P2 - Dynamic Remediation Factor Values in Scots Pine
The Method of Dynamic Factors in Bioindication and Phytoremediation
Edita Baltrenaite, et. al.
How can we tell if a plant is good for phyto-applications? What do we test? How do we look at its soil and contaminant interactions?
It’s complicated. Plants have specific positions, environmental composition, and special diversity even within species that can cause different individuals to behave differently under different conditions of light, soil, water, nutrients, density, etc. These differences can be broadly categorized as internal (physiological) or external (ecological) factors and then further differentiated as biotic or abiotic conditions. The chemical and geochemical characteristics of soils, for example may be a combination of all of these factors and may change within just a few meters of a prospective plant. Biochemical differences in species can also be widely varied. What is a usable hyperaccumulation genetic adaptation for one plant, could be a metabolic weakness in another. Even the same adaptation can impact one metabolic process positively and another negatively, affecting the overall fitness of the plant significantly!
So, as we can see, this is a hideously complex and interrelated question that is highly dependent on a number of conditions-only a few of which we, as scientists or engineers can control. Even the separate organs of the individual plant can respond to different contaminants in different ways and impact overall health in a series of complex interactive webs that may or may not serve our goals in clean-up.
Today, we will be looking at the Scots pine (Pinus sylvestris L.) as a case study, not just in metal hyperaccumulation and growth, but as a way to understand how to look for useful bioindicators and how to test for metal remediation using a quantifiable and repeatable method applicable to most test conditions and species.
Trees are the darling of phytoremediation specialists because of their large biomass, genetic variation, ease of forestry management, biomass conversion into energy using anaerobic digestion (a common and relatively inexpensive method), positive societal response towards them, and resilience in the face of wind and water erosion. We like trees. So, if we can make the remediation plot pretty, healthy, and cheap, we can maximize the effective return on investment from the remediation activities into a social and economic pay-off in a couple different sectors, which will ultimately make phytoremediation more acceptable and more common for use, as opposed to current practice where managers seem to view it with quite a bit of suspicion and recalcitrant acceptance.
But the point of phytoremediation is to remediate. We want our plants to take up specific chemicals of interest, sequester them in their organs and then let us come and harvest them out in some way, without negatively impacting animals or people. There are several ways plants can do this: bioaccumulation, biophilicity, phytoremediation, and translocation – collectively known as dynamic factors in this article. We can also use trees in a rather unique way. Trees, as most of us know, provide a little timeline of growth in their rings. They can be used as dendrochronological and dendroindicational methods for all kinds of useful information such as fire season year and length, drought year and length, anthropogenic effects on forests, and the selective uptake of contaminants of interest. We can quantify uptake better than other species.
And the easiest and most prominent contaminant needing remediation in many industries and federal lands is metals. Metals are deposited by pretty much everything. Every time we build a powerline, run a train, build a building, a small proportion of the metals in that capital project end up going to the soil every time it rains or the wind blows hard. For some government facilities, like ranges, those metals are highly concentrated little packets of leaking toxins that just get worse and worse every year. Living things cannot usually tolerate metals. Our systems don’t like anymore than a tiny trace of things like nickel, cadmium, and lead. Anymore, and living organs start shutting down or the metals start interfering with enzymatic or biochemical functions that result in a high amount of cellular waste with nowhere to go or an immune response that eventually leads to a cytokine storm and full system shut down. It’s unpleasant for living things to uptake metals.
That’s why something like the Scots pine is so remarkable and so interesting to us. Here is a plant that has adapted ectomychorrizal and microbial symbioses along with biochemical processes that allow for a huge (relatively speaking) amount of metals to be safely sequestered in the plant without impacting its health too much. In this case study, Scots pine was grown near a shooting range that contained 3x higher zinc, 5x concentration nickel, and 3x higher copper concentrations than surrounding soils. This local deviation from metal baseline was also confirmed nickel and copper concentrations in a nearby water body exceeded the highest permissible concentration for drinking water by 19 and 22 times, respectively. But these pines were thriving. Researchers discovered 1.3x background concentration of copper and 2x concentration of nickel in the bark, identifying it as a hyperaccumulator candidate and excellent test subject for dynamic factor methodologies.
Some additional studies were conducted using metal-contaminated sewage sludge over soil and measured uptake rates were conducted for six years. Scots pine was able to sequester up to five times higher values of nickel, lead, and copper than other local species, with an astounding 87% greater biomass production than other species.
How did the Scots pine do this?
Researchers identified a couple major conditions. The first was that the Scots pine had an increased specific root length, a reduction in root-shoot ratio and wider root branching. Measuring sequestered heavy metals was facilitated by its lower height, trunk diameter, and dry biomass measurements by weight – all easily accessible with the Scots pine. The bark and trunk were amenable to biomonitoring aerogenic metals through repeated borings and leaf sampling by GCL and the pine family is happy to share its roots with a wide variety of symbiotic fungal species that can increase resilience, nutrient transfer, and even water transfer, making it a hardy and protected root system. This symbiosis may explain why Scots pine seemed to suffer less from H. annosum infection (a dangerous root rot pathogen that likes to eat conifers). There seems to be some evidence that the greater uptake of nickel and chromium in the test plot may have been a further defense against this pathogen. That the pine itself was preferentially selecting metals to imbue into its wood to prevent the fungus from being able to get into the wood and destroy the lignin. This lignin is the tree’s first line of defense to protect its cellulose stores, which is how it powers itself. Without the cellulose, it will die. So it is interesting to see the fungal symbiosis working both to uptake more metals, increase the plant’s resilience to it and facilitate the defense against another fungal pathogen.
