海鮮污染物甲基汞 科學家證實形成原因 | 台灣環境資訊協會-環境資訊中心

海鮮污染物甲基汞 科學家證實形成原因

2009年05月07日
摘譯自2009年5月4日ENS美國,華盛頓特區報導;謝芳怡編譯;蔡麗伶審校

圖片來源:Zadors科學家首次證實了有毒化學物質甲基汞是如何在海洋中形成--在此之前,科學家已知道從大氣沈澱到淡水生態系統的汞能夠轉換為甲基汞,但要證明其在海洋生態系統中的平行循環卻相當困難。

這種有毒形態的汞會累積在魚類體內,透過小魚到大魚不斷累積的食物鏈,其濃度也會越來越高,到了人類食用鮪魚壽司時,人們所暴露的高劑量有毒汞物質則已達到警戒值。而食用到汞成份的懷孕婦女,也會對小孩的發育造成終生的影響。

過去研究顯示,全球75%人體所接觸到的汞來自於食用海洋魚貝類。撰寫此份研究報告的科學家桑德蘭(Elsie Sunderland)則表示,在美國約有40%的人是因為食用在太平洋捕獲的鮪魚而接觸到汞。

此份研究詳細說明金屬汞如何透過煤炭和廢棄物的燃燒,以及亞洲工廠所排放的廢氣,轉變為北太平洋的甲基汞。

因為進入北太平洋的汞大多來自於大氣,科學家預測,如果汞排放率繼續以目前的速度提高,到了2050年太平洋中的汞含量將增加50%。

美國內政部長沙拉薩(Ken Salazar)表示:「美國地質勘探局這項空前的研究,對於美國人民和地球上野生動物的健康安全相當重要,它幫助我們了解大氣中汞排放和海洋魚類體內汞劑量兩者之間的關係。我們必須減少汞排放量,藉此降低海洋中的汞含量。」

現在海洋中的汞含量正在逐漸提高,研究中引用的水質取樣顯示,2006年的汞含量較1990年代中期所測量的高出30%。

此研究報告也首次證實了甲基汞是在中層深度海水中,透過作者所謂的「海洋雨」過程所形成--在海平面陽光水域所形成的海藻,快速死亡後如同下雨般向下沈入更深的海域。在深層海域中,沈澱的海藻被細菌分解,而分解過程和海水中的汞成分交互產生作用後,就形成了甲基汞。之後經過不斷向上延展的的食物鏈,像鮪魚這類的肉食動物體內,就會因為吃了較小的魚類而吸收了甲基汞成分。

美國環保署長潔克森(Lisa Jackson)表示:「這份研究讓我們更深入地了解到危害人體劑量的汞如何進入到空氣、水源和食物之中,並且對美國人民和全世界人類造成嚴重的健康威脅。」

Ocean Origin of Seafood Contaminant Methylmercury Identified
WASHINGTON, DC, May 4, 2009 (ENS)

For the first time, scientists have documented how toxic methylmercury is formed in the ocean. Scientists have known that mercury deposited from the atmosphere to freshwater ecosystems can be transformed into methylmercury, but identifying the parallel cycles in marine ecosystems has remained elusive.

This poisonous form of mercury bioaccumulates in fish, becoming more and more concentrated as it moves up the food chain from smaller to larger fish until people eating tuna sushi are exposed to levels of mercury high enough to warrant warning signs. Pregnant women who consume mercury can pass on life-long developmental effects to their children.

Previous studies have shown that 75 percent of human exposure worldwide to mercury is from the consumption of marine fish and shellfish. In the United States, about 40 percent of all human exposure to mercury is from tuna harvested in the Pacific Ocean, according to Elsie Sunderland, a coauthor of the study.

This study shed some light on how metal mercury emitted by burning coal and waste and from factories in Asia beomes methylmercury in the North Pacific Ocean.

Because much of the mercury that enters the North Pacific comes from the atmosphere, scientists have predicted an additional 50 percent increase in mercury in the Pacific by 2050 if mercury emission rates continue as projected.

"This unprecedented USGS study is critically important to the health and safety of the American people and our wildlife because it helps us understand the relationship between atmospheric emissions of mercury and concentrations of mercury in marine fish," said Secretary of the Interior Ken Salazar. "We need to reduce the mercury emissions so that we can reduce the ocean mercury levels."

Currently, ocean mercury levels are rising. Water sampling cited in the study shows that mercury levels in 2006 were approximately 30 percent higher than those measured in the mid-1990s.

This study documents for the first time that methylmercury is produced in mid-depth ocean waters by processes linked to what the authors call "ocean rain."

Algae, which are produced in sunlit waters near the surface, die quickly and "rain" downward to greater water depths.

At depth, the settling algae are decomposed by bacteria and the interaction of this decomposition process in the presence of mercury results in the formation of methylmercury.

Many steps up the food chain later, predators like tuna receive methylmercury from the smaller fish they consume.
"This study gives us a better understanding of how dangerous levels of mercury move into our air, our water, and the food we eat, and shines new light on a major health threat to Americans and people all across the world," said EPA Administrator Lisa Jackson.