Mercury in freshwater ecosystems

Mercury in aquatic systems

Mercury is a naturally occurring element, but is also a persistent, bioaccumulative toxin (PBT). It is liquid at room temperature and has a density even greater than that of lead. That makes it also a highly unusual metal. It can be classified in three forms:
-Inorganic salts in two ionic states: Hg⁺and Hg²⁺;
-As a metallic element;
-Organic compounds.

The charateristics of the different forms

-Mercury(II), or mercuric, salts are much more common than mercury(I) salts, and therefore it is mercuric salts which will be mainly considered here.
-Organicmetallic compounds are covalently bonded. Mercury bound to proteins or organic acids salts are not included in this group. These organometallic compounds are stable, though some are readily broken down by living organisms, while others are not readily biodegraded. They are usually found in industrial and agricultural use.
-Elemental mercury gives rise to a vapour, which dissolves only slightly in water.

When inorganic mercury spreads out in the water, naturally occurring bacteria absorb it and convert it to a form called methyl mercury. This methylated mercury gets locked up in the bottom sediments.

Which are the different sources of mercury in freshwater ecosystems?

Mercury is used in many different products. It is mined by humans and till the end of the 20th century, mercury was mainly used in thermometers. Nowadays, it is estimated that 22% of the yearly world consumption of mercury is used in electrical and electronic equipment, as well as in numerous household products, dental fillings, industrial processes, in barometers, diffusion pumps, and many other laboratory instruments. It is also a trace element in fossil fuels (primarily coal). Furthermore, it is used in medical equipment, data transmission, telecommunications, and mobile phones. Mercury is also used in batteries, switches/housing, and printed wiring boards. Gaseous mercury is used in mercury-vapor lamps and advertising signs.
Although this amount is small for any single component, 315 million obsolete computers by the year 2004 represent more than 400,000 pounds ^{(pounds into kg ?)}of mercury in total.
The greatest anthropogenic source of mercury in water appears to be emissions from coal fired electric plants. About 6x10³ tonnes of mercury are released to the environment each year as a direct result of human activities. The greatest pollutants are power stations in Asia and Africa. But also the small-scale extraction of gold and silver contributes to the mercury pollution. The other sources include waste incineration, and improper disposal of mercury-containing products. The chloralkali industry and, previously, the wood pulping industry also released significant amounts of mercury.
Although the use of mercury is been reduced, high concentrations of the metal are still present in sediments associated with the industrial applications of mercury. Some mercury compounds have been used in agriculture, principally as fungicides.

Beside the anthropogenic sources, the natural sources are probably more. Natural sources of mercury contamination include the degassing of the earth’s crust through volcanic activity and by evaporation from the oceans. The amount of mercury coming from natural sources is between 25 000 and 125 000 tonnes per year.

When mercury is released into the air, it can travel long distances and then be deposited into streams and lakes through atmospheric deposition. The conversion of inorganic mercury to methyl mercury in aquatic environments implies that recycling of mercury from sediment to water, to air and back could be a rapid process.

How is mercury taken up, accumulated and eliminated by organisms?

Mercuric salts and organic mercury are readily taken up by organisms in water. Aquatic invertebrates, and most particularly aquatic insects, accumulate mercury to high concentrations (BCFs up to 19 600). Fish also take up the metal and retain it in tissues principally as methylmercury (BCF = 200-8000), although most of the environmental mercury to which they are exposed is inorganic.
Environmental levels of methylmercury depend upon the balance between bacterial methylation and demethylation. The indications are that methylmercury in fish arises from this bacterial methylation of inorganic mercury, either in the environment or in bacteria associated with fish gills, surface, or gut. There is little indication that fish themselves either methylate or demethylate mercury. Elimination of methylmercury is slow from fish (with half times in the order of months or years) and from other aquatic organisms. Loss of inorganic mercury is more rapid and so most of the mercury in fish is retained in the form of methylmercury.

Toxicity to microorganisms

Mercury is toxic to microorganisms, because inorganic mercury has been reported to have effects at concentrations of 5 ug/litre, and organomercury compounds at concentrations at least 10 times lower than this.
One factor affecting the toxicity of the organometal is the rate of uptake of the metal by cells. Mercury is bound to the cell walls or cell membranes of microorganisms, apparently to a limited number of binding sites. This means that effects are related to cell density as well as to the concentration of mercury in the substrate. These effects are often irreversible, and mercury at low concentrations represents a major hazard to microorganisms.

Toxicity to freshwater organisms
The organic forms of mercury are generally more toxic to aquatic organisms than the inorganic forms. Aquatic plants are affected by mercury in the water at concentrations approaching 1 mg/litre for inorganic mercury but at much lower concentrations of organic mercury. Aquatic invertebrates vary greatly in their susceptibility to mercury. Generally, larval stages are more sensitive than adults (1-10 ug/l normally causes acute toxicity).
The 96-h LC50s vary between 33 and 400 ug/litre for freshwater fish (that amount is higher for seawater fish). But organic mercury compounds are more toxic. Toxicity is affected by temperature, salinity, dissolved oxygen, and water hardness. A wide variety of physiological and biochemical abnormalities has been reported after fish have been exposed to sublethal concentrations of mercury, although the environmental significance of these effects is difficult to assess. Reproduction is also affected adversely by mercury.

Toxicity to humans
When mercury occurs in methylated form, it is taken up more readily in the body than the unmethylated form. There are a lot of species which are sensitive to mercury poisoning.
The EPA recently estimated that as many as 1 in 6 American women of child bearing age already have unsafe levels of mercury in their bodies, leaving approximately 630,000 newborns at risk each year.

Because mercury biomagnifies up to 100,000 times in predatory fish, the consumption of methylated mercury contaminated fish led to the poisoning of Japanese fisherman and their families in Minamata, Japan, in the 1950s. The US Food and Drug Administration (FDA) recommends that pregnant women and those who may become pregnant avoid eating shark, swordfish, king mackerel, and tile fish known to contain elevated levels of methyl mercury, an organic form of mercury that can accumulate in the food chain.

How are we exposed to mercury?
The level of toxicity from mercury exposure is dependent on many variables, including the type of mercury involved, how much of the substance enters the body, how it enters the body, and how old the individual is when exposed to the metal. All of us are exposed to extremely small amounts of mercury whether we like it or not. The most common ways through which we come into contact with mercury are consumption of certain fish that we eat, as well as mercury contained in silver dental fillings (silver fillings are being used less and less). Though mercury exposure from these is low and should not pose a health risk, overexposure should certainly cause concern.
Organic mercury expresses its toxicity principally in the central nervous system. It may also affect important organs such as the kidneys, and systems such as the immune system. One example of mercury’s dangerous effects is if a pregnant woman consumes fish or seed grain that is highly contaminated with mercury. Fetuses have been known to develop serious development and neurological problems in response to this. The reason is that the brain and body of fetuses, infants and small children are developing rapidly and are more susceptible to toxicity than is the adult brain.
Organic mercury compounds can be absorbed either when they are ingested orally or when they come into contact with the skin. When inside the body, methyl mercury spreads to all the tissues of the body but it seems to concentrate in the blood and in the brain.

For information about lead in aquatic systems, please check out lead this page.

Mercury poisoning in freshwater

Mercury in aquatic systems