科學家重新探討海洋生物和海洋氧含量的共同演化
雪城大學地球科學系的研究人員證實數億年前，海洋和大氣的氧濃度上升跟海洋生物有共同演化的關係
這篇刊登在期刊《科學》(Science，美國科學促進協會出版，2018))的突破性研究的主要作者是雪城大學文理學院的博士候選人Wanyi Lu，其指導教授為Zunli Lu副教授。
Zunli Lu多年來領導的跨國研究工作產生了這篇論文，他們重新探討了在5.億4200萬年前開始並持續至今的顯生元中，大陸棚氧含量增加的原因以及帶來的影響。
「研究氧氣的歷史變化時，研究人員大多著重於大氣和深海，以及它們對生物演化的可能影響。」Zunli
Lu表示，「我們認為在大陸棚(也就是海洋淺層)以上的水層中，海洋氧濃度的面貌可能完全不同。」
團隊的研究核心為Lu在2010年率先使用的一種地球化學代用指標。他和他的同事根據碘的地球化學性質開發出新的研究方法，其量測了礦物和化石的碳酸鈣組成中，碘和鈣之間的比例。
加州大學河濱分校的生物地球化學特聘教授Timothy Lyons認為，碘的地球化學性質在建立古代海洋表面到表層附近的含氧狀況時，是一種相當「強而有力的工具」。「最早的動物首次出現的地點便是海洋表層，接著演化並發展出越來越複雜的生態系。」他說，「此研究的結果呈現出古代海洋表層的環境動盪是之前未曾料想到的，而動物勢必會受到這些狀況影響。」
Lu以平常心看待這些稱讚，但他強調團隊的這些發現確實有新穎之處。他表示：「海洋上層充分氧化的時間比原先認為的還要晚得多。」
這位雪城大學的地球化學家在說明他的觀點時如此描述：地球最初被一層厚重的甲烷迷霧覆蓋住，使得大氣中的氧氣非常地少，甚至根本就沒有。最後，行使光合作用的微生物產生了足夠的化學能，使得大氣中的自由氧逐漸累積。他表示：「這為發生在大約23億年前的大氧化事件揭開了序幕。」
在接下來的10億年之間，由於氧氣濃度提高，多細胞生物也開始出現。其中包含了真核生物，它們的遺傳信息是儲存在一個或多個被膜包覆起來的細胞核當中。
而眾所關注的問題，同時也是Wanyi
Lu特別想知道的，便是全球海洋在什麼時候，透過什麼方式而含有夠多的氧氣可供各式各樣的海洋生物居住，其中有些種類時至今日依然存活著。
Lu表示：「大約在4億年前大氣中的氧氣含量大幅提高，我們從碘得到的數據也與此一致。」他的博士研究和低溫地球化學與全球環境變遷有關。「然而，海洋上層的氧含量直到2億年前才穩定下來至跟現在差不多的情況，此時大型真核浮游生物開始主宰全世界的海洋。這個時間點相當合理。」
若要解讀岩石紀錄中這類現象的意義，就必須要先了解生物地球化學和海洋的大尺度作用如何進行，以及大氣的化學組成。Zunli
Lu表示：「我們利用了十分先進的地球系統模型(Earth
System Model，ESM)來探討在海洋上層這兩種控制因子如何作用。此模型有個相當有趣的名字：GENIE(精靈)，其為『運用網格整合地球』(Grid-ENabled
Integrated Earth)的縮寫。」
加州大學河濱分校的地球科學教授Andy Ridgwell負責研發GENIE代表性的模擬架構，其可以在不同的時間尺度下進行各類ESM的模擬。他表示：「雪城大學的研究團隊運用新穎的方式來把他們對古代岩石的量測數據，和全球氣候系統以及碳循環的數值模型結合在一起。這令人印象十分深刻。」
Ridgwell讚賞團隊最終分析出來的主要結論：真核生物在基本層面上出現的變化，造成有機物再礦化的深度變深，最後讓海洋上層的氧濃度「具有可回復的韌性」。專長為研究生物地球化學模擬和長期氣候變遷的Ridgwell表示：「我們對於造就今日地球的關鍵演化步驟瞭解得越來越多，而這項結論也完美符合我們目前的理解。」
賓州州立大學地球與礦產科學學院的教務長Lee Kump表示，團隊的這項發現強烈提醒我們達爾文的演化論也許只對了一半。「環境變化理所當然地會影響到生物的演化，但是生物的演化革新也會影響到環境，規模甚至可以廣及全球。」這位知名的古氣候學家如此表示。
然而，故事並沒有在此畫下句點。牛津大學的地球化學教授Ros Rickaby表示這些發現也加強了氧化和海洋動物體型之間的關聯。「這些在微觀尺度下進行礦化作用的浮游生物在海洋當中取得進一步的成功，竟然會對整個地球系統產生深遠的作用，使得動物的平均體型增大。想想真是讓人感到不可思議。」她說，「這提醒了我們海洋生態系統當中，每個部份之間都有錯綜複雜的連結。」
Zunli Lu補充：「這是一個絕佳案例顯示生命和地球之間有共同演化的關係。」
Scientists rethink co-evolution of marine life, oxygenated
oceans
Researchers
in the Department of Earth Sciences at Syracuse University have confirmed that rising
oceanic and atmospheric oxygen levels co-evolved with marine life hundreds of
millions of years ago.
