Tributyltin (TBT) is an umbrella term for a class of organotin compounds which contain the (C4H9)3Sn group, with a prominent example being tributyltin oxide. For 40 years TBT was used as a biocide in anti-fouling paint, commonly known as bottom paint, which was applied to the hulls of ocean going vessles. Bottom paint improves ship performance and durability as it reduces the rate of biofouling, which is the growth of organisms on the ship's hull. Although such paints are effective, the TBT slowly leaches out into the marine environment where it is highly toxic to a wide range of organisms. TBT pollution is of serious concern as it has led to collapse of whole populations of organisms.
TBT compounds are organotin compounds, with 3 butyl groups covalently bonded to a tin(IV) centre. A general formula for these compounds is (n-C4H9)3Sn-X. The X group is typically an electronegative "leaving group" such as chloride or carboxylate.. When introduced into a marine or aquatic environment, TBT adheres to bed sediments because of its high specific gravity and low solubility. However, the adsorption of TBT to sediments is reversible and depends on pH. Studies have shown that 95% of TBT can be released from the sediments back into the aquatic environment. This release makes it difficult to quantify the amount of TBT in an environment, since its concentration in the water is not representative of its availability.
The antifouling properties of TBT compounds were discovered in the 1950s in the Netherlands by van der Kerk and coworkers. The function of the biocide in the anti-fouling paint is to prevent the settling of organisms on the hull and to poison the organisms that do. Although an effective biocide, tributyltin was wrongly deemed safe environmentally. By the mid 1960s it became the most popular anti-fouling paint worldwide. TBT was mixed into paints to extend the life of antifouling coatings, and ships were able to continue operations for a longer time frame. The paints ensured fuel efficiency and delayed costly ship repairs. It is also an ingredient in some disinfectants, for example in combination with quaternary ammonium compounds.
The effects of antifouling paint go beyond the organisms that it is intended to kill. By poisoning barnacles, algae, and other organisms at the bottom of the food chain, TBT is biomagnified up the marine predators' food web. It has been shown to harmfully affect many layers of the ecosystem, including invertebrates and vertebrates, even humans. Toxic effects in some species occur at 1 nano-gram per liter of water.
Even with its ban, TBT presents a danger to the environment. One of the most problematic aspects of TBT is its accumulation in sediments and its long half life of about 2 years. TBT often bonds to suspended material and sediments to the bottom, where it can remain and be released for up to 30 years.
TBT has been shown to affect invertebrate development. One of the most studied organisms are marine snails, of which the dog whelk (Nucella lapillus) has often been used as an Indicator species. TBT disrupts their endocrine system by inhibiting Cytochrome P450 molecule. Among its myriad functions, P450 converts androgen, which has male-hormone properties, into oestrogen, which has female-hormone properties. This inhibition leads to masculinization in females, because the androgen levels are higher than normal. Since fewer fertile females are then available for mating, the population begins to decline, thereby seriously impacting the balance of the ecosystem.
Another indicator species is Chironomus riparius, a species of non-biting midge, which has been used to test the effects of TBT on development and reproduction at sublethal concentrations found in marine environments. It was found that only 0.05 ng/ml range is enough to have developmental effects on their larvae, and 10-100 ng/l was enough to seriously offset the female to male ratio in the population. At 10 ng/l females were at 55.6% of the population and 85.7% at 100 ng/l. These results are interesting because unlike the masculinization of the stengoglassan gastropods, this experiment shows feminization.
Vertebrates become affected by the waters contamined with TBT as well as by consuming organisms that have already been poisoned. Oryzias latipes, commonly called Japanese rice fish, has been used as a model vertebrate organism to test for effects of TBT at developmental stages of the embryo. It was observed that developmental rate was slowed by TBT in a concentration-related manner and that tail abnormalities occurred.
Illustrating the infiltration of TBT in the food chain, one study showed that most samples of skipjack tuna tested positive for presence of TBT. Tuna from waters around developing Asian nations had particularly high levels of TBT. Regulation of TBT is not enforced in Asia as rigorously as in Europe or US.
Studies have shown that TBT is detrimental to the immune system. Research shows that TBT reduces resistance to infection in fish which live on the seabed and are exposed to high levels of TBT. These areas tend to have silty sediment like harbours and estuaries.
TBT compounds have been described to interfere with glucocorticoid metabolism in the liver, by inhibiting the activity of the enzyme 11beta-hydroxysteroiddehydrogenase type 2, which converts cortisol to cortisone.
Mammals, even humans, are exposed to TBT through their diet. TBT has been shown to lead to immunosuppression in sea-otters and dolphins. High levels of tributyltin have been detected in the livers of sea otters (Enhydra lutris) and stranded bottlenose dolphins. In addition, otters dying of infectious causes tended to have higher levels of tissue butyltins than those dying of trauma or other causes. TBT has also been blamed by hearing experts for causing hearing loss in mammalian top predators such as toothed whales. Because hearing is important for mating and predation in these animals, long term consequences could be drastic.
TBT compounds are banned and are included in the Rotterdam Convention and have been banned by the International Convention on the Control of Harmful Anti-fouling Systems on Ships of the International Maritime Organization.
Bans on TBT on boats less than 25 metres long first started in the 1980s. In 1990, the Marine Environment Protection Committee adopted Resolution MEPC 46(30), which recommended that the Government eliminate the use of TBT-containing antifouling paints on smaller vessels. This resolution was intended to be a temporary restriction until the International Maritime Organization could implement a complete ban of TBT anti-fouling agents for ships. Several countries followed with a ban of use, and in 1997 Japan banned the production of TBT-based anti-fouling paints.
The use of organotin compounds acting as biocide in anti-fouling paint was completely banned in 2008 by the International Convention on the Control of Harmful Anti-fouling Systems on Ships of the International Maritime Organization. It states that ships cannot bear organotin compounds on their hulls or external parts or surfaces unless there is a coating that forms a barrier so that organotin compounds cannot leach out. This measure helps reduce exposure by allowing recovery to occur. Despite the ban, TBT will most likely be present in the water column and sediment for up to twenty years because of its long half-life.
Violations of the ban on TBT
Even though banned by some international agencies, TBT anti-fouling paints are still being used in countries with poor regulation enforcement, such as countries in the Caribbean. TBT can remain in the ecosystem for up to 30 years.
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