130歲的腦珊瑚對大洋的狀況提供了好消息

原文網址：https://www.princeton.edu/news/2018/10/01/130-year-old-brain-coral-reveals-encouraging-news-open-ocean

130歲的腦珊瑚對大洋的狀況提供了好消息

Catherine Zandonella

氮肥流到水體之後可能會對沿岸的海洋生物造成致命的後果，不過氮汙染對離陸地相當遙遠的大洋會造成什麼影響？

由普林斯頓大學進行的研究發現美國東岸大洋區的氮汙染比先去預估的還要輕微。研究人員測量這顆活的腦珊瑚(Diploria labyrinthiformis)的碳酸鈣骨骼中含有多少氮。圖片來源：研究人員

一顆130歲的腦珊瑚提供了解答――至少可以顯示美國東岸外海的北大西洋狀況是如何。由普林斯頓大學領導的研究團隊測量了珊瑚骨骼含有的氮，發現氮污染比先前預估的輕微許多。這項研究發表在《美國科學院院刊》(Proceedings of the National Academy of Sciences)的線上版。

現為加州理工學院博士後學者的Xingchen (Tony) Wang表示：「出乎意料的是，我們並沒有看到證據顯示北大西洋在過去數十年氮汙染有加劇的情形。」這是他在普林斯頓大學地質科學系攻讀博士學位時進行的一部份研究。

不過，之前以普林斯頓大學為主的團隊在大洋另一個地方――南中國海進行的研究卻發現氮汙染有加劇的趨勢，與中國過去20年來大幅成長的碳產量和肥料用量有對應關係。此研究2017年發表於《科學》(Science )期刊。

研究人員在新研究中探討的珊瑚骨骼樣品採自百慕達島附近的大洋區域，大約位在北美大陸東方1000公里。科學家認為美國本土的氮汙染源，像是交通工具和工廠排放的廢氣會經由空氣傳播到此區而造成強烈影響。

Wang表示團隊沒有發現證據指出人為產生的氮有所增加，不過他們注意到氮的變化趨勢可以對應到一種自然氣候現象――北大西洋震盪(North Atlantic Oscillation)預估中的強度。

之前發表的研究利用電腦模型預估北大西洋人為造成的氮汙染會劇烈增加，不過這篇研究結果卻得到相反的結論。

研究成果或許代表美國的汙染控制對策，成功限制了人為排放的氮進入到大洋當中。

「我們的發現在探討人類未來對北大西洋氮汙染的影響時有很重要的意義。」Wang表示。「由於最近數十年來汙染控制的技術有大幅進展，美國的人為氮排放量呈現出穩定甚至下降的趨勢。我們的結果顯示如果氮排放量可以維持穩定，那麼北大西洋的大洋區在接下來數十年都只會受到輕微氮污染的影響。」

當生物可以利用的氮在水體中的含量過高，植物和藻類會過度生長而嚴重破壞生態系，比方海洋中的「死區」形成原因就是微生物把水裡的氧氣全部消耗掉，使魚類完全無法生存。20世紀初由於肥料產量和化石燃料用量大幅增加，生物可利用的氮(固定氮，fixed nitrogen)也水漲船高。

固定氮排放到大氣之後可以影響到離陸地相當遠的海洋。然而，包括長期監測大洋在內的諸多挑戰，使得固定氮對大洋的影響並不容易研究。

珊瑚在此可以起到幫助。壽命相當長的石珊瑚(Scleractinian coral)成長時會利用碳酸鈣建構骨骼。它們會把從周遭海水吸收的氮的一部份堆積在骨骼當中，因此珊瑚的骨骼便可以成為有多少氮排放到環境中的自然紀錄。

為了區別人類和自然產生的氮，研究人員運用了氮的重量有兩種的特性。比較重的氮-15較氮-14多了一顆中子，而人類產生的氮裡面氮-15跟氮-14的比例比海洋中的低。

普林斯頓大學地質和地球物理系的教授Daniel Sigman說：「長久以來我都希望可以利用珊瑚骨骼中的碳來重建過去的環境變化。托Tony幫忙我們現在有辦法做到了。」

Wang在普林斯頓大學讀博士時發展出一套靈敏精確的方法測量氮-15和氮-14的比例。他用的工具――質譜儀就像是可以測量分子重量的體重計一樣。

為了採集北大西洋的珊瑚樣品，Wang和伍茲霍爾海洋研究所的地質和地球物理副研究員Anne Cohen在2014年組織研究團隊前往百慕達。他們在距主島大約10公里處的Hog Reef深度3公尺左右的地方，採集了一株活的腦珊瑚(Diploria labyrinthiformis)的碳酸鈣骨骼樣品。研究人員透過測量上方海洋浮游生物中的氮含量，確定百慕達地表逕流中的氮不會對此區構成重要影響。

除了Wang、Cohen、Sigman，參與研究的科學家還有普林斯頓大學地質科學系的研究生Victoria Luu、國立台灣大學的任昊佳、美國加州理工學院的Zhan Su以及德國馬克斯•普朗克化學研究所的Gerald Haug。

130-year-old brain coral reveals encouraging news for open ocean

When nitrogen-based fertilizers flow into water bodies, the result can be deadly for marine life near shore, but what is the effect of nitrogen pollution far out in the open ocean?

