新發現改變了科學家對於海床為何缺乏氫氣的想法

 原文網址：https://www2.lehigh.edu/news/discovery-transforms-understanding-of-hydrogen-depletion-at-the-seafloor

新發現改變了科學家對於海床為何缺乏氫氣的想法

之前認為海底熱泉噴出的流體缺乏氫氣，是因為海床之下的微生物群集會迅速把它們用掉。但科學家從開曼中部隆起的皮卡爾德熱泉採集流體並進行分析之後，發現缺乏氫氣的原因其實是由非生物作用造成。

在皮卡爾德熱泉區，5℃的冰冷海水和398℃的高溫熱泉流體(以附近的Beebe黑煙囪為代表)在海床底下互相混和，形成了溫度範圍介於44-149℃的流體。圖片來源：Chris German/ROV Jason (Copyright WHOI)

海底熱泉是海底火山在海床噴出高熱流體的地方，其溫度可以高達攝氏350度。在1970年代發現它們的存在徹底改變了我們對於地球和生命的認知。不過時至今日，海床表面和下方的生物仍然有許多尚未解開的謎團。

深入瞭解這些火山活動活躍的地區相當重要，因為海底熱泉的化學成分會影響到更大範圍的海水化學性質。此外，這些位在海底的獨特環境也讓某些生物作用和非生物作用得以進行，從中我們可以推測地球的生命最初是如何誕生，又是如何經過時間的考驗，甚至還能讓我們瞭解其他星體上可能的生命形式。

理海大學地球與環境科學系的地球化學教授Jill McDermott表示，過去對於海底熱泉的研究已經得出此處某些種類的氣體含量較少，像是氫分子。科學家認為這種缺失是由住在海床淺層的微生物群集，統稱為海底生物圈的生物造成。

然而，McDermott和同僚最新的研究成果卻跟此假設相悖。這些研究人員從世上最深的熱泉區――位在開曼中部隆起，4970公尺深的皮卡爾德熱泉區以氣密法採集了熱泉流體樣本並加以分析，結果察到樣品具有氫分子大量減少以及其他的化學性質變化。他們認為只有非生物與熱成因(高溫造成的分解)作用能造成這類變化，因為流體的溫度遠遠超過生命目前所知的生存極限――122℃。

這項結果今日發表在《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)的文章「在熱液系統混和帶的非生物氧化還原作用：減少地下生物圈可以取用的能量」。其他作者包括伍茲霍爾海洋研究所的三名科學家：地質和地球物理學部門的資深科學家Christopher German、海洋化學和地球化學部門的資深科學家Jeffrey Seewald、海洋化學和地球化學部門的三等副研究員Sean Sylva；另外還有麻省理工學院的副教授Shuhei Ono。

「我們的研究發現化學成分的能量在被微生物群集利用之前，就已經先被非生物作用給減少才是化學成分改變的原因，」McDermott表示。「這項結果在預測全球的地球化學循環可以支撐生物圈到地下多深的地方，以及推測全球氫氣儲量有多少時具有相當重要的意涵。」

她繼續說道：「這也代表地下生物圈可以得到的能量可能比過去認為的都還要少。未來研究的一大目標是評估居住在海床下的生物所能發揮的影響力，會因為海洋地殼中消耗氫氣的非生物作用而降低多少。」

研究人員在開曼中部隆起的皮卡爾德熱泉運用複合式遠端遙控載具涅柔斯號，以及遠端遙控載具傑森二號來採集樣品。圖片來源：Jill McDermott

分析溶解其中的氣體、無機化合物和有機化合物的化學成分後，研究團隊發現低溫的流體樣品是由海水跟附近Beebe熱泉的「黑煙囪」流出的液體混合而成。會叫做「黑煙囪」是因為噴出流體的熱泉看起來就像冒著黑煙的煙囪。McDermott表示海水混到熱泉的流體樣本中，許多種化學物質的含量都會變高或變低。而氣體含量變化最大的樣品對應到的海床溫度為149℃，這對生物來說已經熱到無法生存，因此他們的結論認為造成這些變化的地球化學作用不可能有生物直接牽涉其中。

他們辨識出造成化學成分變化的非生物反應包括了硫酸鹽還原以及生質受熱降解，考慮質量平衡、測量穩定同位素以及計算化學能之後也支持了此說法。

這些樣品是他們在兩班研究航次中運用可以遠端操靠的兩台載具――傑森二號和涅柔斯號來採集。它們都是被設計來探索全世界的深海，同時還能進行各式各樣的科學研究。

「這真的是令人相當興奮的實地考察計畫。它讓我們有難得的機會來探討自然環境以及生存其中的生命之間發生的複雜化學反應，」Seewald表示。「我們現在可以更加準確地估計海床下方可能有多少微生物。」

皮卡爾德熱泉區發現於2010年，地點位於加勒比海大開曼島的南方不遠處。研究人員從熱泉採集到的樣品溫度範圍為44到149℃，這讓他們有難得的機會探討從生命可以生存的環境逐漸轉變至生命無法生存的環境時，會有什麼樣的變化發生。

「這項研究最棒的一點是我們找到了幾座熱泉，它們的溫度範圍跨足了對生物來說熱到無法生存以及恰到好處的區間，」German說。「這幾座特別討人喜愛的熱泉讓我們有機會可以更加瞭解海床之下的生物有辦法(以及沒辦法)做到的事。」

科學家已經知道對於全世界海洋地殼裡的微生物群集來說，決定它們的構成以及運作方式時很重要的一個因素是海底熱泉的溫度以及化學成分。

「會有這種關係存在是因為海底熱泉的流體可以提供能量給微生物進行特定的代謝反應，」McDermott解釋。「反過來說，熱泉流體的化學成分是否會受到生物作用而改變，或者非生物作用才是主因，卻是一個相當重要但很少人探討的問題。」

團隊的發現開啟了新的研究方向來讓科學家探討生物可以取用多少能量時，除了微生物作用之外，非生物作用是否也是一個相當重要的變因。

 

Discovery Transforms Understanding of Hydrogen Depletion at the Seafloor

Scientists analyzing hydrothermal fluid collected at the Piccard vents at Mid-Cayman Rise have found non-biological processes deplete hydrogen that was thought to be readily available to subseafloor microbial communities.

