Posted by: Green Knight | December 15, 2010

Arsenic, Old Lace, Phytoremediation, and Vexing Vetch

My last blog on selenium (Se) coincided with NASA’s report on bacteria in Mono Lake that apparently can use arsenic (As) in place of phosphorus (P) in constructing the nucleotide backbones of their DNA. There are skeptics on this issue, and “Discover” magazine’s website has an interesting blog discussion about it.

What got me thinking is that chief among the 30 or so primary plant absorbers of Se, and converters of it to more toxic forms, are a number of plants in the genus Astragalus, vetch being a common common name, and all in the pea family. Astragalus bisulcatus, aka two-grooved milk vetch, or Gray’s vetch, is the main culprit, but there are several others, as well as unrelated plants such as woody aster, that all require Se to grow, making them good indicator species for the presence of seleniferous marine shale bedrock and derived soils. It is these plants that convert the Se to forms absorbable by most other plants. Livestock which eat any of them suffer the toxic effects.

Ironically, the various vetches also serve two positive environmental purposes. First, crown vetch, along with fescue and other plants, is a weed of choice for providing vegetative cover on old landfills, to help resist erosion and keep the soil cap intact. Second, vetch, along with sunflowers and others, is used in a type of bioremediation known as “phytoremediation,” from the Greek for plant. This is a method used for brownfields contaminated with heavy metals…rather than excavate the soil and bury it somewhere else, these plants are sown onsite. They happen to be good at extracting and absorbing metals from the soil (like Astragalus and selenium, remember?).  However, in this case, wildlife and livestock are kept away.  When the plants are mature, they’re cut down, baled up, and sent off to an incinerator.  The toxic metals are concentrated in the ash, which is chemically stabilized to an insoluble form, and disposed of at a hazardous waste facility.  This minimizes the volume of waste since the soil remains in place.  And the contaminated site is ready for another crop of vetch, sunflowers, or whatever.  The soil structure is even preserved.  This works best for brownfields, which are sites where threats to health or the environment aren’t imminent and therefore cleanup can take longer.

Obviously we can’t do this in all the high-selenium sites in locations from the Canadian prairies to Texas and every state west of that line, but the technique could be used in high-threat areas like wildlife refuges, grazing land, etc.  This is a task that will require a multiagency approach, with BLM, the Bureau of Reclamation, Fish & Wildlife, USGS, USDA, EPA, the National Park Service and Forest Service, and related State agencies to cooperate in varying degrees depending on conditions at each affected site, but overall guidelines should be established and lessons learned as high-priority sites are remediated.  “Death in the Marsh” was published in 1991, and I couldn’t find any information on strategies to address the selenium situation since that time; wheels are apparently still spinning, with little road contact.

[Interestingly, vetch of the genus Vicia is used at some wineries, such as Frog’s Leap, as a cover crop to attract beneficial insects and fix nitrogen in the soil.]

Back to arsenic (As).  We already use natural microbes, and sometimes genetically-engineered ones, to eat up oil and chemical spills, so the possibility that an extremophile or three beat our gengineers to the punch is stimulating to consider.  It raises all sorts of possibilities in terms of coming up with custom bugs to handle specific types of contamination.  To Carl Zimmer and the other skeptics over at “Discover,” well, call me an As-hole, but I find that life is nothing if not opportunistic.

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