Triphenyltin (TPT)
Researchers Find New POP

KELLYN S. BETTS / Environmental Science & Technology 1nov02

Triphenyltin (TPT) appears to be a persistent organic pollutant (POP), according to scientists at Spain’s Institute of Chemical and Environmental Research (IIQABCSIC). In the October 15 issue of ES&T (pp. 4224–4228), they report finding unexpectedly high levels of TPT, a compound used in fungicides and marine antifouling paints, in what they believe is the first evidence of the compound in deep-sea organisms.

Overall, the TPT levels in the deep-sea fish were “surprising”, because they were so much higher than the levels of tributyltin (TBT), another organotin compound that has received much more attention because of its toxic properties, says Cinta Porte-Visa, a researcher with IIQAB-CSIS’s Environmental Chemistry Department and the paper’s corresponding author. Mora moro, a species of codfish that lives more than 1000 meters below the Mediterranean Sea’s surface, contained the highest TPT levels, up to 1430 nanograms per gram (ng/g wet weight as tin) of tissue.

The levels of TPT in the tested fish varied substantially, however: The Lepidion lepidion, or Mediterranean codling, had up to 176.5 ng/g of TPT, while the Alepocephalus rostratus contained less than 1.4 ng/g. Still, the TPT levels found in these deep-sea fish are much higher than levels recorded in organisms from the Mediterranean coast, Porte-Visa says, and on a par with the highest levels reported to date, which were recorded in horseshoe crabs on the Japanese coast.

The TPT discovery contrasts sharply with the TBT levels in the same fish, Porte-Visa says. TBT levels tend to be highest in those fish living near contamination sources such as harbors and decrease sharply with distance and depth, she explains. Levels of polychlorinated biphenyls (PCBs) and DDT were also low in the deep-sea organisms, showing that these fish are “clearly less contaminated than coastal ones,” she says.

The unusual TPT distribution indicates that the chemical persists in the environment and can be transported transported long distances, Porte-Visa explains. In her paper, she notes that TPT has a high sediment–water partition coefficient; the organotin can therefore bind to sediment particles and drift out to the deep sea undiluted. The ability of some organisms to metabolize TPT may also be very low, she adds.

“TPTs just haven’t been looked at as much, in terms of their levels,” says Margaret Whalen, an assistant professor at Tennessee State University’s department of chemistry who is studying the impacts of organotin compounds on humans. “People are definitely accumulating TBT in blood,” she says, but she hasn’t found measurable TPT accumulation in the small group of 13 individuals that she has tested thus far. Porte’s study nonetheless implies that TPT may not be breaking down as rapidly as scientists previously believed it would, she adds.

Organotins are “considered to be amongst the most toxic chemicals ever released into the marine environment”, according to the World Wild Fund for Nature (WWF International), an environmental group, but most of the regulatory action to date has focused on TBT. The International Maritime Organization has agreed to ban the use of organotins in antifouling paints by 2008, largely because of TBT, but the treaty has yet to enter into force. Moreover, the U.S. EPA reviewed TPT’s use as a fungicide in 2001 and determined that its benefits continue to outweigh its risks.

Laboratory exposure studies have shown TPT produces a wide spectrum of toxic effects in aquatic and terrestrial organisms, including effects on the immune system, reproductive/ developmental effects, and cancers of the endocrine system organs, according to the United Nations’ Food and Agriculture Organization. Brief exposures to TPT can impair human immune system function, according to studies conducted using human blood, reported Sharnise Wilson, one of Whalen’s students, at the spring 2002 American Chemical Society meeting. Very low levels—on the order of a few nanograms per liter— of TPT (and TBT) can induce reproductive disorders, Porte says.

Unlike many other POP chemicals, TPT does not appear to be bioaccumulating in the fatty tissue of the tested fish. Instead, it is concentrating in the liver, Porte says. She suspects that it has an affinity for proteins, but more study will be needed to determine which ones. Metallothioneins certainly have a role in sequestering metals, she says, but it is as yet unclear how much of a role they play with organometallics like organotins. Although most of the fish evaluated in the study are not widely consumed, a recent study by The Irish Republic’s National Food Center ranked M. moro as a good tasting fish that could be acceptable to the country’s population to replace the dwindling supplies of coastal species.