原文網址：https://news.umich.edu/lake-huron-sinkhole-surprise-the-rise-of-oxygen-on-early-earth-linked-to-changing-planetary-rotation-rate/
發生在地球歷史早期的氧氣含量升高為日後動物驚人的多樣性打下了基礎。但是這道歷時將近二十億年、逐步發展的過程是受到哪些因素控制，數十年來科學家還是難以解釋。
休倫湖中島滲穴的紫色微生物蓆，攝於2019年六月。當微生物蓆下方產生的氫氣與硫化氫等氣體，變成氣泡往上升的時候便會在微生物蓆上形成小丘陵與類似這張照片中的「指頭」。圖片來源：Phil Hartmeyer，NOAA雷霆灣國家海洋保護區
最近一組國際研究團隊提出地球年輕時的轉動速率隨著時間逐漸變慢，使得一天變得更長，或許促進了行光合作用的藍綠菌釋放出更多氧氣，因而決定了地球氧化的時程。
他們的這項結論啟發自研究現代微生物群落的成果。在休倫湖水面下方80英尺的中島滲穴擁有相當極端的環境，此處湖水氧含量很低且有大量的硫，不過某些顏色明亮的細菌卻能在此繁榮生長。科學家認為它們是很好的類比，可以用來探討數十億年前覆蓋在陸地與海床表面，像是毯子一般的單細胞生物群落。
研究人員證實日長度增加會讓行光合作用的微生物蓆產生更多氧氣。他們根據這項發現繼而導出地球氧氣含量的歷史與自轉速率之間，有著從來沒人想到的關聯。雖然地球現在每24小時就會自轉一圈，但在地球幼年的時候一天的長度可能短到只有6個小時。
研究團隊的發現8月2日發表在期刊《自然―地球科學》(Nature Geoscience)的線上版。
論文主要作者為馬克斯．普朗克海洋微生物學研究所的Judith
Klatt以及萊布尼茲熱帶海洋研究中心的Arjun
Chennu。Klatt之前曾在密西根大學的地質微生物學家Gregory
Dick的實驗室擔任博士後研究員，他也是這篇研究的二位通訊作者之一。其他共同作者則來自密西根大學以及大峽谷州立大學。
「地球科學當中一個存在已久的問題是：地球的大氣如何得到氧氣，而這道氧化過程的發生時間又是受控於那些因素，」正在研究船「風暴號」上的Dick表示。這艘50英尺長、隸屬美國國家海洋暨大氣總署(NOAA)的研究船載有一組科學家以及潛水員，他們從密西根州的阿爾皮納出發，前往離岸數英里遠的中島滲穴進行採樣。
「我們的研究提出地球的自轉速率，也就是日長度，對於地球氧化過程的模式以及時程可能具有重大影響，」密西根大學地球與環境科學系的教授Dick表示。
研究人員模擬了地球自轉速率逐漸變慢的過程，結果顯示日長度增加會讓早期的藍綠菌蓆釋放出更多氧氣，這種關係有助於解釋地球過去兩次大氧化事件的發生原因。
此計畫的契機為共同作者，密西根大學地球與環境科學系的海洋物理學家Brian
Arbic在一場公開演講中聽聞了Klatt的成果。Dick的研究室當時正在建構光合作用的歷史，而Arbic注意到從地質時間來看，日長度的變化或許有所影響。
藍綠菌現在這段日子的名聲並不佳，因為侵擾伊利湖以及世界各地的水體之中有毒且破壞景觀的藻華，主要的嫌犯便是藍綠菌。
不過這些微生物(以前被稱為藍綠藻)其實已經存在了數億年之久，也是第一批發明光合作用從陽光中獲取能量的生物。它們藉此來製造有機化合物，並且釋放出一種副產物――氧氣。
科學家認為原始海洋裡這些數量繁多的簡單生物是把氧氣釋放出來的功臣，使得多細胞動物接下來得以出現。這讓地球從一顆幾乎找不到氧氣，漸漸轉變成現今大氣氧含量約有21%的星球。
在休倫湖的中島滲穴有一種可以產生氧氣的紫色藍綠菌，與之競爭的是另一種不用陽光，而是以硫為主要能量來源的白色硫氧化菌。
中島滲穴底部日復一日上演著微生物之舞――這些紫色和白色的細菌形成的薄毯，隨著一天當中的時間推移與環境條件的逐漸變化，不斷爭奪著對方的領土。攝取硫的白色細菌會在早晨與晚上覆蓋住紫色的藍綠菌，阻擋它們獲得陽光而無法進行產生氧氣的光合作用。
但是陽光強度增加到一個特定門檻之後，這些硫氧化菌就會退縮到行光合作用的藍綠菌下方，使它們可以開始產生氧氣。
雖然之前已經有科學家觀察到硫氧化菌的垂直遷移現象，但是這篇發表在《自然―地球科學》的研究，是首次將微生物的移動以及對產氧速率的影響，跟地球歷史上不斷改變的日長度給連結起來。
