Tributyltin

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, applied to the hulls of ocean going vessels. Bottom paint improves ship performance and durability as it reduces the rate of biofouling (the growth of organisms on the ship's hull). The TBT slowly leaches out into the marine environment where it is highly toxic toward nontarget organisms. TBT is also an obesogen. After it led to collapse of local populations of organisms, TBT was banned. 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, applied to the hulls of ocean going vessels. Bottom paint improves ship performance and durability as it reduces the rate of biofouling (the growth of organisms on the ship's hull). The TBT slowly leaches out into the marine environment where it is highly toxic toward nontarget organisms. TBT is also an obesogen. After it led to collapse of local populations of organisms, TBT was banned. TBT compounds are organotin compounds, with three 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, such as chloride, hydroxide, 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 absorption process can complicate quantification of TBT in an environment, since its concentration in the water is not representative of its availability. Tributyltin compounds are biocides. TBT´s antifouling properties were discovered in the 1950s in the Netherlands by van der Kerk and coworkers. It prevents microrganisms to settle on the hull of a ship and poisons the organisms that do. By the mid 1960s it had become the most popular anti-fouling paint around the globe. 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 relatively inexpensive. TBT 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. Although an effective biocide, tributyltin was wrongly deemed safe environmentally. TBT has a half-life of one or two weeks in marine water. When it accumulates in sediments its half life is about 2 years. TBT often bonds to suspended material and sediments, where it can remain and be released for up to 30 years. TBT has been shown to affect invertebrate development. Marine snails, such as 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.