微生物或許看管了地球深部的碳質
一項發表在《自然》(Nature)的研究有了重大突破。研究結果顯示微生物可以消耗地球深處的碳――重要的是，這有助於封存一小部分沉降到地球深處的碳。這項發現可以讓我們更加瞭解地球運作的基本原理，並闡明自然界的哪些作用或許可以減緩氣候變遷。
在哥斯大黎加一座瀑布沉積出來的方解石。圖片來源：Peter
Barry
隱沒帶連結了地球表面與地球內部。當兩個板塊相撞，密度較高的一方下沉的時候，也會把地表的物質帶到地球內部。研究人員發現碳和其他元素被封存在地殼的過程中，地表附近的微生物發揮了重要的作用，這讓我們對地球的運作過程有了新的重大理解，也有助於研究人員模擬地球內部的演變過程。
劍橋大學地球科學系的主任Chris
Ballentine教授是研究共同作者，他說：「化學物質回到地球內部的過程中，大型隱沒帶具有十分重要的地位。我們的研究結果指出在大型隱沒帶，生物正把碳給封存起來。從地質時間尺度來看，生物或許控制了地球表面化學物質的分布，並且可以把碳這類的元素儲存在地殼當中。」
這是首度有證據表明地下生物在把碳從隱沒帶移除的過程中具有重要地位。之前科學家已經確認微生物可以攝取溶解在水中的碳，並轉化成岩石裡的礦物。這種自然作用可以封存二氧化碳，因此控制了地球表面有多少碳。而這項研究指出有條隱沒帶中正在大規模發生此作用。
主要作者Peter
Barry博士在牛津大學地球科學系時進行了這項研究，他說：「我們發現這條隱沒帶上的許多碳並未經過火山返回大氣或是進入地球內部，而是被封鎖在沒有火山活動的地區。」
「在此之前，科學家都假設海洋裡的碳在進入地函的過程中，生物並未參與其中或是無關緊要。但我們發現生物和化學作用聯合起來，成為碳在進入地函路途上的守門員。」
由25名科學家組成的跨領域研究團隊，在12天的實地考察過程中，於哥斯大黎加各處採集了溫泉水的樣本。科學家長久以來猜測這些溫泉水中冒出的含碳分子十分古老，是在距今數百萬年前隱沒下去的。他們比較了兩種不同的碳(碳同位素)的含量，證實了這項預測，也代表在隱沒帶上方的地殼內部，正在進行某種之前未知的作用，使得大量的碳被封鎖在這個地方。
科學家進行分析之後，估計約有94%的碳被轉換成方解石和微生物的生物量。
美國田納西大學諾克斯維爾分校的微生物學副教授Karen
Lloyd,是研究資深作者，她說：「這些微生物進行的作用就是所謂的『碳封存』。許多科學家正努力研究碳封存以減緩氣候變遷，或是研發碳捕集來把溫室氣體長期埋在地底很長一段時間。我們的研究是個絕佳範例，顯示這類作用正在自然界發生，而且是在從來沒有人發現的地方。研究也顯示此處碳封存的規模相當大，足以成為一種碳的源區(reservoir)。」
哥斯大黎加國立大學火山學與沉積學觀測站的教授Maarten
de Moor是研究共同作者，他說：「如此微小的微生物對地質作用的影響力，竟然可以跟直通地球內部、威力十分強大且極為壯觀的火山相比，實在是相當驚人。我們在研究中證明出來的作用雖然不太顯眼，但範圍相對於火山來說更加廣大，因此影響也不容小覷。」
研究人員接下來的計畫是探討其他隱沒帶，觀察此現象是否廣泛分布。如果這些生物和地球化學作用在世界各地都會發生，代表進入地函深處的碳比之前預估的還要少19%。
研究共同作者Donato
Giovannelli是義大利那不勒斯腓特烈二世大學的助理教授，也是美國羅格斯大學和義大利CNR-IRBIM的合作科學家，他說：「生物或許還能透過更多方式來對地質造成巨大的影響，只是我們還沒發現到而已。」
Peter Barry博士現為伍茲霍爾海洋研究所的副研究員，他補充說：「我們的研究團隊集結了三個不同領域的科學家齊力合作，唯有透過這種跨領域的研究方法才能得到如此重大的突破。未來更加深入的研究勢必能改變人類對這些系統的看法，這實在是令我感到相當興奮。」
Microbes may act as gatekeepers of
Earth's deep carbon
This groundbreaking study, published in Nature, shows that microbes consume and
- crucially - help trap a small amount of sinking carbon in this zone. This
finding has important implications for understanding Earth's fundamental
processes and for revealing how nature can potentially help mitigate climate
change.