Klatt表示：「中島滲穴的兩種微生物蓆彼此競爭著最上方的位置，有時候硫氧化菌會遮住可以進行光合作用的藍綠菌。」此時她正在阿爾皮納的實驗室，處理從中島滲穴的微生物蓆鑽出來的岩芯樣品，「微生物之間的這類競爭關係，也許耽擱了早期地球的氧氣產生進度。」
在理解這項日長度變化為什麼與地球氧化有關的假設時，關鍵是一天的時間越長，中午日照強烈的時數也會增加，使得行光合作用的藍綠菌產生更多氧氣。
「我們的想法是一天長度較短，也就是中午日照強烈時數較少的時候，那些攝取硫的白色細菌一天大部分的時間都會蓋住行光合作用的細菌，使得氧氣產量減少，」Dick表示。此時船隻正停在距中島數百碼、波濤洶湧的湖面上。
一條江鱈停在休倫湖中島滲穴的石頭上，這些石頭表面覆滿了紫色與白色的微生物蓆。圖片來源：Phil
Hartmeyer，NOAA雷霆灣國家海洋保護區
研究人員相信現今休倫湖裡的微生物可以做為古代生物的良好類比，一部份的原因是中島滲穴底部的極端環境很可能類似早期地球淺海常見的嚴苛條件。
休倫湖下方的底岩是年代為四億年的石灰岩、白雲岩與石膏，它們形成於曾經覆蓋這片大陸的含鹽海水。地下水的流動經年累月下來溶解了一部份的底岩而形成洞穴或裂縫，崩塌之後便產生了阿爾皮納附近位在陸上與水底的滲穴。
現今半徑為300英尺的中島滲穴底部，冰冷、缺氧、富含硫的地下水持續地滲透進來，驅離了大部分的動物與植物，卻也創造出對某些特化的微生物來說相當理想的家園。
大峽谷州立大學水資源研究所的Bopi
Biddanda是共同作者之一，多年來他和
Dick的團隊合作，利用各種不同的技術來研究中島滲穴底部的微生物蓆。在NOAA雷霆灣國家海洋保護區――雖然這裡最知名的為沉船殘骸，但中島滲穴和其他幾個類似的滲穴也是坐落於此――藉由潛水員的幫忙，研究人員得以在湖床設置儀器來研究此處的化學性質與生物。
一名潛水員正在觀察休倫湖中島滲穴的石頭，表面覆滿了紫色、白色與綠色的微生物。圖片來源：Phil
Hartmeyer，NOAA雷霆灣國家海洋保護區
他們也把微生物蓆的樣品帶回實驗室，在條件可控的環境下進行實驗。
根據氧氣傳輸的物理定律，Klatt推測日長度和氧氣釋放量的關係可以推及至任何一種微生物蓆生態系。她聯合Chennu進行了詳細的模擬，探討在地質時間尺度下，微生物蓆的作用和地球整體的變化模式是否有關。
模擬研究顯示日長度的確可以影響微生物蓆釋放出的氧氣多寡。
「簡單來說，一天長度越短，氧氣可以從微生物蓆離開的時間就越少，」Klatt表示。
這讓研究人員推論一天長度變長可能和大氣氧含量提升有關。模型顯示他們提出的機制或許有助於解釋地球氧化過程分段進行的特殊模式，以及地球歷史中為什麼大部分的時間氧氣都很少。
綜觀地球大部分的歷史，空氣裡的氧氣都非常稀少。科學家相信氧氣上升過程中有兩個大幅躍進的階段：首先是發生在24億年前左右的大氧化事件，一般歸功於最初能行光合作用的藍綠菌；然後過了約莫20億年之後氧氣才第二次劇烈增加，又稱作新基生代氧化事件。
地球從46億年前形成之後，就一直受到月球重力造成的潮汐摩擦(tidal
friction)拖拉，使得自轉速率一直在緩緩變慢。
研究經費來自美國國家科學基金、馬克斯．普朗克學會、密西根大學的透納獎學金。野外作業則由NOAA的大湖環境研究實驗室以及雷霆灣國家海洋保護區支持。
Lake Huron sinkhole surprise: The rise
of oxygen on early Earth linked to changing planetary rotation rate
The rise of oxygen levels early in
Earth’s history paved the way for the spectacular diversity of animal life. But
for decades, scientists have struggled to explain the factors that controlled
this gradual and stepwise process, which unfolded over nearly 2 billion years.
