Mercury: biological functions

First biological function of mercury discovered

Purple Non Sulfur Bacteria may play a role in reducing the amount of methyl-mercury in the oceans by converting it to a less harmful form.Sincerely,

The element mercury (Hg) is extremely toxic to most organisms, including humans.  It’s deadly effects are thought to be due to it’s ability to block the function of certain key metabolic enzymes.  Being so toxic, it has long been thought that mercury had no biological functions in the living world at all.  At least that was presumption until a research team published the first evidence that a unique group of organisms can not only stand being around the stuff, but actually benefit by the presence of Mercury.   In a paper published this month in Nature Geoscience, D. S. Gregoire and A. J. Poulain show that photosynthetic microorganisms called purple non-sulfur bacteria can use mercury as an electron acceptor during photosynthesis.  These bacteria rely on a primitive form of photosynthesis that differs from the type common to plants.  In the case of photosynthesis in plants, water is used as an electron donor, with carbon dioxide the electron acceptor.  The result of this process is the production of sugars, the release of oxygen, and the removal of carbon dioxide from the air.  Purple non-sulfur bacteria, on the other hand, usually prefer to live in watery environments where light is available to them, but the oxygen levels are low.

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They use hydrogen as the electron donor, and an organic molecule such as glycerol or fatty acids, as the electron acceptor.  This also results in the production of sugars, but does not release oxygen or remove carbon dioxide from the atmosphere.  This process also generates too many electrons for for their organic electron donor to handle, leading to the potential for damage to other molecules in the cell.

The researcher showed that purple non-sulfur bacteria grow better when mercury is in their environment.  The reason seems to be that the bacteria use the mercury to accept those extra electrons, reducing mercury from a high oxidation state to a low one.  The oxidation state refers to the number of electrons that an atom can gain or lose.  In the case of mercury, when it goes to its low oxidation state after gaining the extra electrons, it becomes a vapor and evaporates away into the atmosphere.  In mercury’s high oxidation state it can form the soluble compound methyl-mercury, which can be toxic to other organisms.

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It’s quite possible that the impact of mercury reduction by photosynthesis may extend far beyond the health of these unusual little microbes.  Jeffry K. Schaefer, in the Department of Environmental Sciences at Rutgers University speculates that, “By limiting methyl-mercury formation and accumulation in aquatic food webs from microorganisms to fish, this process may even contribute to less toxic mercury ultimately ending up on our dinner plates.”

Journal Reference:

A physiological role for HgII during phototrophic growth.  Nature Geoscience.  February 2016, Volume 9 No 2  pp121 – 125  D. S. Grégoire & A. J. Poulain  doi:10.1038/ngeo2629

Biogeochemistry: Better living through mercury.  Jeffry K. Schaefer.  Nature Geoscience: News and Views.  18, January 2016.

. Author manuscript; available in PMC 2016 Dec 23.
Published in final edited form as:
PMCID: PMC5181110
NIHMSID: NIHMS832433
PMID: 28018837

Recent Advances in Mercury Research

Introduction

Mercury (Hg) is a naturally occurring element present in the Earth’s crust. Although present in the environment and atmosphere in small quantities, it is a source of concern due to its considerable release through anthropogenic emissions. Mercury toxicity is not limited to humans; it is also detrimental to animals and plants []. The mechanisms through which it causes cell death are of particular interest and this has received considerable attention []. Mercury has a rich history; its applications have varied from extracting gold (which remains a common practice in developing countries) to treating everything from constipation to syphilis. Many believe that the character of the ‘Mad Hatter’ in Alice in Wonderland was based on English hatters who were poisoned by mercuric nitrate used in the felting process [].

Natural sources of mercury in the environment include elemental mercury vapor from volcanoes and forest fires and the release of inorganic mercury by weathering and movement of water []. More recent anthropogenic sources include burning of coal and fossil fuels, mining of mercury, precious metal refinement, electrical and automotive part manufacture, and chemical processing, and release through waste incineration, landfills, and industrial contamination of water systems.

Some of the best-known environmental effects of mercury on human health come from Minamata, Japan, and Iraq secondary to consumption of tainted fish and wheat, respectively. In Minamata, over the course of several years industrial waste-water containing mercury was released into the environment and bioaccumulated in fish. In the 1950s, adults who consumed the mercury-laced fish developed Minamata disease, which was eventually characterized by paresthesia, sensory deficits, slurred speech, unsteady gait, muscle weakness, irritability, memory loss, and depression []. Similar symptoms were observed in Iraq after consumption of bread made from grain treated with ethylmercury []. Methylmercury (MeHg) contamination of fish is still a significant source of human exposure to mercury compounds in fish-eating populations.

Because mercury in the environment poses a significant risk to human health it, is regulated by the United States Environmental Protection Agency (US EPA) and the Occupation Health and Safety Agency (OSHA). These agencies take recommendations from the National Academy of Sciences expert panels that review available scientific studies and make safe level recommendations. It is recommended that levels of mercury not exceed 5.0 μg/L in whole blood or 1.0 μg/g hair, which corresponds to a reference dose of 0.1 μg/kg body weight/day [].

The FDA also limits the amount of mercury in cosmetic products to 1 ppm. In a study conducted in 2013, 549 skin-lightening products, manufactured in 32 countries, purchased both online and in stores in the USA, Taiwan, Japan, China, Thailand, and Sri Lanka were tested for mercury levels. Cosmetics were screened for mercury content above 200 ppm using a low-cost portable x-ray fluorescence spectrometer. Of the 549 tested products, 33, representing ~6.0 %, contained mercury above 1,000 ppm. Forty-five percent contained mercury in excess of 10,000 ppm and 3.3 % of skin-lightening products purchased in the USA were found to contain mercury in excess of 1,000 ppm [].

Mercury is a very potent toxicant, with the nervous system, particularly the developing nervous system, being the most affected. Various mercury research studies have emerged in the last few years and the number is too vast to review all of them here, but common ‘themes’ have appeared, such as studies of low-level mercury exposure, and the contribution of genetics to mercury toxicity. The most recent of these studies are reviewed herein. A background of mercury administration, distribution, metabolism, and excretion is provided for context.

Mercury Species

Mercury, also known as quicksilver, is the only metal that is liquid at room temperature. There are three primary forms of mercury: elemental or metallic mercury (Hg0), inorganic mercury, and organic mercury. Hg0 is a silver-colored liquid at room temperature but forms mercury vapors at ambient temperatures due to its very high vapor pressure. These characteristics have made mercury particularly valuable for use in production of chlorine gas and caustic soda, extraction of gold and silver from ore, and as a component of batteries, dental amalgam, thermometers, barometers, and electrical switches (Table 1). Elemental mercury is rarely found in nature but can be heat-extracted from inorganic cinnabar ores. Mercuric mercury (Hg2+) is the oxidation state that is most commonly found.