At a subduction zone there is communication between
Earth's surface and interior. Two plates collide and the denser plate sinks,
transporting material from the surface into Earth's interior. Showing that the
microbes at the near-surface are playing a fundamental role in how carbon and
other elements are being locked up into the crust provides a profound new
understanding of Earth processes and helps researchers model how Earth's
interior may develop over time.
Co-author, Professor Chris Ballentine, Head of the
Department of Earth Sciences at the University of Oxford, said: 'What we've
shown in this study is that in areas that are critically important for putting
chemicals back down into the planet - these big subduction zones - life is
sequestering carbon. On geological timescales life might be controlling the
chemicals at the surface and storing elements like carbon in the crust.'
This is the first evidence that subterranean life
plays a role in removing carbon from subduction zones. It has been well
established that microbes are capable of taking carbon dissolved in water and
converting it into a mineral within the rocks. This study demonstrates that the
process is happening on a large scale across a subduction zone. It is a natural
CO2 sequestration process which can control the availability of
carbon on Earth's surface.
Lead author, Dr Peter Barry, who carried out the
research while at the Department of Earth Sciences, Oxford University, said:
'We found that a substantial amount of carbon is being trapped in non-volcanic
areas instead of escaping through volcanoes or sinking into Earth's interior.
'Until this point scientists had assumed that life
plays little to no role in whether this oceanic carbon is transported all the
way into the mantle, but we found that life and chemical processes work
together to be the gatekeepers of carbon delivery to the mantle.'
During the 12-day expedition, the 25-person group of
multi-disciplinary scientists collected water samples from thermal springs
throughout Costa Rica. Scientists have long predicted that these thermal waters
spit out ancient carbon molecules, subducted millions of years before. By
comparing the relative amounts of two different kinds of carbon - called
isotopes - the scientists showed that the predictions were true and that
previously unrecognized processes were at work in the crust above the
subduction zone, acting to trap large amounts of carbon.
Following their analyses, the scientists estimated
that about 94 percent of that carbon transforms into calcite minerals and
microbial biomass.
Senior author, Karen Lloyd, Associate Professor of Microbiology
at the University of Tennessee, Knoxville, said: 'These microbes are literally
sequestering carbon. Scientists are actively working on carbon sequestration to
mitigate climate change and carbon capture and storage as a means to bury
greenhouse gases over long time periods. Our study is a really good example of
where this is happening naturally, and it was previously unrecognised. This
study shows that this happens on a big, reservoir scale.'
Maarten de Moor, co-author and professor at the
National University of Costa Rica's Observatory of Volcanology and Seismology,
said: 'It is amazing to consider that tiny microbes can potentially influence
geological processes on similar scales as these powerful and visually
impressive volcanoes, which are direct conduits to Earth's interior. The
processes that we have identified in this study are less obvious, but they are
important because they are operating over huge spatial areas in comparison to
volcanoes.'
The researchers now plan to investigate other
subduction zones to see if this trend is widespread. If these biological and
geochemical processes occur worldwide, they would translate to 19 percent less
carbon entering the deep mantle than previously estimated.
Co-author Donato Giovannelli, Assistant Professor at
the University of Naples Federico II and affiliated scientist at the CNR-IRBIM
and Rutgers University, said: 'There are likely even more ways that biology has
had an outsized impact on geology, we just haven't discovered them yet.'
Dr Peter Barry, now an Associate Scientist at Woods
Hole Oceanographic Institution, added: 'We have people from three different
fields working together and only with such an interdisciplinary approach can
you make such breakthroughs. Moving forward, this will change how people look
at these systems. For me that is thrilling.'
原始論文：P. H. Barry,
J. M. de Moor, D. Giovannelli, M. Schrenk, D. R. Hummer, T. Lopez, C. A. Pratt,
Y. Alpízar Segura, A. Battaglia, P. Beaudry, G. Bini, M. Cascante, G. d’Errico,
M. di Carlo, D. Fattorini, K. Fullerton, E. Gazel, G. González, S. A.
Halldórsson, K. Iacovino, J. T. Kulongoski, E. Manini, M. Martínez, H. Miller,
M. Nakagawa, S. Ono, S. Patwardhan, C. J. Ramírez, F. Regoli, F. Smedile, S.
Turner, C. Vetriani, M. Yücel, C. J. Ballentine, T. P. Fischer, D. R. Hilton,
K. G. Lloyd. Forearc carbon sink reduces long-term volatile recycling
into the mantle. Nature, 2019; 568 (7753): 487 DOI: 10.1038/s41586-019-1131-5
引用自：University
of Oxford. “Microbes may act as gatekeepers of Earth's deep carbon.”
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