Now an international research team is proposing that
increasing day length on the early Earth—the spinning of the young planet
gradually slowed over time, making the days longer—may have boosted the amount
of oxygen released by photosynthetic cyanobacteria, thereby shaping the timing
of Earth’s oxygenation.
Their conclusion was inspired by a study of
present-day microbial communities growing under extreme conditions at the
bottom of a submerged Lake Huron sinkhole, 80 feet below the water’s surface.
The water in the Middle Island Sinkhole is rich in sulfur and low in oxygen,
and the brightly colored bacteria that thrive there are considered good analogs
for the single-celled organisms that formed mat-like colonies billions of years
ago, carpeting both land and seafloor surfaces.
The researchers show that longer day length increases
the amount of oxygen released by photosynthetic microbial mats. That finding,
in turn, points to a previously unconsidered link between Earth’s oxygenation
history and its rotation rate. While the Earth now spins on its axis once every
24 hours, day length was possibly as brief as 6 hours during the planet’s
infancy.
The team’s findings were published online Aug. 2 in
the journal Nature Geoscience.
Lead authors are Judith Klatt of the Max Planck
Institute for Marine Microbiology and Arjun Chennu of the Leibniz Centre for
Tropical Marine Research. Klatt is a former postdoctoral researcher in the lab
of University of Michigan geomicrobiologist Gregory Dick, who is one of the
study’s two corresponding authors. The other co-authors are from U-M and Grand
Valley State University.
“An enduring question in the Earth sciences has been
how did Earth’s atmosphere get its oxygen, and what factors controlled when
this oxygenation took place,” Dick said from the deck of the R/V Storm, a
50-foot NOAA research vessel that carried a team of scientists and scuba divers
on a sample-collection trip from the town of Alpena, Michigan, to the Middle
Island Sinkhole, several miles offshore.
“Our research suggests that the rate at which the
Earth is spinning—in other words, its day length—may have had an important
effect on the pattern and timing of Earth’s oxygenation,” said Dick, a professor
in the U-M Department of Earth and Environmental Sciences.
The researchers simulated the gradual slowing of
Earth’s rotation rate and showed that longer days would have boosted the amount
of oxygen released by early cyanobacterial mats in a manner that helps explain
the planet’s two great oxygenation events.
The project began when co-author Brian Arbic, a
physical oceanographer in the U-M Department of Earth and Environmental
Sciences, heard a public lecture about Klatt’s work and noted that day length
changes could play a role, over geological time, in the photosynthesis story
that Dick’s lab was developing.
Cyanobacteria get a bad rap these days because they
are the main culprits behind the unsightly and toxic algal blooms that plague Lake
Erie and other water bodies around the world.
But these microbes, formerly known as blue-green
algae, have been around for billions of years and were the first organisms to
figure out how to capture energy from sunlight and use it to produce organic
compounds through photosynthesis—releasing oxygen as a byproduct.
Masses of these simple organisms living in primeval
seas are credited with releasing oxygen that later allowed for the emergence of
multicellular animals. The planet was slowly transformed from one with
vanishingly small amounts of oxygen to present-day atmospheric levels of around
21%.
At the Middle Island Sinkhole in Lake Huron, purple
oxygen-producing cyanobacteria compete with white sulfur-oxidizing bacteria
that use sulfur, not sunlight, as their main energy source.
In a microbial dance repeated daily at the bottom of
the Middle Island Sinkhole, filmy sheets of purple and white microbes jockey
for position as the day progresses and as environmental conditions slowly
shift. The white sulfur-eating bacteria physically cover the purple
cyanobacteria in the morning and evening, blocking their access to sunlight and
preventing them from carrying out oxygen-producing photosynthesis.
But when sunlight levels increase to a critical
threshold, the sulfur-oxidizing bacteria migrate back down below the
photosynthetic cyanobacteria, enabling them to start producing oxygen.
