氨讓地球早期的生命蓬勃發展
英國聖安德魯大學、美國雪城大學和英國倫敦大學皇家哈洛威學院組成的國際科學團隊，找出了另外一種地球早期生命的食物來源。
早期地球的想像圖。圖片來源：©
Pavel Parmenov / Adobe Stock
地球上的生命需要某些重要元素才能生存，比方說氮與磷。這兩種營養鹽元素在所有生物體內都相當豐富，它們是生命的藍圖與元件――DNA與蛋白質的組成材料。氮與磷的根本來源為大氣和岩石，因此在地質時間中，地表環境的化學性質出現重大變化的時候，生物可以取得多少氮和磷也會隨之波動。
發表在《自然―地球科學》(Nature
Geoscience)的研究，闡明了這些元素的供給量如何直接影響到地球大氣累積氧氣的過程，同時在地球早期生命的演化過程中也占據了關鍵地位。
產氧光合作用的出現造成了地球歷史上最劇烈的變化，從根本層面上改造了整個地球。產氧光合作用為生物圈帶來了新的碳源，也為大氣層提供了氧氣。後者在23億年前左右的「大氧化事件」達到高峰。
雖然氮和磷對生物來說至關重要，但科學家一直不太瞭解大氧化事件之前海中有多少氮和磷，尤其是這些元素的供給量和地球氧化之間的因果關係。
研究團隊採集了保存情況相當良好的岩石樣品，它們含有27億年前早期產氧光合作用留下的證據。分析這些岩石之後，研究團隊得以探討早期地球的氮循環，並加以瞭解氮循環和地球氧化的初始階段之間，有什麼樣的回饋作用。
雪城大學人文及科學學院的地球科學副教授Christopher
Junium說：「地球歷史上這段時期留下的岩石中，只有極為少數適合我們用來進行這項分析。經過27億年來板塊運動造成的變形和加熱之後，大部分年代如此久遠的岩石原來含有的生命跡象都已經消失無蹤了。」
岩石樣品含有的證據，首度指出大氧化之前的海洋，某個地方累積了大量的氨。這可以成為一座十分充足的氮源，使早期的生物圈蓬勃成長，連帶產生氧氣。
領導此研究團隊，聖安德魯大學地球與環境科學院的教授Aubrey
Zerkle表示：「現在我們談到氨，會想到清潔用品中刺鼻的臭味。但是對地球最初的產氧生物來說，高濃度的氨就像是吃到飽的自助餐，和更早以前的地球歷史中它們只能靠廚餘桶維生相比，無疑是有了重大改善。」
研究成果除了讓科學家更加瞭解氮循環在全球氧化過程中發揮的作用，也提供了其他營養鹽的回饋作用在地球早期演化過程中的大致變化。
Junium表示：「越來越多證據顯示在地球歷史當中，隨著生物演化和環境變化，營養鹽的取得難易度也跟著來回漲落。」
驚人的是大氣劇烈氧化的證據要等到4億年後才開始出現，代表像磷之類的其他營養鹽在決定演化的道路時，勢必也具有相當重要的影響。
Ammonium fertilized early life
on Earth
A team of international
scientists—including researchers at the University of St. Andrews, Syracuse
University and Royal Holloway, University of London—have demonstrated a new
source of food for early life on the planet.
Life on Earth relies on the availability of critical
elements such as nitrogen and phosphorus. These nutrient elements are
ubiquitous to all life, as they are required for the formation of DNA, the
blueprints of life, and proteins, the machinery. They are originally sourced
from rocks and the atmosphere, so their availability to life has fluctuated
alongside significant changes in the chemistry of Earth’s surface environments
over geologic time.
The research, published in Nature Geoscience, reveals how the supply of these elements
directly impacted the growth of Earth’s oxygen-rich atmosphere and were key to
the evolution of early life on Earth.
The most dramatic change in Earth history followed
the evolution of oxygenic photosynthesis, which fundamentally transformed the
planet by providing a source of carbon to the biosphere and a source of oxygen
to the atmosphere, the latter culminating in the Great Oxidation Event (GOE)
some 2.3 billion years ago.
Despite the critical importance of nutrients to life,
the availability of nitrogen and phosphorus in pre-GOE oceans is not well
understood, particularly how the supply of these elements drove and/or
responded to planetary oxygenation.
Using samples of exceptionally well-preserved rocks
that have been associated with early evidence for oxygenic photosynthesis 2.7
billion year ago, the team of researchers examined Earth’s early nitrogen cycle
to decipher feedbacks associated with the initial stages of planetary
oxygenation.
“There is precious little rock available from this
time interval that is suitable for the type of analyses we performed. Most
rocks that are this old have been deformed and heated during 2.7 billion years
of plate tectonic activity, rendering the original signals of life lost,” says
Christopher Junium, associate professor of Earth sciences in the College of
Arts and Sciences.
The rock samples showed the first direct evidence of
the build-up of a large pool of ammonium in the pre-GOE oceans. This ammonium
would have provided an ample source of nitrogen to fuel the early biosphere and
associated oxygen production.
Research team leader Aubrey Zerkle, reader in the
School of Earth and Environmental Sciences at the University of St Andrews,
says: “Today we think of ammonium as the unpleasant odor in our cleaning
supplies, but it would’ve served as an all-you-can-eat buffet for the first
oxygen-generating organisms, a significant improvement on the dumpster scraps
they relied on earlier in Earth’s history.”
As well as helping scientists better understand the
role of the nitrogen cycle in global oxygenation, the new findings also provide
context for other nutrient feedbacks during early planetary evolution.
“It is becoming ever more clear that the game of
nutrient limitation has tipped back and forth through Earth’s history as life
has evolved and as conditions have changed,” Junium says.
Surprisingly, evidence for significant atmospheric
oxygenation does not appear until 400 million years later, meaning that some
other nutrient, such as phosphorus, must have been important in setting the
evolutionary pace.
原始論文：J. Yang, C. K.
Junium, N. V. Grassineau, E. G. Nisbet, G. Izon, C. Mettam, A. Martin, A. L.
Zerkle. Ammonium availability in the Late Archaean nitrogen cycle. Nature
Geoscience, 2019; DOI: 10.1038/s41561-019-0371-1
引用自：Syracuse University. "Ammonium fertilized
early life on Earth."
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