不起眼的苔蘚卻幫助地球形成了富含氧氣的大氣層

原始網址：www.sciencedaily.com/releases/2016/08/160815185610.htm

Humble moss helped create our oxygen-rich atmosphere

不起眼的苔蘚卻幫助地球形成了富含氧氣的大氣層

The evolution of the first land plants including mosses may explain a long-standing mystery of how Earth's atmosphere became enriched with oxygen, according to an international study led by the University of Exeter.

根據一則由英國艾希特大學進行的國際研究，包含蘚苔在內的植物成功演化出在陸地上生活的能力，或許能解釋一項長久以來的疑問：地球大氣如何變得富含氧氣？

Oxygen in its current form first appeared in Earth's atmosphere some 2.4 billion years ago, in an incident known as the Great Oxidation Event. However, it was not until roughly 400 million years ago that this vital compound first approached modern levels in the atmosphere. This shift steered the trajectory of life on Earth and researchers have long debated how oxygen rose to modern concentrations.

大約在24億年前發生了稱作「大氧化事件(Great Oxidation Event)」的劇變後，地球大氣層的組成當中第一次出現了氧氣。然而，還要再等到4億年前左右，這個重要成分在大氣中的濃度才會首度攀升至跟現今十分接近。這個轉變引領了地球生物的演化方向，而研究人員對氧氣如何提升至現今的濃度也已經爭論許久。

In a study published in the journal Proceedings of the National Academy of Sciences, Professor Tim Lenton, of the University of Exeter, and his colleagues theorised that the earliest land plants, which colonised the land from 470 million years ago onwards, are responsible for the levels of oxygen that sustains our lives today. Their emergence and evolution permanently increased the flux of organic carbon into sedimentary rocks, the primary source for atmospheric oxygen, thus driving up oxygen levels in a second oxygenation event and establishing a new, stable oxygen cycle.

艾希特大學的Tim Lenton和他的同僚在刊登於《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)的研究中，提出了一項理論認為自4億7000萬年前開始最早往陸地拓殖的植物，讓氧氣濃度上升至今日人類得以維生的程度。它們的出現和演化永久增加了有機碳進入沉積岩的速率，由於這個過程是大氣中氧氣的主要來源，因此這促使第二次氧化事件發生並提升了大氣氧濃度，使得氧循環進入了一種全新的穩定狀態。

Earth's early plant biosphere consisted of simple bryophytes, such as moss, which are non-vascular -- meaning they do not have vein-like systems to conduct water and minerals around the plant. Using computer simulations, the researchers first estimated that these plants could have generated roughly 30% of today's global terrestrial net primary productivity by about 445 million years ago.

地球最早的植物生物圈成員之一為簡單的苔蘚植物門(bryophyte)，包含蘚苔在內的這些植物缺乏維管束(vascular)構造，意味著它們不具有連通整株植物可以用來傳輸礦物質和水分的脈絡狀系統。研究人員利用電腦模擬的結果，最初預估約莫在4億4500萬年前，這些植物的生產力已經可以達到當今全球陸地淨初級生產量(net primary productivity)的30%左右。

When the properties of modern bryophytes were taken into account, including their elemental composition and effects on rock weathering, they found that modern levels of atmospheric oxygen were achieved by 420 to 400 million years ago, consistent with independent evidence.

當研究人員將現存苔蘚植物的性質，像是元素組成以及對岩石風化作用的影響納入模型當中，他們發現大氣氧濃度在4億2000萬年至4億年前到達與現在相同的程度，這跟從另一種獨立證據推估出來的時間點相符。

These findings therefore suggest that the first land plants, such as the humble moss, created the stable oxygen-rich atmosphere that allowed large, mobile, intelligent animal life, including humans, to evolve.

因此，這些發現指出最初登陸的植物，像是不起眼的苔蘚卻是形成富含氧氣的穩定大氣層的幕後推手，並進一步讓大型、活動力強、有心智能力的動物，包含人類在內的生命得以演化出來。

Professor Tim Lenton, of the University of Exeter, said: "It's exciting to think that without the evolution of the humble moss, none of us would be here today. Our research suggests that the earliest land plants were surprisingly productive and caused a major rise in the oxygen content of Earth's atmosphere."

艾希特大學的 Tim Lenton教授說：「若這些不起眼的苔蘚從未演化出來，則今日我們之中的任何一位都不會站在這裡，這個想法相當具有啟發性。我們的研究主張最初的陸生植物生產力其實相當驚人，而促使地球大氣層的氧含量大幅提升。」

引用自：University of Exeter. "Humble moss helped create our oxygen-rich atmosphere." ScienceDaily. ScienceDaily, 15 August 2016. 

論文來源：Timothy M. Lenton, Tais W. Dahl, Stuart J. Daines, Benjamin J. W. Mills, Kazumi Ozaki, Matthew R. Saltzman, Philipp Porada. Earliest land plants created modern levels of atmospheric oxygen. Proceedings of the National Academy of Sciences, 2016