岩石紀錄揭開了氧氣的變化歷史

 原文網址：https://artsandsciences.syracuse.edu/earth-sciences-department/news/rock-record-illuminates-oxygen-history

地球歷史上有幾個關鍵時刻，可以讓科學家更加深入地瞭解生物如何適應環境裡物理化學條件的變化，進而幫助他們回答這道問題：「人類如何走到現在這一步？」而這些時刻也闡明了下一個問題：「我們的未來會是什麼樣子？」。其中一個發生在20幾億年前，對演化產生了深遠的影響，稱為「大氧化事件」(Great Oxidation Event，GOE)。它標記了人類以及其他許多生命形式不可或缺的氣體，也就是光合作用產生的氧氣首次在大氣中累積到一定程度的時間。

研究團隊從南非採來進行這項研究的沉積岩。圖片來源：Benjamin Uveges

如果你搭乘時光機到大氧化事件之前(大約是早於24億年前)，會處在幾乎沒有氧氣的環境當中。那時活躍的生物稱作厭氧生物，意味著它們不需要氧氣，而是依靠其他作用(像是發酵)來產生能量。這些生物有些仍然存活在今日的極端環境，像是酸性的溫泉與海底熱泉。

大氧化事件引發了地球表面歷史上最劇烈的的化學性質轉變之一。它象徵了一顆大氣近乎缺乏氧氣，因此不適合複雜生命居住的星球，轉變成一顆大氣含氧的星球，而能支持我們現今所見的生物圈。

大氣氧含量出現重大變化的確切時間與原因，長久以來都是科學家深感興趣的主題，因為要瞭解包含人類在內的複雜生命如何誕生，就要先瞭解大氣中的氧氣。雖然我們對於此關鍵時期的認識尚未完全成形，雪城大學和麻省理工學院組成的研究團隊則決定更加「深入」——他們從南非的地下鑽取出遠古的岩芯，從中挖掘有關大氧化事件發生時間的線索。他們的成果所帶出的全新見解不只是關於氧含量升高如何影響生物演化的步調，還有真核生物(此類生物的細胞具有被膜包覆起來的核心)漫長且複雜的崛起過程。

此研究發表在期刊《美國國家科學院院刊》。計畫主持人Benjamin Uveges在2018年於雪城大學取得博士學位，並在麻省理工學院擔任博士後研究員的期間完成了這項計畫。而雪城大學的地球科學教授Christopher Junium則協助了化學分析的部分。

埋藏在岩石中的答案

為了回顧過去那段時間，研究團隊從南非幾個地方採集了沉積岩岩芯並進行分析。這些精挑細選出來的地點具有年代為22億至25億年前的岩石，是保存大氧化事件相關證據的理想年代區間。透過分析岩石中的穩定同位素比例，團隊找到的證據顯示當時海洋發生了要有硝酸鹽才能進行的作用，而硝酸鹽正是環境中氧氣變多的指標。

為了分析這些古代沉積物，Uveges找上了雪城大學地球與環境科學系的副教授Junium。他的專長是研究過去的環境演變，藉此更加瞭解未來的全球變遷。要精準測定岩石中含量稀少的氮，他的尖端儀器是不可或缺的。

「我們在此研究中分析的岩石含有的氮極為稀少，一般用於這類研究的儀器是無法測量出來的，」Uveges表示。「由Chris建造測量氮同位素濃度的儀器，需要的最低氮濃度是平常的百分之一到千分之一，世上只有幾台儀器有這種能耐。」

團隊在Junium的實驗室運用「同位素比值質譜儀」(Isotope Ratio Mass Spectrometer，IRMS)分析南非岩石樣品中的氮同位素比例。他們先把樣品磨碎，利用化學處理萃取出特定的成分，然後將其轉化成氣體。接著將氣體離子化(轉變成帶電粒子)並加速穿過磁場，過程中同位素會因為質量不同而分離開來。最後IRMS會測量¹⁵N與¹⁴N的比例，這項數值可以顯示氮元素經歷了什麼作用。

那麼對於過去的氧含量，這些作用可以透露出什麼樣的訊息？在沉積物變成岩石之前，其中的微生物會影響其化學組成，留下的同位素訊號可以顯示氮經歷的作用與生物利用氮的方式。科學家可以藉由追蹤¹⁵N和¹⁴N隨著時間的變化，而瞭解地球的環境演變，特別是氧濃度。