A 130-year-old brain coral has provided the answer, at least for the North Atlantic Ocean off the East Coast of the United States. By measuring the nitrogen in the coral’s skeleton, a team of researchers led by Princeton University found significantly less nitrogen pollution than previously estimated. The study was published online in the Proceedings of the National Academy of Sciences.

“To our surprise, we did not see evidence of increased nitrogen pollution in the North Atlantic Ocean over the past several decades,” said Xingchen (Tony) Wang, who conducted the work as part of his doctorate in geosciences at Princeton and is now a postdoctoral scholar at the California Institute of Technology.

Earlier work by the Princeton-based team, however, did find elevated nitrogen pollution in another open ocean site in the South China Sea, coinciding with the dramatic increase in coal production and fertilizer usage in China over the past two decades. The finding was reported in the journal Science in 2017.

In the new study, the researchers looked at coral skeleton samples collected in the open ocean about 620 miles east of the North American continent near the island of Bermuda, a region thought to be strongly influenced by airborne nitrogen released from U.S. mainland sources such as vehicle exhaust and power plants.

Although the team found no evidence that human-made nitrogen was on the rise, the researchers noted variations in nitrogen that corresponded to levels expected from a natural climate phenomenon called the North Atlantic Oscillation, Wang said.

The result is in contrast to previously published computer models that predicted a significant increase in human-made nitrogen pollution in the North Atlantic.

The work may indicate that U.S. pollution control measures are successfully limiting the amount of human-generated nitrogen emissions that enter the ocean.

“Our finding has important implications for the future of human nitrogen impact on the North Atlantic Ocean,” said Wang. “Largely due to advances in pollution technology, human nitrogen emissions from the U.S. have held steady or even declined in recent decades,” he said. “If emissions continue at this level, our results imply that the open North Atlantic will remain minimally affected by nitrogen pollution in coming decades.”

Nitrogen, when in its biologically available form and supplied in excess, can cause overgrowth of plants and algae and lead to severe ecosystem harm, including marine “dead zones” that form when microorganisms consume all the oxygen in the water, leaving none for fish. Fertilizer production and fossil fuel burning have greatly increased the production of biologically available, or “fixed,” nitrogen since the early 20th century.

When emitted to the atmosphere, fixed nitrogen can influence the ocean far from land. However, the impacts on the ocean are difficult to study because of the challenges involved in making long-term observations in the open ocean.

Corals can help. Stony or “Scleractinian” corals are long-lived organisms that build a calcium carbonate skeleton as they grow. The corals soak up nitrogen from the surrounding water and deposit a small portion in their skeletons. The skeletons provide a natural record of nitrogen emissions.

To distinguish human-made, or anthropogenic, nitrogen from the naturally occurring kind, the researchers take advantage of the fact that nitrogen comes in two weights. The heavier version, known as ¹⁵N, contains one more neutron than the lighter ¹⁴N. Anthropogenic nitrogen has a lower ratio of ¹⁵N to ¹⁴N than does the nitrogen in the ocean.

“It has long been my dream to use the nitrogen in coral skeletons to reconstruct past environmental changes; thanks to Tony, we are now doing it,” said Daniel Sigman, the Dusenbury Professor of Geological and Geophysical Sciences at Princeton.

While a graduate student at Princeton, Wang developed a sensitive and precise method to measure the ¹⁵N-to-¹⁴N ratio using a mass spectrometer, which is like a bathroom scale for weighing molecules.

To collect coral samples in the North Atlantic Ocean, Wang and Anne Cohen, an associate scientist in geology and geophysics at Woods Hole Oceanographic Institution, led a team in 2014 to Bermuda. The investigators removed a sample of the calcium carbonate skeleton from a living brain coral (Diploria labyrinthiformis) about 10 feet below the surface on Hog Reef, about six miles from the main island. The researchers confirmed that Bermuda’s nitrogen run-off was not a factor at the site by measuring nitrogen levels in plankton floating nearby.

In addition to Wang, Cohen and Sigman, the research featured contributions from Princeton graduate student in geosciences Victoria Luu, Haojia Ren of National Taiwan University, Zhan Su of Caltech and Gerald Haug of the Max Planck Institute for Chemistry.

原始論文：Xingchen Tony Wang, Anne L. Cohen, Victoria Luu, Haojia Ren, Zhan Su, Gerald H. Haug, Daniel M. Sigman. Natural forcing of the North Atlantic nitrogen cycle in the Anthropocene. Proceedings of the National Academy of Sciences, 2018. DOI: 10.1073/pnas.1801049115

引用自：Princeton University. "130-year-old brain coral reveals encouraging news for open ocean."