Wanyi
Lu, a Ph.D. candidate studying under Associate
Professor Zunli Lu (no relation) in
the College of Arts and Sciences, is the lead author of a groundbreaking paper
in Science magazine (American Association for the
Advancement of Sciences, 2018).
The paper stems from a multi-year,
multinational research effort led by Zunli Lu that rethinks the causes and
impacts of increased oxygenation on the continental shelves during the current
Phanerozoic Eon, which began more than 542 million years ago.
“Most studies of oxygen history
focus on the atmosphere and deep oceans, with implications on the evolution of
life,” Zunli Lu says. “We believe the oceanic oxygen level in the water column
above the continental shelves [i.e., the upper ocean] may have been a different
beast.”
Central to the team’s research was a
geochemical proxy that Lu pioneered in 2010. Using a novel approach based on
iodine geochemistry, he and his colleagues measured the ratio of iodine to
calcium in calcium carbonate minerals and fossils.
Timothy
Lyons, Distinguished Professor of
Biogeochemistry at the University of California, Riverside (UCR), considers
iodine geochemistry a “powerful tool” for constraining oxygen conditions in
surface-to-near-surface conditions of the ancient ocean. “These are the waters
in which the earliest animals first appeared, evolved and advanced toward
complex ecologies,” he says. “The results from this study reveal previously
unimagined environmental dynamics in those early waters, and those conditions
must have impacted animals.”
Lu takes the praise in stride, but
insists the group’s findings are novel. “The upper ocean became well-oxygenated
much later than originally thought,” he says.
The Syracuse geochemist illustrates
his point by describing a thick haze of methane that originally enveloped the
planet, leaving little to no oxygen in the atmosphere. Photosynthesizing
microbes eventually produced enough chemical energy, causing free oxygen to
accumulate in the atmosphere. “This set the stage for the Great Oxidation Event
about 2.3 billion years ago,” he says.
With oxygenation came the rise of
multicellular life forms over the next billion years. Among them were
eukaryotes, whose genetic information was stored within a membrane-bound
nucleus or nuclei.
The question on everyone’s mind,
notably Wanyi Lu’s, was how and when the global ocean became oxygenated enough
to accommodate diverse marine life forms, including those alive today.
“Our iodine data is consistent with
a major rise in the atmospheric oxygen level that occurred around 400 million
years ago,” says Lu, whose doctoral studies involve low-temperature geochemistry
and global environmental changes. “Nevertheless, upper-ocean oxygen levels did
not stabilize at near-modern conditions until 200 million years ago, when
larger eukaryotic plankton dominated the world’s oceans. The timing makes
perfect sense.”
To understand such observations in
the rock record, one must appreciate large-scale biogeochemical and
oceanographic processes, as well as atmospheric chemical composition. “We
examined the roles of these two controls in the upper ocean, using a sophisticated
Earth System Model [ESM] with an interesting name: GENIE, which is short for
‘Grid-ENabled Integrated Earth,'” Zunli Lu says.
Andy
Ridgwell, professor of Earth Sciences at UCR,
developed GENIE’s signature modeling framework, which composes a range of ESM
simulations over various timescales. “The innovative way that the Syracuse team
combined measurements of ancient rocks with a complex, mathematical model of
the global climate system and carbon cycle was impressive,” he says.
Ridgwell lauds the main conclusion
of the team’s final analysis—that a fundamental change in eukaryotes led to
greater re-mineralization depth of organic matter and, ultimately, a
“resiliently oxygenated” upper ocean. “This fits perfectly with our developing
understanding of the key evolutionary steps taken to create the planet we have
today,” says Ridgwell, who studies biogeochemical modeling and long-term
climate change.
Lee
Kump, dean of the College of Earth and
Mineral Sciences at Penn State, says the group’s findings are a potent reminder
of how Darwin’s theory of evolution may be only half-right. “Changes in the
environment affect biological evolution, to be sure, but biological innovation
can affect the environment, even at the global scale,” says the renowned
paleoclimatologist.
That is not the end of the story,
however. Ros Rickaby, professor of
geochemistry at the University of Oxford (U.K.), says the findings also
reinforce the link between oxygenation and marine animal body size. “It is
incredible to think that the increasing success of microscopic mineralising
plankton out in the ocean, through the change in oxygen distribution, could
have had such far-reaching effects across the Earth system to boost the average
body size of animals,” she says. “It reminds us of the intricate
interconnection between every part of the marine ecosystem.”
Adds Zunli Lu: “It is a prime
example of the co-evolution of life and the planet.”
原始論文：Wanyi Lu, Andy Ridgwell, Ellen Thomas, Dalton S.
Hardisty, Genming Luo, Thomas J. Algeo, Matthew R. Saltzman, Benjamin C. Gill,
Yanan Shen, Hong-Fei Ling, Cole T. Edwards, Michael T. Whalen, Xiaoli Zhou,
Kristina M. Gutchess, Li Jin, Rosalind E. M. Rickaby, Hugh C. Jenkyns, Timothy
W. Lyons, Timothy M. Lenton, Lee R. Kump, Zunli Lu. Late inception of a
resiliently oxygenated upper ocean. Science, 2018; eaar5372
DOI: 10.1126/science.aar5372
引用自：Syracuse University. "Scientists rethink co-evolution of
marine life, oxygenated oceans: Breathing new life into study of planet's
oxygen history."
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