改寫氧濃度變化的時間線

Uveges表示他們最驚人的發現是改寫了海洋含氧氮循環的開始時間。證據顯示氮循環變得容易受到溶解氧影響的時間，比之前認為的大約早了一億年，意味著海洋與大氣的氧氣累積到一定程度的時間有著明顯的差距。

Junium指出這些結果代表氮循環出現了重大的轉捩點：生物必須要更新它們體內生化作用的機器來處理氧化程度更高的氮，因為此種形式的氮以往是較難吸收利用的。

「這些發現全都符合一項正在興起的看法：大氧化事件對生物來說是個漫長的考驗。因為生物在利用產氧光合作用帶來更多能量的同時，也要逐漸適應並處理氧氣這種副產物，它們必須要在兩者之間取得平衡，」Junium表示。

隨著產氧光合作用讓大氣累積越來越多氧氣，氧含量的升高也造成許多厭氧生物滅亡，並且成為需氧呼吸作用演化出來的基礎。人類以及其他動物都是透過此作用，運用氧氣分解葡萄糖並產生能量，供應肌肉運動、腦部活動，並且維持細胞的功能。

「地球歷史最初的二十幾億年，海洋與大氣裡幾乎沒有氧氣存在，」Uveges表示。「相較之下，今日的氧氣則佔了大氣組成的五分之一，而且我們所知的複雜多細胞生物基本上都需要氧氣來呼吸。因此從某種程度上來說，研究氧氣如何出現，以及它對地球的化學、地質與生物造成的影響，也是在研究地球與生物如何共同演化成現今的樣貌。」

產氧光合作用演化出來之後，地球表面的環境過了多久才變成富含氧氣？他們的發現改寫了我們對此的理解。不只如此，他們的研究也找出了生物地球化學上的一個關鍵標記，可以用來幫助科學家模擬大氧化前後不同生命形式的演化過程。

「我希望我們的發現能夠啟發更多人來研究這段有趣的時光，」Uveges表示。「我們利用新的地球化學技術來研究這些岩芯，結果對大氧化事件的全貌建立了更為詳細的圖像，也更加了解它對地球生物所造成的衝擊。」

研究經費來源如下：美國國家科學基金會CAREER獎(授予雪城大學的Christopher Junium)、西蒙斯基金會的生命起源基金(授予麻省理工學院的Benjamin Uveges、Gareth Izon、Roger Summons)。

 

Rock record illuminates oxygen history

Several key moments in Earth's history help us humans answer the question, "How did we get here?" These moments also shed light on the question, "Where are we going"? -- offering scientists deeper insight into how organisms adapt to physical and chemical changes in their environment. Among them is an extended evolutionary occurrence over 2 billion years ago, known as the Great Oxidation Event (GOE). This marked the first time that oxygen produced by photosynthesis -- essential for the survival of humans and many other life forms -- began to accumulate in significant amounts in the atmosphere.

If you traveled back in time to before the GOE (more than ~2.4 billion years ago), you would encounter a largely anoxic (oxygen-free) environment. The organisms that thrived then were anaerobic, meaning they didn't require oxygen and relied on processes like fermentation to generate energy. Some of these organisms still exist today in extreme environments such as acidic hot springs and hydrothermal vents.

The GOE triggered one of the most profound chemical transformations in Earth's surface history. It marked the transition from a planet effectively devoid of atmospheric oxygen -- and inhospitable to complex life -- to one with an oxygenated atmosphere that supports the biosphere we know today.

Scientists have long been interested in pinpointing the timing and causes of major shifts in atmospheric oxygen because they are fundamental to understanding how complex life, including humans, came to be. While our understanding of this critical period is still taking shape, a team of researchers from Syracuse University and MIT is digging deep -- literally -- into ancient rock cores from beneath South Africa to unearth clues about the timing of the GOE. Their work provides new insight into the pace of biological evolution in response to rising oxygen levels -- and the long, complex journey toward the emergence of eukaryotes (organisms whose cells contain a nucleus enclosed within a membrane).

The study, published in the journal Proceedings of the National Academy of Sciences, was led by Benjamin Uveges '18 Ph.D., who completed the project as a postdoctoral associate at MIT and collaborated with Syracuse University Earth sciences professor Christopher Junium on the chemical analyses.

