深海熱泉擁有生命誕生的理想條件
倫敦大學學院的研究團隊在高溫的鹼性海水中創造出原始細胞，這項新的證據支持相較於淺水池塘，生命更有可能起源自深海熱泉。
馬里亞納群島附近的深海熱泉。圖片來源：NOAA
Ocean Exploration & Research
在以細胞為基礎的生命發展過程中，出現原始細胞被視為關鍵的前置步驟。雖然之前的實驗無法在深海熱泉環境中培養出原始細胞，不過這篇發表在《自然―生態學與演化》(Nature
Ecology & Evolution)的新研究，卻發現高溫與鹼性環境不只可以讓生命誕生，更有可能是必要條件。
倫敦大學學院基因、演化與環境研究部門的教授Nick
Lane是研究主要作者，他說：「關於生命起源的場所與方式有幾種互相競爭的理論，其中最有可能孕育出生命的地點之一便是海底熱泉。而我們的發現讓這項理論有可靠的實驗證據作為支持。」
海底熱泉位於地球的海洋深處。此處的海水進入地殼之後跟礦物接觸而發生化學反應，形成了含有氫氣的高溫鹼性環境。這些過程會產生含有大量礦物的煙囪，它們噴出的酸性或鹼性液體可以提供能量，使得氫氣和二氧化碳更容易發生化學反應，進而形成越來越複雜的有機物。
倫敦大學學院領導的團隊曾經發現某些世上最古老的化石，它們就形成在類似的海底熱泉之中。
研究生命起源的科學家之前發現的原始化學反應，重現了基本的細胞可能如何形成，這是一項十分重大的進展。原始細胞僅有一副雙層膜來包覆其中的水溶液，也就是說它們已經有明確的邊界來包覆內容物，因此可以視為細胞最基本的形式，所以創造出原始細胞是相當重要的一步。
之前從自然界的簡單分子(精確來說為脂肪酸)創造出原始細胞的實驗是在低溫的淡水中進行，但只能在相當嚴格控制的條件下才能成功；不過在海底熱泉環境中進行的原始細胞實驗，卻是以分崩離析坐收。
任職於倫敦大學學院基因、演化與環境研究部門的Sean
Jordan博士是研究第一作者，他說他和同事發現之前的研究有所缺陷：「其他實驗裡用到的分子類型只有幾種而已，大部分都是大小類似的脂肪酸。但是在自然環境中應該會看到各式各樣的分子才對。」
在這項研究中，團隊以之前沒人用過的方式混和不同類型的脂肪酸與脂肪醇，試著從中創造出原始細胞。
研究人員發現碳鍊較長的分子需要經過加熱才能自行組裝成囊泡(原始細胞)，同時鹼性溶液可以幫助新生的囊泡維持它們的電荷。此外，他們也證明處在鹹水環境有所助益，因為在含鹽溶液中脂肪分子會形成更加緊密的鍵結，形成的囊泡也就更加穩定。
這是研究人員首次在類似深海熱泉的環境中，成功創造出自行組裝的原始細胞。他們發現高溫、鹼性和鹽分並不會阻礙原始細胞形成，反而還有所助益。
Jordan博士說：「我們的實驗成功創造出生命的基本要素之一，而且實驗條件比起其他實驗室試驗來說，更能反映出古代地球的環境。」
「雖然我們仍然不曉得第一個生命誕生自何處，但我們的研究顯示不能否決深海熱泉這個可能。」
研究人員也強調並非只有地球擁有深海熱泉。
Lane教授表示：「太空任務得到的證據指出木星和土星的冰封衛星上，海中可能具有類似的鹼性海底熱泉。雖然我們還沒有找到任何證據支持這些衛星上頭有生命存在，但如果我們想要在其他行星或衛星上面找到生命，像我們這樣的研究可以幫助人們決定應該從何找起。」
參與這項研究的人員來自倫敦大學學院和倫敦大學柏貝克學院。經費來自於英國生物技術暨生物科學研究委員會和bgC3。
Deep sea vents had ideal conditions
for origin of life
By creating protocells in hot, alkaline
seawater, a UCL-led research team has added to evidence that the origin of life
could have been in deep-sea hydrothermal vents rather than shallow pools.
