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When mercury first enters into waterways and ecosystems, it is considered elemental inorganic mercury. It is not until being in the ecosystems that the elemental inorganic mercury is then converted into methylmercury by the inhabitant organisms and various forms of bacteria. These bacteria are known as anaerobic bacteria species, which convert elemental mercury into methylmercury when the bacteria methylate oxidized mercury into methylmercury.

The environments that methylmercury levels are most commonly found at high levels, or future high risks are wetlands, newly flooded reservoirs, aquatic areas close to mining sites or factories, bays, and waterways with low pH levels. These factors when presented with high mercury levels, are home to the bacteria that process it. These methylmercury containing bacteria are what are then eaten by the next higher level in the food chain. Some methylmercury containing bacteria also excrete the toxin directly into the water.

This newly transformed form of mercury (methylmercury) then begins to bio-accumulate in the many species inhabiting the contaminated waterways and ecosystems. plans and autotrophic organism passively absorb Methylmercury throughout their life, and even long-term exposure does not significantly reduce their fitness. Animals accumulate methylmercury in their body's faster as they consume other mercury-containing organisms, resulting in each successive level in the food chain consuming higher levels of methylmercury. The bio-accumulation levels of toxic methylmercury are highest by larger predator fish species, fish eating animals, and humans which fish being a prominent food source for humans, causes much additional worry. Though methylmercury traces have also been found in reptiles, amphibians, invertebrates, flora, birds, and even the surrounding soil.

The many health effects of methylmercury has on wildlife include reproductive problems and reductions, enzyme and immune system problems, developmental issues, and genetic alterations. These problems have had the greatest effects on waterfowl because of their high fish diets. Some of the waterfowl experiencing the most problems with methylmercury are the Great Egret, Diving Duck, Herons, and Loons. Research and scientific surveys have shown a dramatic reduction in Loon chicks and a change in juvenile Great Egrets with a direct correlation with high levels of methylmercury. Ecosystems with both methylmercury and the element selenium can produce an even more toxic and potentially deadly mixture for wildlife.

Human health impacts[edit]
Areas in California with fish consumption advisories due to mercury contamination.

There have been many studies and negative health impacts, directly relating to mercury contamination in both food sources and the environment. One of the main sources of mercury contamination and over consumption is through fish, already contaminated with high levels of mercury. Mercury can impair, damage, and even destroy functioning nerve tissue-much like lead. The over consumption of methylmercury can also reduce immune system response, damage the nervous system, including coordination, sense of touch, taste, and sight.

One of the major health threats presented from the over consumption of these high mercury levels is the development of learning disabilities and developmental problems in children. This is the result of over mercury exposure after birth, and/or over consumption of high mercury levels of the mother during pregnancy. The high concern is because mercury can be passed between the pregnant mother through the placenta to the unborn fetus. The form that mercury that is being consumed is Methylmercury, which the federal government has classified as a neurotoxin, which is described as a poisonous substance that attacks the nervous system, and impairs the function of the nerve and nerve tissue. Even small amounts of this neurotoxin can cause brain and nervous system development problems. The effects of this utero (before birth in the uterus) transfer can also take between a number of months or even years before signs appear, making it difficult to trace back. The forms that methylmercury exposure will show itself is that the child will have shorter attention spans, poor fine motor skills, slow language development, visual-spatial abilities (like drawing), and memory. A mothers exposure and consumption of methylmercury prior to pregnancy can also be just as serious as exposure during pregnancy, because methylmercury is slowly excreted from the body, sometimes taking months to fully leave an individuals system. This can greatly effect the fetuses development, as many important developmental stages of the nervous systems and brain occur during the first two months of pregnancy. Health experts suspect that children are more susceptible to methylmercury than adults, because they eat more food relative to their total body weight causing a higher contamination percentage. It has been concluded that 60,000 children born each year are at risk for neuro-developmental effects, due to in utero exposure to methylmercury. Mercury poisoning can secondarily effect young babies in lesser amounts through breastfeeding as well; Methlymercury absorbed by the mothers body can then be excreted through breastmilk, meaning that mothers who eat negatively impacting babies health. The U.S. Food and Drug Administration posts warnings on what type of fish or shellfish are higher risk, and provide on their website charts of how much fish can be safely eaten during different stages of breastfeeding and pregnancy, as well as what servings of fish are safe for consumption by different age ranges of children1.

Another form of human exposure to elemental mercury is the inhalation of the vaporized form of the element directly from its source in the environment. This form of exposure is especially common in and around old mining sites. This form of exposure can cause gingivitis, tremors, damages to the gastrointestinal tract, less efficient enzyme productivity, and in rare cases kidney failure.

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1Advice about Eating Fish | FDA https://www.fda.gov/food/consumers/advice-about-eating-fish

2San Francisco Bay Mercury TMDL (ca.gov) https://www.waterboards.ca.gov/sanfranciscobay/water_issues/programs/TMDLs/sfbaymercurytmdl.html#:~:text=The%20San%20Francisco%20Bay%20Mercury%20TMDL%20Project%20examines,of%20mercury-laden%20sediment%20already%20in%20San%20Francisco%20Bay.

3https://www.usgs.gov/centers/california-water-science-center/science/mercury#:~:text=Numerous%20water%20bodies%20in%20California%20have%20fish-consumption%20advisories,Sierra