The vertical migration of sulfur-oxidizing bacteria
has been observed before. What’s new is that the authors of the Nature Geoscience study are the first to
link these microbial movements, and the resultant rates of oxygen production,
to changing day length throughout Earth’s history.
“Two groups of microbes in the Middle Island Sinkhole
mats compete for the uppermost position, with sulfur-oxidizing bacteria
sometimes shading the photosynthetically active cyanobacteria,” Klatt said
while processing a core sample from Middle Island Sinkhole microbial mats in an
Alpena laboratory. “It’s possible that a similar type of competition between
microbes contributed to the delay in oxygen production on the early Earth.”
A key to understanding the proposed link between
changing day length and Earth’s oxygenation is that longer days extend the
afternoon high-light period, allowing photosynthetic cyanobacteria to crank out
more oxygen.
“The idea is that with a shorter day length and
shorter window for high-light conditions in the afternoon, those white
sulfur-eating bacteria would be on top of the photosynthetic bacteria for
larger portions of the day, limiting oxygen production,” Dick said as the boat
rocked on choppy waters, moored a couple hundred yards from Middle Island.
The present-day Lake Huron microbes are believed to
be good analogs for ancient organisms in part because the extreme environment
at the bottom of the Middle Island Sinkhole likely resembles the harsh
conditions that prevailed in the shallow seas of early Earth.
Lake Huron is underlain by 400-million-year-old
limestone, dolomite and gypsum bedrock that formed from the saltwater seas that
once covered the continent. Over time, the movement of groundwater dissolved
some of that bedrock, forming caves and cracks that later collapsed to create
both on-land and submerged sinkholes near Alpena.
Cold, oxygen-poor, sulfur-rich groundwater seeps into
the bottom of the 300-foot-diameter Middle Island Sinkhole today, driving away
most plants and animals but creating an ideal home for certain specialized
microbes.
Dick’s team, in collaboration with co-author Bopi
Biddanda of the Annis Water Resources Institute at Grand Valley State
University, has been studying the microbial mats on the floor of Middle Island
Sinkhole for several years, using a variety of techniques. With the help of
scuba divers from NOAA’s Thunder Bay National Marine Sanctuary—which is best
known for its shipwrecks but is also home to the Middle Island Sinkhole and
several others like it—the researchers deployed instruments to the lake floor
to study the chemistry and biology there.
They also brought mat samples to the lab to conduct
experiments under controlled conditions.
Klatt hypothesized that the link between day length
and oxygen release can be generalized to any given mat ecosystem, based on the
physics of oxygen transport. She teamed up with Chennu to conduct detailed
modeling studies to relate microbial mat processes to Earth-scale patterns over
geological timescales.
The modeling studies revealed that day length does,
in fact, shape oxygen release from the mats.
“Simply speaking, there is just less time for the
oxygen to leave the mat in shorter days,” Klatt said.
This led the researchers to posit a possible link
between longer day lengths and increasing atmospheric oxygen levels. The models
show that this proposed mechanism might help explain the distinctive stepwise
pattern of Earth’s oxygenation, as well as the persistence of low-oxygen
periods through most of the planet’s history.
Throughout most of Earth’s history, atmospheric
oxygen was only sparsely available and is believed to have increased in two
broad steps. The Great Oxidation Event occurred about 2.4 billion years ago and
has generally been credited to the earliest photosynthesizing cyanobacteria.
Nearly 2 billion years later a second surge in oxygen, known as the Neoproterozoic
Oxygenation Event, occurred.
Earth’s rotation rate has been slowly decreasing
since the planet formed about 4.6 billion years ago due to the relentless tug
of the moon’s gravity, which creates tidal friction.
The study was funded by grants from the National
Science Foundation, the Max Planck Society and the University of Michigan
Turner Fellowship. Field operations were supported by the NOAA Great Lakes
Environmental Research Laboratory and NOAA’s Thunder Bay National Marine
Sanctuary.
原始論文：J. M. Klatt,
A. Chennu, B. K. Arbic, B. A. Biddanda & G. J. Dick. Possible link
between Earth's rotation rate and oxygenation. Nature Geoscience,
2021 DOI: 10.1038/s41561-021-00784-3
引用自：University of Michigan. "Lake Huron
sinkhole surprise: The rise of oxygen on early Earth linked to changing
planetary rotation rate."
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