Thursday, 15 February 2018

Arsenic can't kill this fern!

Have you ever seen the greenish hue of treated lumber and wondered what it was? In some cases this color comes from chromated copper arsenate, which has been used since the 1930s as a wood preservative. While this preservative has been studied extensively and appears to be safe, the process by which it is applied can lead to arsenic-contaminated soils. Dr. Lena Ma of the University of Florida was walking around a disused wood treatment site one day when she notice a particular species of fern growing all around the site. By bringing some fronds (a fern's leaves) to the lab, she discovered that this fern, Pteris vittata (Figure 1), is able to accumulate 200 times more arsenic than is present in the surrounding soil! This was an incredible finding, because arsenic is very famous for being lethal to living organisms.
Figure 1: Pteris vittata. This fern sucks arsenic out of the soil (or walls!) in which it grows and can hyper accumulate the arsenic in its fronds (leaves).

Arsenic is a quasi-metal substance called a metalloid, meaning that it exhibits some metal-like properties (it can form alloys with other metals), but also some properties of non-metals (it is brittle). It is also chemically similar to phosphorous, an element that is a component in many important components of all of Earth's organisms, such as DNA and RNA. Since arsenic is similar to phosphorous (it is in the same column in the periodic table), it can sneak into living organisms through phosphorous transporters, then interfere with all the essential cellular processes that require phosphorous. It is this interference that makes arsenic so toxic, and why the ability of the Pteris ferns to hyper-accumulate arsenic so amazing.

Intrigued by this arsenic-tolerant fern, Dr. Ma and her colleagues carried out further research on Pteris vittata. They soon found that when this fern grows in healthy soils it does not have such high levels of arsenic, but if arsenic is added the fern rapidly absorbs the arsenic through its roots and stores it in its fronds. After further study, scientists discovered that these ferns seem to have new versions of phosphorous transporters that, instead of moving phosphorous, can selectively move arsenic and quarantine this metal in a special plant cell storage container called the vacuole. It seems likely that this fern does this to poison any animals or insects that attempt to eat it.

Naturally, Pteris vittata (and its relatives that also hyper-accumulate arsenic) have great potential in bioremediation - the use of biological organisms to clean up a polluted environment. Perhaps these ferns could be grown in poisoned landscapes to soak up the arsenic and then be removed and disposed of safely. This is important for all humans since arsenic in soils makes its way into our food and water, causing illness, cancer, and death in high doses. Efforts are underway to see whether what we have learned about Pteris arsenic tolerance can be applied to creating crops that will be able to prevent arsenic from entering their roots via phosphorous channels, thereby reducing the amount of arsenic in our diets. It is fantastic to learn such an important biological tactic from a fern. Ferns have been on earth longer than all animals and insects, and have clearly developed some tricks during this period - only time will tell what other wisdom they may share with us.

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