Previous experiments had failed to foster the
formation of protocells – seen as a key stepping stone to the development of
cell-based life – in such environments, but the new study, published in Nature Ecology & Evolution, finds
that heat and alkalinity might not just be acceptable, but necessary to get
life started.
“There are multiple competing theories as to where
and how life started. Underwater hydrothermal vents are among most promising
locations for life’s beginnings – our findings now add weight to that theory
with solid experimental evidence,” said the study’s lead author, Professor Nick
Lane (UCL Genetics, Evolution & Environment).
Deep under the Earth’s seas, there are vents where
seawater comes into contact with minerals from the planet’s crust, reacting to
create a warm, alkaline (high on the pH scale) environment containing hydrogen.
The process creates mineral-rich chimneys with alkaline and acidic fluids,
providing a source of energy that facilitates chemical reactions between
hydrogen and carbon dioxide to form increasingly complex organic compounds.
Some of the world’s oldest fossils, discovered by a
UCL-led team, originated in such underwater vents.
Scientists researching the origins of life have made
great progress with experiments to recreate the early chemical processes in
which basic cell formations would have developed. The creation of protocells
has been an important step, as they can be seen as the most basic form of a
cell, consisting of just a bilayer membrane around an aqueous solution – a cell
with a defined boundary and inner compartment.
Previous experiments to create protocells from
naturally-occurring simple molecules – specifically, fatty acids – have
succeeded in cool, fresh water, but only under very tightly controlled
conditions, whereas the protocells have fallen apart in experiments in hydrothermal
vent environments.
The study’s first author, Dr (UCL Genetics, Evolution
& Environment), said he and his colleagues identified a flaw in the
previous work: “Other experiments had all used a small number of molecule
types, mostly with fatty acids of the same size, whereas in natural
environments, you would expect to see a wider array of molecules.”
For the current study, the research team tried
creating protocells with a mixture of different fatty acids and fatty alcohols
that had not previously been used.
The researchers found that molecules with longer
carbon chains needed heat in order to form themselves into a vesicle
(protocell). An alkaline solution helped the fledgling vesicles keep their
electric charge. A saltwater environment also proved helpful, as the fat
molecules banded together more tightly in a salty fluid, forming more stable
vesicles.
For the first time, the researchers succeeded at
creating self-assembling protocells in an environment similar to that of
hydrothermal vents. They found that the heat, alkalinity and salt did not
impede the protocell formation, but actively favoured it.
“In our experiments, we have created one of the
essential components of life under conditions that are more reflective of
ancient environments than many other laboratory studies,” Dr Jordan said.
“We still don’t know where life first formed, but our
study shows that you cannot rule out the possibility of deep-sea hydrothermal
vents.”
The researchers also point out that deep-sea
hydrothermal vents are not unique to Earth.
Professor Lane said: “Space missions have found
evidence that icy moons of Jupiter and Saturn might also have similarly
alkaline hydrothermal vents in their seas. While we have never seen any
evidence of life on those moons, if we want to find life on other planets or
moons, studies like ours can help us decide where to look.”
The study involved researchers from UCL and Birkbeck,
University of London, and was funded by the BBSRC and bgC3.
原始論文：Sean F. Jordan, Hanadi Rammu, Ivan N. Zheludev,
Andrew M. Hartley, Amandine Maréchal, Nick Lane. Promotion of protocell
self-assembly from mixed amphiphiles at the origin of life. Nature Ecology & Evolution,
2019; DOI: 10.1038/s41559-019-1015-y
引用自：University College London. "Deep sea vents
had ideal conditions for origin of life."
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