Context

A new study found mercury (Hg) in birds concentrated around two thermal power plants (TPP) in Maharashtra’s Nagpur could lead to bioaccumulation and biomagnification.

Key-highlights of the Study

• Traces of Hg on the feathers of the avifauna (birds of a particular region) in this area was linked to the Hg present in the ash ponds of the TPPs. 
• Coal-based TPPsare the major source of mercury — the ninth most toxic element found on earth — emissions into the environment.
• Mercury contributed over 80 per cent of emissions released by the TPPs, according to 2016 estimates by Delhi-based think-tank Centre for Science and Environment.
• Fly ash generated by TPPs was one of the ways through which Hg was released into the environment.
• Fly ash is usually disposed in ash ponds in the form of ash slurry, forming an aquatic ecosystem of its own.
• In a wetland, Hg was found in two forms: organic (MeHg) and inorganic (iHg).
• MeHg has more bioconcentration than its counterpart, resulting in bioconcentration — the process by which a chemical concentration in an aquatic organism exceeds that in water — and subsequent physiological, reproductive and behavioral changes.

Hg in Bird feathers samples

• Bird feathers sample of eight bird species, like Indian pond heron, darters, Moorhen, etc, were collected and observed. It was seen that some of the species had a higher concentration of Hg.
• This could be because of feeding on fishes and insects in ash ponds. A majority of the bird species, however, had extremely low traces of Hg.
• In conclusion, it was seen that except in soil, other samples collected from fly ash, plants, water and bird feathers had low quantities of Hg in them.
• The purpose of the study was to establish new ways to analyse the presence of toxic elements like Hg. One such unique method was the non-invasive biomonitoring of bird feathers. Bird feathers turned out to be an ideal specimen, as they did not hurt the organism or require the physical presence of the bird.

About Mercury

• Mercury is one of the most toxic elements and a threat to wildlife because it accumulates and magnifies to unsafe levels in aquatic food chains
• Mercury exists naturally and as a man-made contaminant. Mercury is classified as a heavy metal and is well known as being among the most toxic of metals.
• Mercury is a non-transition metal and is an extremely rare element in the earth’s crust.
• Hg has three valence states (0, I and II), and exists in three main forms-
  • elemental mercury or Quick silver (Hg°, metallic mercury, and mercury vapor)
  • inorganic mercury (Hg+ and Hg2+)
  • organic mercury such as methylmercury (CH3Hg, MeHg) and ethylmercury (C2H5Hg, EtHg).

How does mercury accumulate in organisms?

• Mercury has the ability to build up in organisms and up along the food chain.
• Although all forms of mercury can accumulateto some degree, methylmercury is absorbed and accumulates to a greater extent than other forms. 
• Inorganic mercurycan also be absorbed, but is generally taken up at a slower rate and with lower efficiency than is methylmercury.
• The biomagnificationof methylmercury has a most significant influence on the impact on animals and humans.

What is Bioaccumulation and Biomagnification?

• Bioaccumulation: The term bioaccumulationrefers to the net accumulation over time of metals within an organism from both biotic (other organisms) and abiotic (soil, air, and water) sources.
• Biomagnification: The term biomagnificationrefers to the progressive build up of some heavy metals (and some other persistent substances) by successive trophic levels – meaning that it relates to the concentration ratio in a tissue of a predator organism as compared to that in its prey.

Toxic Effects of Mercury

The toxic effects of mercury depend on its chemical form and the route of exposure.

• Methylmercury: Methylmercury [CH3Hg] is the most toxic form.
  • It affects the immune system, alters genetic and enzyme systems, and damages the nervous system, including coordination and the senses of touch, taste, and sight.
  • Methylmercury is particularly damaging to developing embryos, which are five to ten times more sensitive than adults.
  • Exposure to methylmercury is usually by ingestion, and it is absorbed more readily and excreted more slowly than other forms of mercury.
• Elemental mercury: Elemental mercury, Hg(0), the form released from broken thermometers, causes tremors, gingivitis, and excitability when vapors are inhaled over a long period of time.
  • Although it is less toxic than methylmercury, elemental mercury may be found in higher concentrations in environments such as gold mine sites, where it has been used to extract gold.
  • If elemental mercury is ingested, it is absorbed relatively slowly and may pass through the digestive system without causing damage.
• Ingestion of other common forms of mercury, such as the salt HgCl2, which damages the gastrointestinal tract and causes kidney failure, is unlikely from environmental sources.
• When released into the environment, it accumulates in water laid sediments where it converts into toxic methylmercury and enters the food chain. 
• Mercury contamination is a significant public health and environmental problem because methylmercury easily enters the bloodstream and affects the brain.

Conclusion

 The new findings call for additional research to advance the very limited understating of the threats posed to migrating birds by mercury pollution. Uncovering the impacts of mercury pollution to wildlife has been and will continue to be a critical driver of improved mercury emissions regulations around the world.