These strong relationships seem to give the tree quite an advantage in compromised soils. The researchers wanted to identify all of these relationships and codify them into a more standard way of describing phytoremediation potential and using bioindicators for other contaminants of concern. This means evaluating the process of absorption of chemical elements and comparing how one plant does to another. In order to describe the interface of soil and plant, these researchers selected pH; Eh; CEC (cation exchange capacity); the content of clay particles and organic material in soil; concentrations of Fe, Mn, and Al oxides and hydroxides; the diversity and population of microorganisms; and what forms of the metals are being mobilized in the various plant bits. These factors all come together in a factor called the Biological Absorption Coefficient (BAC) or Index of Bioaccumulation (IBA). When this ratio is over 1.00, plants are accumulators. When not quite equal to 1.00, they are considered indicators. And if they are less than 1.00, they are excluders. Living things tend to preferentially uptake P, S. Cl, Br, I. Those elements tend to be necessary for growth and existing metabolic systems support high levels of these chemicals. Ca, K, Mg, Zn, Se tend to be selected for uptake, but in much smaller quantities or only under certain conditions and these are known as ‘strong’ uptake chemicals. Mn, Ni, Cu, Co, Pb, As, Hg are difficult for living things to use in any quantities and have a retarding effect on most growth. They can cause tissue poisoning and/or death for most organisms in medium quantities. These are known as ‘medium’ uptake since trace amounts are still necessary for some biochemical processes or they can be managed or excreted. And finally, V, Cr, Sb, Cd are not useful for any biochemical processes and cause poisoning in even small dosages. These are known as ‘weak/very weak’ uptake chemicals and it is very rare to find any plants able to manage these contaminants without some sort of molecular genetic engineering to help them. But, we will be addressing that more in a later essay.
In order to represent the dynamic factor of metal bioaccumulation to define these uptake indicators, the authors of this article used an equation comparing the concentration of the metal in tree wood ash on the treated site, to the concentration of observed metal in the treated soil against the concentration of metals in a control soil to concentration of metal in control tree wood ash.
They used a dynamic factor of metal translocation to reflect metal that was moved into vegetative organs but not necessarily bioaccumulated since vegetative organs have a habit of falling to the ground and decomposing in situ, thus, not really remediating anything, just moving it around. This is important in calculating the overall remediation value since translocated metals can’t be used to adjust the total value of metals removed from the soil and will bring down the overall dynamic factor of remediation.
The dynamic factor of metal biophilicity was developed to reflect changes in metal usage within plant metabolism. This is important to identify where the metal goes, either in woody biomass as sequestered accumulation, vegetative organs as translocated removal, or causing death/disease in root and branch systems that result in a decrease to overall survivability, again, dropping the total dynamic remediation value. It’s measured by relating the ratio of metal accumulation in living biomass against the expected metal concentration in the Earth’s crust, as defined by geological measurements conducted locally and globally. This is important to compare many different species from many different locations to equalize performance across those complex metabolic patterns.
At the end, we get a dynamic bioremediation factor for the Scots pine that looks like this:
This is a description of the interface between bioaccumulation, translocation, and biophilicity to integrate four types of information and make the whole relationship much more easy to see. Because the equation is dimensionless, it’s easier to compare multiple species of metals or alternate contaminants and it helps level the playing field for different plant types around the world. This is a complicated process and having a balanced relationship that can make those interactions more accessible is a huge step in making phytoremediation workable for managers and project leaders trying to find good candidates for diverse contaminants on sub-optimal soils.
The researchers tested this set of relationships in 2006 in Lithuania with Scots pines, silver birches (Betula pendula) and black alders (Alnus glutinosa) to see if they could confirm Scots pine’s utility in metal remediation and if they could estimate its utility in comparison to two other local species using this dynamic factor model.
The results confirmed the empirical evidence of Scots pine’s utility. There were some unexpected results in the dynamic translocation factor, favoring black alder for Mn and Ni uptake, but these results seemed to show a decreased metal translocation in the contaminated site versus the control site. However, the preponderance of the evidence showed Scots pine meeting or exceeding its dynamic factor expectations in comparison to the other species and the equation correctly identifying estimated uptake levels and species selection utility for metals uptake. An interesting finding from this was also that the more metals the Scots pine took up, the weaker its overall protective functions became, without being reflected in the remediation factor value. At the end of the experiment, the Scots pine showed a reduced photosynthesis intensity, decreased immunity to air contaminants, and lower health when compared to the silver birch and black alder. So, although the tree was able to uptake the metals of interest in high concentrations (5x control uptake and an average of .07-.15% metal elimination per year), there were consequences to the trees’ health and fitness. This should be factored into any pilot study attempting to define dynamic factors for species selection. It may be that a lower remediation factor distributed among more species to promote ecosystem resilience would be a more effective strategy, not just monoculture selection for maximum uptake. Thinking of a long-term, sustainable and healthy forest would be a wiser criteria than just maximizing the dynamic factor values, in my opinion.