Human activities often have a number of consequences. In addition to the pollution that our industries and general course of living and expanding bring about, even more minor activities can cause significant environmental contamination. The mere act of digging a deep hole can expose materials and metals that are then able to get into the water table and increase the contaminants found in downstream rivers.
Because of situations similar to that and due to the nature of byproducts from chemical processes used in industrial manufacturing, medicine, and even just construction, heavy metals are some of the biggest concerning pollutants in the world. It is quite easy for them to build up in the soil or in the water supply and without proper filtering or oversight of our water or food, they can have toxic effects when either are consumed.
And that’s all without considering the direct effect such changes have on the environment and ecosystems dealing with the heavy metal buildup. Unfortunately, the methods commonly and most effectively used to filter out heavy metals from something like water are labor intensive and require the use of materials derived from fossil fuels and that put out significant CO2 when consumed for their filtering purpose. That isn’t the most ideal scenario or outcome, so scientists have long been looking for alternative options.
That is where bioremediation comes in. But a specific sub-class of it is needed for dealing with filtering of water and the removal of heavy metal contamination. Phytoremediation is the use of plants in order to use their natural filtering abilities to clean up contaminants. This phytofiltering then has the specific nature known as rhizofiltration, where the roots specifically of plants are used for filtering water sources. Mangrove trees are among the most well-known examples of a natural filtering capability found in nature. But rather than dealing with roots, we’ll be discussing an even more unique style of filtering.
The Power of the Moss
Scientists at Japan’s RIKEN research institute announced a new discovery in their studies on bryophytes, mosses, to uncover the mechanisms by which they are able to grow in a variety of soil pH levels and in places with high concentrations of metals. A previous hypothesis suggested that they are able to detoxify or otherwise sequester the metals inside of their cells while avoiding negative impacts. Several bryophytes are already known to be capable of accumulating copper, but the researchers wanted to see if other metals, especially more concerning ones, were able to be used by other species.
For their study, they chose Funaria hygrometrica, a moss known to grow particularly on metal heavy sources, such as areas around metal mines. Cells of the moss were grown on agar plates and then placed into a suspension liquid medium. After culturing, experimental groups of the moss cells were placed into liquid tubes with a variety of metals and pH levels. This mixture was then filtered through a method similar to column chromatography. This filtration lasted 22 hours.
The moss cells that had been used were at the protonemal stage, the fastest proliferating and earliest life stage for the moss. After filtration with the metals, the samples were washed repeatedly with multiple substances in order to remove all residual metals not involved with the cells. Then spectroscopy was used to observe the cells and find which metals they adsorbed the best. Adsorption is the process by which molecules are able to adhere to a surface.
Only two of the metals out of the 15 used were found to accumulate to any meaningful amount. Gold was adsorbed up to the level where it made up 11.3% of the dry weight of the cells. But that was the second best and it was nowhere near the level of the top contender. What the researchers found is that this moss species is capable of adsorbing lead to the point of where it makes up 74.1% of the dry weight of the cells. Considering lead has only slightly more than half the density of gold, that makes this feat even more impressive.
Upon further analysis under an electron microscope, the scientists found that the lead is primarily concentrated within the walls of the cells, making them substantially and visibly thicker in the process. The lead was also seen to accumulate on the membrane of the endoplasmic reticulum and on the surface of thylakoids within the chloroplasts. But nearly 90% of the total was within the cell walls.
A follow up test made an even more interesting find. Taking one of the control cells without metal on them, they removed a portion of the cell wall and placed it in a solution containing lead. Upon inspection afterward, the cell wall was found to still have accumulated the metal, even though it wasn’t attached to a living cell at the time, implying that the process is a chemical one based in the inherent structure of the cell wall and not by activity of the cell itself.
A spectral analysis of the composition of the cell wall in comparison to the cell walls of two other moss species showed that it was the inclusion of polygalacturonic acid (PGA) in the walls of this species that allowed for the high adsorption of lead. How this molecule is able to facilitate this and why it is only able to do so in high volume for only lead remains unknown.
The final piece of experimental data they recovered is that Funaria hygrometrica cells were still able to adsorb the metal even under pH conditions ranging from 3 to 9. They were also able to adsorb platinum-group metals decently well within this wide range. Whether the latter capability is just a general metal adsorption method independent of the lead capabilities is still unclear and requires further study.
Regardless, the RIKEN scientists were able to showcase the abilities of this moss species and their use as a biomaterial for adsorption of heavy metals in liquids especially. Since they actively take up the metals and incorporate them into their cells, this makes the moss a great option for cleanup without any byproduct or conversion products to be concerned about.
This also means that the species has the potential to take over the reins for the cleanup of heavy metal contaminated waters, especially those contaminated with lead, and will allow us to set aside our use of techniques like chemical sedimentation that cause so many harmful side-effects to the climate.
Whether there will be research into also using the cell wall materials of these mosses to independently adsorb lead, only time will tell. But it will certainly take quite a bit more experimentation before we can truly know how this wonderful little moss does what it does.
Photo CCs: Funaria hygrometrica (d, 153418-482420) 4917 from Wikimedia Commons