The discovery of hydrothermal vents―where volcanoes at the seafloor produce hot fluid exceeding 350 degrees Celsius or 662 degrees Fahrenheit―fundamentally changed our understanding about Earth and life in the 1970s. Yet, life at and underneath the seafloor is still very much a mystery today. 

Gaining a better understanding of these volcanically active areas is important, as the chemistry at seafloor vents impacts ocean chemistry more generally. In addition, the seafloor’s unique environment supports biological and non-biological processes that offer clues as to how life on Earth first began, how it is sustained over time―and the potential for life on other planetary bodies.

According to geochemist Jill McDermott, a professor in the Department of Earth and Environmental Sciences at Lehigh University, past studies of the chemistry of hydrothermal vent fluids have revealed reductions in certain gas species, such as molecular hydrogen. These depletions were thought to be caused by microbiological communities living in the shallow seafloor, collectively called the subseafloor biosphere.

However, results of a new study by McDermott and colleagues contradict that assumption. The researchers analyzed gas-tight hydrothermal fluid samples from the world's deepest known vent field, the Piccard hydrothermal field at the Mid-Cayman Rise, which is at a depth of 4970 meters, or about 16,000 feet below sea level. They observed chemical shifts in their samples, including a large loss of molecular hydrogen, that could only be the result of abiotic (non-biological) and thermogenic (thermal breakdown) processes, because the fluid temperatures were beyond the limits that support life―understood to be 122 degrees Celsius or around 250 degrees Fahrenheit, or lower.

The results were published online today in an article “Abiotic redox reactions in hydrothermal mixing zones: decreased energy availability for the subsurface biosphere” in the Proceedings of the National Academy of Sciences. Additional authors include: Christopher German, Senior Scientist in Geology & Geophysics and Jeffrey Seewald, Senior Scientist in Marine Chemistry & Geochemistry and Sean Sylva, Research Associate III, in Marine Chemistry & Geochemistry from the Woods Hole Oceanographic Institution; and Shuhei Ono, Associate Professor, Massachusetts Institute of Technology.

“Our study finds that these shifts in chemistry are driven by non-biological processes that remove energy before microbial communities gain access to it,” says McDermott. “This could have critical implications for constraining the extent to which global geochemical cycles can sustain a deep biosphere, and for the global hydrogen budget.”

She adds “This also means the subsurface biosphere is likely receiving less energy than anyone had realized previously. The degree to which non-biological hydrogen consumption in the oceanic crust may reduce the impact of life inhabiting the seafloor is a great target for future studies."

Using chemical analysis of dissolved gases, inorganic compounds, and organic compounds, the team found that the low-temperature fluid samples originated from mixing between seawater and the nearby Beebe Vents black smokers, so named because the fluid expelled from the vents resembles black smoke from a chimney. In these mixed fluid samples, many chemical species are either high or low in abundance, according to McDermott. The sample with the largest shifts in the amount of gas had a seafloor temperature of 149 degrees Celsius or 300 degrees Fahrenheit, a temperature that is too hot to host life. Thus, they concluded, the process responsible for the geochemical changes could not directly involve life.

The non-biological reactions they identified as responsible for these chemical shifts include sulfate reduction and the thermal degradation of biomass, and are supported by mass balance considerations, stable isotope measurements, and chemical energetics calculations.

The samples were collected during two research expeditions using two remotely operated vehicles, Jason II and Nereus, both designed for deep-water exploration and to conduct a diverse range of scientific investigations in the world's oceans.   

"This was a really exciting field program that provided a rare opportunity for us to explore the complex interplay between the chemistry of a natural environment and the life that it supports,” said Seewald. “We are now in a much better position to estimate the amount of microbial life that may exist beneath the seafloor."

Discovered in 2010, the Piccard Hydrothermal Field is located just south of Grand Cayman in the Caribbean. The fluid samples the researchers examined vented at 44 to 149 degrees Celsius (111 to 300 degrees Fahrenheit), providing a rare opportunity for the team to study the transition between life-supporting and non-life-supporting environments.

“The cool (hot) thing about this study is that we were able to find a set of vents that spanned from where it was too hot for life, to where it was just right,” says German. “That particularly cute set of circumstances opened up the possibility to gain new insights into what life might (and might not) be able to do, down beneath the seafloor."

Shifts in hydrothermal vent fluid temperature and chemical composition are known to serve as an important control on microbial community structure and function in the oceanic crust throughout the world's oceans.

“This relationship exists because hydrothermal fluids provide energy for specific microbial metabolic reactions,” says McDermott. “However, the reverse question of whether vent fluid chemistry is modified by life itself, or instead by non-living processes, is an important one that is rarely addressed.”

The team’s discovery may serve to open up a new path of exploration toward assessing whether non-biological processes serve as important controls on energy availability, in addition to microbial processes.

原始論文：Jill M. McDermott, Sean P. Sylva, Shuhei Ono, Christopher R. German, Jeffrey S. Seewald. Abiotic redox reactions in hydrothermal mixing zones: Decreased energy availability for the subsurface biosphere. Proceedings of the National Academy of Sciences, August 12, 2020; DOI: 10.1073/pnas.2003108117

引用自：Lehigh University. “Discovery Transforms Understanding of Hydrogen Depletion at the Seafloor.”