By David Batcheller

 Heavy, silvery metal  Only metal to exist as liquid at room temperature  Elemental state, Hg 0, found in waters and atmosphere  Exists in two main oxidation states  Mercury (I)  Mercury (II) – most common

 Toxic at high doses  Organic compounds of mercury are more toxic than inorganic compounds  Hg(II) and Methylmercury are the two most common toxic forms of mercury in the marine environment

 Natural (accounts for 1/3 of mercury):  Volcanic Activity  Forest fires  Erosion  Anthropogenic (accounts for 2/3 of mercury):  Metal production  Chlor-alkali and pulp industries  Waste handling and treatment  Coal, peat, and wood burning

Natural waters supersaturated with Hg 0 compared to atmosphere  Flux from water to atmosphere Hg 0 oxidized to Hg(II) in atmosphere  Returns to Earth’s surface  Global spread  Residence time Over time, Hg(II) is reduced back to Hg 0 and returns to the atmosphere

 Most toxic form of mercury  Organic  Insoluble  Bioaccumulates  Mercury methylated by sulfate-reducing bacteria to form MeHg  Limited amount produced from anthropogenic sources

 MeHg is not easily or quickly excreted from marine organisms  All the MeHg that remains in an organism upon being consumed is transferred to the predatory species  Higher trophic levels feed more often and thus accumulate more MeHg at a faster rate

 Biomagnification continues up the food chain until the highest trophic levels where mercury levels can exist at toxic levels  The more trophic levels, the more mercury accumulated at the top  Mercury concentrations can range from under.001ppm in plankton to over 1.0ppm in large fish and mammals

 Entry of mercury into the aquatic food chain  Mercury first travels across the lipid membrane of unicellular organisms  At high concentrations, Hg(II) transported into the cell via specialized MerT transport protein  At low concentrations, lipid soluble mercury complexes diffuse across membrane

 Mercury taken up is then methylated by the bacteria creating methylmercury  Methylmercury is either retained in the bacteria and travels up the food chain or is released by the bacteria to be absorbed by phytoplankton to proceed up the food chain

 MeHg is retained in the fatty and muscle tissues of higher trophic level animals due to its lipid solubility  Fish still uptake MeHg from the water but majority taken from food  Hg(II) is absorbed at the microvilli interface  Low uptake rate  MeHg proportion over total Hg increases from 10% in the water column to 15% in phytoplankton, 30% in zooplankton, and 95% in fish

 The affect of high mercury levels on fish has not been well researched  Some studies have found high mercury levels to decrease fecundity and in some cases increase homosexuality in fish

 Marine mammals are at the top of the marine food chain yet don’t seem to exhibit effects of mercury poisoning  Majority of mercury concentrated in liver  Only a small amount as MeHg  Avoid effects of mercury toxicity due to abundant amount of selenium in liver and kidney  Selenium in marine mammals is positively correlated to mercury in liver and kidney

 Mercury in birds is concentrated in the liver and kidneys  Marine and fish-eating birds have higher mercury levels than land-dwelling birds  Causes reproductive impairment, reduced hatchability, and deformations during development  Mercury levels in eggs have been found up to 3ppm

 High levels of mercury in humans can harm the development of the nervous system in unborn babies and young children  In extreme cases, high levels of mercury can lead to mercury poisoning

 Extremely high levels of mercury in humans can cause severe damage to brain, kidneys, and lungs  Diseases associated with mercury poisoning are acrodynia, Hunter-Russell syndrome, and Minamata disease  Symptoms of mercury poisoning can include:  Itching or burning of skin  Sensory impairment  Muscle weakness and paralysis  Mental instability  Death Minamata Disease

 Due to rising concerns about mercury in seafood, many restaurants have started testing their food  Many restaurants were found to be selling fish above 1ppm in mercury which is the FDA’s “action level”  Now more restaurants are disposing of fish with levels higher than the “action level” and have quit buying from companies in other countries that sell fish above this level

 One study found that cage-rearing fish could be a step towards reducing mercury levels in fish  The study tested the mercury levels of 4 kinds of carnivorous and 1 kind of herbivorous caged fish from 5 different sites  These fish were on exclusive diets of either dried pellet feed or fresh fish forage fish or fish viscera feed  Results showed decreased Hg and MeHg levels in fish on the dried pellet feed diet

 Cut back on anthropogenic sources of mercury through cleaner and more efficient industrial factories  Cage-rear more of the fish we eat, as shown in the study

 Avoid eating too much fish from higher trophic levels such as shark, swordfish, king mackerel, or tilefish  Eat up to 12 ounces per week of fish that are lower in mercury such as shrimp, canned light tuna, salmon, pollock, and catfish  Make sure the fish you eat has a mercury concentration under the FDA’s “action level” of 1ppm  Inquire as to the mercury levels of fish at your local restaurants  Ask local advisors about the safety of fish caught in your area

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