. These are not, for the most part, showy plants—switchgrasses in shades of dull-green, ferns that peak out modestly from mountainside cracks, and clumps of pennycress that seem more fitting as part of the background rather than the star of the landscape.

Despite their humble appearance, these plants are quietly performing something many environmental scientists have struggled to understand for decades: hyperaccumulation. The adaptation these plants have made to survive in tough habitats makes then not just tolerate toxic substances, but also collect and break them down.

Now doesn't that just take the fun out of environmental engineering?

Across the US, human activities such as mining, explosives testing, and heavy herbicide use have rendered many otherwise great pieces of land nearly lifeless. In fact, the US Army estimates that more than 538 sites nationwide are contaminated with TNT and RDX, both toxic and explosive chemicals.¹

Scientists already use bacteria² and fungi to bioremediate contaminated areas—which means they basically clean up the sites with living organisms. They've proven to be successful in some cases, and less successful in others. The success varies, because each treated site has different levels of contamination as well as different types of competition from other organisms.

Unlike these bacteria and fungi, plant hyperaccumulators are a bit easier to manage. For one, they're a lot easier to collect and dispose of, which ensures that less of the pollutant gets back into the environment after the hyperaccumulators decompose. Best of all, sometimes native plants can carry out the job, which ultimately reduces the risk of introducing invasive species into an area.

It's true that there are a lot of limitations to this kind of decontamination method. Hyperaccumulators are still plants, so they're generally limited to certain pH ranges. They also take a lot longer to grow and therefore accumulate pollutants more slowly than other methods. And while there's a lack of efficient phytoaccumulators for certain metals such as mercury right now, a better understanding of the process could lead to the development of mercury-hungry plants.

Scientists have known about phytoremediation-the process of using plants in particular for bioremediation—for a while, but the practice is continually improved to make it more efficient, and so that it can clean up a wider variety of pollutants. There's still lots of fun left in phytoremedation for those in biotech, and there's always room for improvement.

Phytoremediation of metal-contaminated soils is relatively easy.  Many plants contain natural enzymes that attract and bind to metals, which makes these metals less dangerous. The gene for an enzyme called mercuric ion reductase, which turns mercury into a less reactive and dangerous form, can actually be introduced into a transgenic plant³-and there you go, a customizable pollutant-gobbler!  There are also some chemicals additives that can go straight into the soil that make it easier for plants to take up harmful metals, such as lead.⁴

What about organic toxins, such as herbicides? Some plants host microbes that assist with bioremediation. These plants actually release chemicals that give resident microbes a boost, which ultimately facilitates this combined plant-microbe function. Other plants can break down the toxic organic compounds in herbicides and store them, or even release them into the air. Scientists are still struggling to keep the latter from reducing their ability to collect toxic materials, but genetic modification of both plants and symbiotic bacteria seems to be the key.⁵

But do the words "transgenic plants" evoke anxiety? Does the much-debated Pandora's box of GMO crops spring to mind? Whether or not you agree with it, GMO is expanding into many applications, and phytoremediation is no exception. If we take precautions, transgenic plants can restore land that has been inhospitable since the Industrial Revolution. And using plants as a long-term method for cleaning up soils doesn't take much investment or energy-with more research, plants can clean up generations' worth of deadly chemical buildup.

Let's hope the lessons learned from unintended consequences of Bt crops and other instances of GMO's hazards will carry over here, and give transgenic phytoremediation the chance to flourish.

²Ryan, Robert P., Monchy, Sebastien, Cardinale, Massimiliano, Taghavi, Safiyh, Crossman, Lisa, Avison, Matthew B., Berg, Gabriele, van der Lelie, Daniel, J. Maxwell Dow. The versatility and adaptation of bacteria from the genus Stenotrophomonas. Nature Reviews Microbiology 7, (July 2009) 514-525.

³Salt, D.E., Smith, R.D., I. Raskin. Phytoremediation. Annual Review of Plant Physiology Plant Molecular Biology 49 (1998) 643-68.

⁴Sas-Nowosielska, A., Kucharski, R., Małkowski, E., Pogrzeba, M., Kuperberg, J.M., Kryński, K. Phytoextraction crop disposal-an unsolved problem. Environmental Pollution 128, issue 3 (Febuary 2004) 373-379. 

⁵Newman, Lee A. and Reynolds, Charles M. Bacteria and phytoremediation: new uses for endophytic bacteria in plants. Trends in Biotechnology 23, issue 1 (January 2005) 6-8. 

Other resources:

This site has an amazing way of presenting the contaminants and other information related to Superfund sites, great for both the science and art-types.

Wikipedia compiled a table of natural hyperaccumulators

Salt, David E., Blaylock, Michael, Kumar, Nanda PBA, Dushenkov, Viatcheslav, Ensley, Burt D., Chet, Ilan, Ilya Raskin. Phytoremediation: A novel strategy for for the removal of toxic metals from the environment using plants. Biotechnology 13 (1995) 468 - 474.

Illustration by Justine Chow