Nature: Hyperaccumulation In Plants
In the fall of 2019, I registered to take a plant anatomy and morphology course at Elmhurst University. One of the independent projects assigned to us was that we were to create a plant biology project about a subject of our own interest within the field. I had none at the time, so I decided to look up, "alchemy," (because I was (am) an anime fan of The Full Metal Alchemist) as an inquiry to inspire a topic of interest.
I came across a few review and peer-reviewed research papers whose subject or research focus was about transgenic and hyperaccumulation in plants, phytoremediation and bioremediation-- I became extremely fascinated by these processes. Plants not only photosynthesize and produce oxygen, some of them can translocation and/or absorb high concentrations of heavy metals from a substrate!
(sounds nice to say, "Years later," but in reality, it was the following semester)…
my fascination continued to peak. The more I studied plant biology and particularly how hyperaccumulation can influence geochemical and nutrient cycling, soil legacies, biodiversity, microbial ecology and other trophic levels inclusive to population and community ecology, systematics and astrobiology, to say the least, the more knowledge and research opportunities I began to seek and eventually conduct.
Since 2019, I have conducted 5 smackeroos of plant biology research: germination, atmospheric carbon dioxide sequestration, native plant conservation, green roof and biotechnology design focused on improving native plant growth (with arbuscular mycorrhizal fungi), native plant specialist vs. generalist ecological function impacted by drought tolerance and microbial factors across time-scales (I am conducting the latter right now).
So, let me share a bit of what I have learned about hyperaccumulation in plants.
In this brief overview, hyperaccumulation is defined as the ability of a plant to uptake heavy metals (toxic concentration) from contaminated soils or substrates and accumulate them within their organs, tissues or cells including their roots, shoots and leaves, or are stabilized in the rhizosphere or through volatilization to a higher level than in the contaminated soil or substrate.
Hyperaccumulating plants uptake greater amounts of toxic concentrations of metals at a higher rate and can translocate and/or store metals in various parts of the body.
Most plants have been found living on metalliferous soils or
Figure 1. Sources of heavy metal pollution in the environment. Rizvi et al. 2020.
substrates such as serpentine (Ultramafic rocks) or old mine spoils (although some research suggest hyperaccumulating plants can grow in non-metalliferous soils or substrates). In reality however, there are various sources of heavy metal (Figure 1) which generates one of many questions such as, "Can hyperaccumulating plant species given their evolutionary adaptation or conservation of certain traits (phylogenetic, functional, taxonomic) hyperaccumulate in any substrate and in various nutrient and chemical concentrations?"
Gene expression and gene regulation has been shown to give rise to this phenomenon over evolutionary time, although to which and the how different genetic expressions and regulating underpinnings function and in which plant species, genus or family are not completely understood or investigated yet, in this expanding plant biology field.
Often, phytoremediation or the utilization of plants that are either toxic metal tolerant or are hyperaccumulators, are used in bioremediation processes given that some plants do quite well at phytoremediating contaminants, which is also a process that is generally said to be cost-effective. Additionally, microbes or chelating chemicals, for example, can be added to the remediation process to help yield higher contaminant uptake and faster reaction kinetic rates.
What are the basic processes you ask? Let's dig in!
Plants have biophysical, biochemical, physiological, and facilitative mechanisms that allow them to adsorb, transport, translocation, mineralize, hyperaccumulate and/or transform heavy metals (high density and atomic weights) in the above-ground and below-ground systems, and might influence plant secondary and micronutrients uptake and cycling (I suspect).
Despite there being various opinions regarding the criteria by which should be used as a standard to identify a toxic metal or title a metal as such for plants, through several natural biophysical and biochemical processes, such as adsorption, transport and translocation, hyperaccumulation or transformation, and mineralization, plants can remediate pollutants (heavy metals and other hydrocarbons). Phytoremediation can decontaminant a substrate such as soil, sludge, water and sediments.
Chandra et al. 2018. Phytoremediation of environmental pollutants.
In no hierarchical order, below is a brief list of primary processes hyperaccumulating plants hyperaccumulate or phytoremediate toxic heavy metals.
Plants absorb, translocate and store toxic metals or contaminants within their organs, such as root and shoot tissues. It generally occurs within the root or below ground system. The bioconcentration factor (BCF) can help researchers understand if a plant has a standard of ratio of metal concentration in the roots compared to substrate to determine if the plant hyperaccumulates (BCF= metal concentration in plant root/metal concentration in substrate).
Plant roots, seedlings, and shoots are used to uptake stored contaminants from aqueous materials. Through rhizofiltration, or the processes where terrestrial and aquatic plants reduce contaminant mobility into groundwater and its bioavailability to other trophic levels, utilizes adsorption, precipitation, and bioaccumulation to uptake and mobilize contaminants on roots.
Plants immobilize contaminants or act to stabilize them within the substrate and often in soils, by binding them to it. Typically, contaminants are stabilized in the rhizosphere around roots and in soil.
Contaminants chemically react (can be converted into nontoxic forms) and are transpired through biophysical volatilization processes into the atmosphere. Henry's Law of volatility and vapor pressure are used to determine organic contaminant volatility ability. It measures the ability of a toxic metal or contaminant to move in hydraulic environments (Pretty cool right!)
In a direct strategy, plants can remove contaminants directly into the atmosphere through the stem, trunk or leaves per transpiration or in an indirect strategy, through the below-ground system's normal function of transporting water and root foraging within soils.
Plants and microbes degrade or transform contaminants through enzymatic actions, that transform them into smaller molecules often used to benefit plant growth. The use of plants and associated microorganisms to degrade organic pollutants.
Substrates are supplied to grow microorganisms in the rhizosphere to breakdown toxic molecules. Plants release exudates within the rhizosphere in which microorganisms consume as nutrients, and enhance organic pollutant degradation in the root zone.
Plant's ecological role within the environment and in relationship to various trophic-level organismal, genetic, physical, geochemical and spatial environments, such as areas where endemic plants verses non-endemic plants live and function in niche ecosystems, and in various soils or substrates plants utilize --- is to me --- beyond awe. Now that I have come to understand more about the different ways plants function in various ecological spaces, the deeper and more expansive I find myself valuing earth: its evolution, the ecosystem services it provides and how deeply important it is for me to do... (Well, what do you think?)
Find more information about phytoremediation (hyperaccumulation and metal tolerating plants) by visiting The U.S. Environmental Protection Agency (EPA).
Norbay Durr is Chief Editor of ILAC, an undergraduate at Elmhurst University majoring in biology, English and seeking a minor in chemistry and loves to do all the plant biology research (and literature and cultural studies research!).