EIACP Centre on Plants and Pollution

Mercury

Mercury in the Environment

Mercury Pollution

Emissions of mercury, a toxic metal, by state, in kilograms

Source: http://www.sourcewatch.org/index.php/Mercury_and_coal

Mercury is one of the most dangerous toxic trace metal pollutants that can affect the growth of many plant species. It accumulates biologically and enters in to the food chain system generating long term health problems. Industrial development has been considered as one of the major factors responsible for increasing levels of mercury accumulations within the plants. Anthropogenic activities have contributed significantly to the mercury adulteration of our atmosphere. Some of these common causes include mining, growing rate of fossil fuel burning, and increased use of raw material rich in mercury. There are several industrial processes that involve mercury rich materials in production.

Source: http://goodgardeningpractices.com/2011/10/08/mercury-accumulation-in-plants-and-its-effects/

Selenium Applied to Reduce Mercury Accumulation in Rice

There are many ways to improve the soil quality, such as changing soil, thermal desorption and other chemical and botanical methods, but all those take much longer time and more money, and under certain circumstances, land use has to be changed. In view of these, scientists put forward an innovative way to improve soil quality without having to change the land purposes, that is to use selenium to reduce mercury accumulation in rice. Selenium is an element that human body can not dispense with. It bears significance to life functions.

Selenium Successfully Applied to Reduce Mercury Accumulation in Rice

Source: http://english.cas.cn/newsroom/research_news/201503/t20150319_145500.shtm

Mercury Accumulator plants

• Eichhornia crassipes -Commonly known as water hyacinth, is an aquatic plant native to the Amazon basin, and is often considered a highly problematic invasive species outside its native range.
• Rapeseed (Brassica napus) -Also known as rape, oilseed rape, rapa, rappi, rapaseed (and, in the case of one particular group of cultivars, canola), is a bright-yellow flowering member of the family Brassicaceae (mustard or cabbage family) consumed in China as a vegetable.
• Hydrilla (Esthwaite Waterweed or Hydrilla) - Is a genus of aquatic plant, usually treated as containing just one species, Hydrilla verticillata, though some botanists divide it into several species. This abundant source of biomass is a known bioremediation hyperaccumulator of Mercury, Cadmium, Chromium and Lead, and as such can be used in phytoremediation.
• Pistia - Is a genus of aquatic plant in the arum family, Araceae. The single species it comprises, Pistia stratiotes, is often called water cabbage, water lettuce, Nile cabbage, or shellflower. Its native distribution is uncertain, but probably pantropical; it was first discovered from the Nile near Lake Victoria in Africa.
• Salix - Willows, sallows, and osiers form the genus Salix, around 400 species of deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions of the Northern Hemisphere. Most species are known as willow, but some narrow-leaved shrub species are called osier.

Mercury accumulates in the food chain

Mercury accumulates in biological tissue through complex reactions (bioaccumulation), many of which are still unknown. We do know that several bacteria incorporate environmental inorganic mercury into their bodies through chemical conversion to several organic mercury compounds, collectively called methyl mercury (Me-Hg). This Me-Hg form is more toxic and more difficult to remove from bacterial systems than inorganic mercury. Any higher-level organisms that consume these bacteria also consume the Me-Hg. This cycle repeats up the food chain, with each higher predator consuming more and more Me-Hg, ultimately arriving in fish. Estimates suggest that Me-Hg can accumulate more than a million-fold in the aquatic food chain.

Movement of metals with in plant roots

Plants require a relatively small number of elements for their growth and survival. Soils can, however, contain non essential elements within the soil solution that despite the selectivity of root cell membranes, still may be taken up and detected in plant tissues in trace amounts. Uptake of solutes takes place through the epidermis of the roots. Since mercury is not required for plant growth, there probably is not a specialized transporter for Hg. Therefore Hg accumulation within plant components is through root cells, via transporters for essential cations. Research suggests the major pathway that ions follow is from the epidermis to the endodermis of the root following a symplastic entry, meaning from cell to cell via. Uptake of metals by plant roots depends on the ionic potential of the metal of interest, soil organic matter content, cation exchange capacity (CEC), and pH. Metals can be absorbed actively or passively, accumulating with macronutrient cations and/or competing for uptake at the plants root tip. Binding of metal ions to root cell walls occurs via high affinity binding sites and plasma membrane localized transport systems. Uptake of metal ions occurs through secondary transporters such as channel proteins and or H+-coupled carrier proteins. Secondary transporters facilitate cation uptake by altering the electrical potential of the plasma membrane, which is negative on the inside of the membrane. There are several types of metal transporter and more than one transport system can exist for one metal. The zinc-iron permease (ZIP) proteins are a family of metal ion transporters that are found in plants, protozoa, fungi, invertebrates and vertebrates involved in metal ion accumulation and homeostasis These proteins have been implicated in the uptake of Zn and Cd in Thlaspi species. In Arabidopsis thaliana, the metal ion transporter IRT1, an iron transporter, has been shown to transport cadmium within the roots. The uptake of Pd2+ and Cd2+ via Ca2+, and Mg2+ transporters have been observed in rice roots, but inhibition of these metals was also observed . Metal transport into plant roots can also be inhibited or enhanced by mycorrhizal fungal association.

Source: (The Effects of Mercury Contamination on Tree,Fungal, and Soil Composition along East Fork Poplar Creek, Anderson and Roane Counties,Tennessee Sharon Jean-Philippe University of Tennessee - Knoxville, jeanphil@utk.edu)

Mercury effects on plants

1. Industrial development has been considered as one of the major factors responsible for increasing levels of mercury accumulations within the plants. Anthropogenic activities have contributed significantly to the mercury adulteration of our atmosphere.
2. By several research studies in this perspective clearly indicate that the pattern of mercury accumulation is never uniform within or among various plants species. There are several responsible factors for such effects including fluctuation in environment like temperature, soil, pH, soil aeration, soil moisture, the root system, presence of elements in the soil, and supply of energy to different parts of the plant. Different plant varieties and species have different rate of mercury and cadmium and other toxic metals absorption rate and accumulation rates within their tissues.
3. The toxic metal pollution dynamics depends greatly upon the uptake and accumulation of chemically active elements and compounds. In simple terms it means that the level of different element’s accumulation would be different in each plant species. For instance, mercury and cadmium, both highly dangerous elements, have high rate of bio-accumulation compared to other elements.
4. The low, moderate, and high levels of mercury accumulations in different parts of plants can easily be correlated with the transportation system through xylem and phloem. Such accumulation rate is affected by factors like lateral transfer, intra-phloem remobilization, and transpiration intensity. This in simple terms is an explanation as why the mercury mobility is greater while entering through stem or leaf.
5. Studies also revealed that the entry of toxic metal pollutants like mercury through plant leaves is more significant because of its aerosol deposition in the plants. Such type of leaf accumulation is directly proportional to the distance from the industrial unit emitting such mercury rich pollutants.

Source: http://goodgardeningpractices.com/2011/10/08/mercury-accumulation-in-plants-and-its-effects/