Answers Embedded in Rock

To step back in time, the research team analyzed sedimentary rock cores collected from several sites across South Africa. These locations were carefully selected because their rocks, dating back 2.2 to 2.5 billion years, fall within the ideal age range for preserving evidence of the GOE. By analyzing stable isotopic ratios embedded in these rocks, the team uncovered evidence of oceanic processes that required the presence of nitrate -- an indicator of more oxygen-rich conditions.

To analyze the ancient sediment, Uveges worked with Junium, an associate professor of Earth and environmental sciences at Syracuse University. Junium specializes in studying how past environments evolved to better understand future global change. His state-of-the-art instruments were essential for obtaining accurate readings of trace nitrogen levels.

"The rocks that we analyzed for this study had very low nitrogen concentrations in them, too low to measure with the traditional instrumentation used for this work," says Uveges. "Chris has built one of only a handful of instruments in the world that can measure nitrogen isotope ratios in samples with 100 to 1,000 times less nitrogen in them than the typical minimum."

In Junium's lab, the team analyzed nitrogen isotope ratios from South African rock samples using an instrument called an Isotope Ratio Mass Spectrometer (IRMS). The samples were first crushed into powder, chemically treated to extract specific components, then converted into gas. This gas was ionized (turned into charged particles) and accelerated through a magnetic field, which separated the isotopes based on their mass. The IRMS then measured the ratio of ¹⁵N to ¹⁴N, which can reveal how nitrogen was processed in the past.

So how does this process reveal past oxygen levels? Microbes (short for microorganisms) influence the chemical makeup of sediments before they become rock, leaving behind isotopic signatures of how nitrogen was being processed and used. Tracking changes in ¹⁵N to ¹⁴N over time helps scientists understand how Earth's environment, particularly oxygen levels, evolved.

Rewriting the Oxygen Timeline

According to Uveges, the most surprising finding is a shift in the timing of the ocean's aerobic nitrogen cycle. Evidence suggests that nitrogen cycling became sensitive to dissolved oxygen roughly 100 million years earlier than previously thought -- indicating a significant delay between oxygen buildup in the ocean and its accumulation in the atmosphere.

Junium notes that these results mark a critical tipping point in the nitrogen cycle, when organisms had to update their biochemical machinery to process nitrogen in a more oxidized form that was harder for them to absorb and use.

"All of this fits with the emerging idea that the GOE was a protracted ordeal where organisms had to find the balance between taking advantage of the energy gains of oxygenic photosynthesis, and the gradual adaptations to dealing with its byproduct, oxygen," says Junium.

As oxygen produced through photosynthesis began to accumulate in the atmosphere, this rise in oxygen led to the extinction of many anaerobic organisms and set the stage for the evolution of aerobic respiration -- a process that uses oxygen to break down glucose and provides the energy needed for functions like muscle movement, brain activity and cellular maintenance in humans and other animals.

"For the first 2 plus billion years of Earth's history there was exceedingly little free oxygen in the oceans or atmosphere," says Uveges. "In contrast, today oxygen makes up one fifth of our atmosphere and essentially all complex multicellular life as we know it relies on it for respiration. So, in a way, studying the rise of oxygen and its chemical, geological and biological impacts is really studying how the planet and life co-evolved to arrive at the current situation."

Their findings reshape our understanding of when Earth's surface environments became oxygen-rich after the evolution of oxygen-producing photosynthesis. The research also identifies a key biogeochemical milestone that can help scientists model how different forms of life evolved before and after the GOE.

"I hope our findings will inspire more research into this fascinating time period," says Uveges. "By applying new geochemical techniques to the rock cores we studied, we can build an even more detailed picture of the GOE and its impact on life on Earth."

This work was funded by grants including: An NSF CAREER award (Syracuse University -- Christopher Junium) and a Simons Foundation Origins of Life Collaboration award (MIT -- Benjamin Uveges, Gareth Izon and Roger Summons).

原始論文：Benjamin T. Uveges, Gareth Izon, Christopher K. Junium, Shuhei Ono, Roger E. Summons. Aerobic nitrogen cycle 100 My before permanent atmospheric oxygenation. Proceedings of the National Academy of Sciences, 2025; 122 (20) DOI: 10.1073/pnas.2423481122

引用自：Syracuse University. "Rock record illuminates oxygen history." 

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