原文網址：www.sciencedaily.com/releases/2017/04/170427091720.htm
科學家在北方湖泊的發現也許是瞭解地球早期生命的關鍵
加拿大滑鐵盧大學的科學家領導由微生物學家、地球化學家和淡水學家組成的團隊，驚訝地發現加拿大北方地盾(Boreal Shield)的許多湖泊也許跟太古代(Archea)的海洋十分相似。太古代距今超過25億年前，當時只有微生物繁榮生長在沒有氧氣的世界。
這項發現的重要之處在於跟太古代的海洋擁有同樣重要性質的湖泊，在加拿大的北方地盾中竟有數以百萬計。直到現在，科學家仰賴的「類比湖泊」(analogue lake)――擁有跟遠古環境類似的湖泊――只有四處，大多是在偏遠地區或者生態系相當脆弱的地點發現。
「有了那麼多的湖泊可供研究，此發現改變了我們研究這塊領域的方法。」共同作者，滑鐵盧大學理學院生命科學系的博士生Jackson Tsuji表示，「令人興奮的是這些湖泊基本上就位在我們的後院，它們遠有的資訊可以對過去與現今的全球氣候，以及水資源管理有所啟發。」
刊登在《科學報導》(Scientific Reports)的這項發現，有望改變科學家研究最初生命形式所用的方法。一般認為它們起源的無氧海洋含有少量的硫與大量的鐵。許多北方地盾的湖泊同樣含有少量的硫與大量的鐵，並且每個夏季都會形成無氧水層。雖然這些水層在春季和秋季會互相混和，但它們可以很快地重新建立起分層狀態。
「過往我們認為要找到太古代海洋的適當類比，意謂著要找到一座不會發生混和作用的湖泊。舉例而言，現今所用的類比對象往往深達數百公尺且處於完全分層的狀態。」生命科學系的Josh Neufeld教授表示，「我們的重要發現之一是儘管會發生混和作用，此處的無氧生物群集仍然相當強盛。」
研究人員可以把這些湖泊當作現生實驗室來研究過去的微生物可能是如何運作。科學家認為在作為樣本的湖泊中找到的微生物，是在陽光協助下代謝鐵化合物。由於在藻華形成過程中鐵扮演了關鍵腳色，因此它們或許能幫助研究人員瞭解該如何預測並控管有害的藻華。
此外，湖中特殊且從未發現的微生物群落，尤其是在湖底會消耗甲烷的微生物，對溫室氣體的排放來說也有深遠的啟示。
這篇研究在水化學、微生物群落的組成以及穩定同位素的層面上，將目前四座類比為太古代海洋的湖泊跟二座北方地盾的湖泊互相比對。研究人員以獨特的方式運用最新的生物與同位素工具，得出跟現今的類比湖泊中相似的生物作用，不僅研究當下存在於北方湖泊中，而是每年都會在湖水裡重新活躍地進行。
地球與環境科學系的教授Sherry Schiff表示：「因為有跨領域團隊帶來的可塑性讓我們能追尋意想不到的結果，加上最尖端的技術和工具才得以讓這個突破性的發現成真。」
北方地盾是加拿大面積最大的生態區，其幅員涵蓋了加拿大陸地的20%，有許多北方地盾湖泊散落於其中各處。類似的湖泊也可以在芬蘭、挪威、瑞典和俄羅斯發現。
Discovery
in northern lakes may be key to understanding early life on Earth
A team of researchers has
discovered that many Canadian lakes can provide new insights into ancient
oceans, and their findings could advance research about greenhouse gas
emissions, harmful algal blooms, and early life forms.
Scientists
from the University of Waterloo led the team of microbiologists, geochemists,
and freshwater specialists in a surprising finding that lakes of the Boreal
Shield may be similar to oceans of the Archean Eon, a period more than 2.5
billion years ago when microbial life thrived in a world without oxygen.
This
finding is important because there are millions of Boreal Shield lakes in
Canada that likely share key properties with the Archean oceans. Until now,
scientists have relied on only four so-called analogue lakes -- ones with
similar primordial conditions -- most of which are found in remote or
ecologically sensitive locations.
"With
so many lakes to study, this discovery changes how we approach this field of
research," said co-author Jackson Tsuji, a doctoral student in the
Department of Biology in the Faculty of Science. "It's exciting that these
lakes, which are basically in our backyard, hold information that could have
implications for global climate, past and present, and water management."
Published
in Scientific Reports,
the findings have the potential to transform how scientists carry out research
about Earth's earliest life forms, which originated in oxygen-free oceans
thought to be low in sulphur and high in iron. Many Boreal Shield lakes, also
low in sulphur and high in iron, develop oxygen-free layers each summer.
Although these layers mix in the spring and fall, they re-establish quickly.
"We
used to think finding a suitable Archean ocean analogue meant that you had to
find a lake that didn't mix. For example, current analogues are hundreds of
metres deep and completely stratified," said Josh Neufeld, a professor in
the Department of Biology. "An important discovery here was how robust
this oxygen-free community is, despite the mixing."
Researchers
can use these lakes as living laboratories to study how microbes of the past
might have functioned. The microbes detected in the sampled lakes are thought
to metabolize iron compounds with the help of sunlight, which may help
researchers understand how to predict and control harmful algal blooms because
iron plays a key role in algal bloom formation.
In
addition, the unique and previously unknown microbial communities, specifically
methane-consuming microbes at the bottom of these lakes, have broad
implications for greenhouse gas emissions.
The
study compared aspects of the four current Archean-ocean analogues to two
Boreal Shield lakes using water chemistry, microbial community profiles, and
stable isotope patterns. The researchers' unique application of the latest
biological and isotopic tools shows that similar biological processes to
existing analogues are not only present, but active in the water, reoccurring
every year.
"This
groundbreaking discovery was possible because we had the flexibility to pursue
some unexpected results with a multi-disciplinary team using state-of-the-art
tools and techniques," said Sherry Schiff, a professor in the Department
of Earth and Environmental Sciences.
Boreal
Shield lakes are widespread across the Boreal Shield, the largest of the Canadian
ecozones, which extends across more than 20 per cent of Canada's land mass.
Similar lakes are found in Finland, Norway, Sweden, and Russia.
原始論文：S. L. Schiff, J. M. Tsuji, L. Wu, J. J. Venkiteswaran, L.
A. Molot, R. J. Elgood, M. J. Paterson, J. D. Neufeld. Millions of
Boreal Shield Lakes can be used to Probe Archaean Ocean Biogeochemistry. Scientific
Reports, 2017; 7: 46708 DOI: 10.1038/srep46708
引用自：University of Waterloo. "Discovery in northern lakes may
be key to understanding early life on Earth." ScienceDaily. ScienceDaily,
27 April 2017.
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