地球深處生命含有的碳總計有150至230億噸――比人類總和的數百倍還多

https://deepcarbon.net/life-deep-earth-totals-15-23-billion-tonnes-carbon

地球深處生命含有的碳總計有150至230億噸――比人類總和的數百倍還多

一項國際合作計畫的科學家表示幾乎不算活著的「殭屍」細菌和其他生命形式構成了地下深處的大量碳質――重量比地表所有人類含有的碳多245到385倍。這項即將結束、為期10年的計畫旨在揭開地球最深處的秘密。

一條線蟲穿梭在微生物形成的生物薄膜之中。這條尚未命名的線蟲(Poikilolaimus sp.)住在南非的Kopanang金礦，位在地表之下1.4公里之處。圖片來源：Gaetan Borgonie (Extreme Life Isyensya, Belgium)

在美國地球物理學會年度會議的前夕，深碳觀測計畫(Deep Carbon Observatory，DCO)的科學家發表了幾項顛覆性的發現，包括在地底深處溫度壓力極大、難以取得能量和養分的環境下，有什麼樣的生命存活以及它們的數量有多少。

團隊鑽探至海床以下2.5公里，並從陸上的礦坑與超過5公里深的鑽井中採集微生物。利用這些結果他們建立了地球深處的生態系模型。

根據從數百個陸地與海洋之下的研究地點獲得的見解，他們估計深層生物圈的大小為20至23億立方公里(幾近所有海洋加起來的兩倍)，而深層生物含有的碳總和為150至230億噸(即每立方公里的地下空間平均至少有7.5噸的碳)。

這項研究也有助於找出哪些類型的地外環境可以供養生命。

以下為幾個關鍵發現與新理解：

• 深層生物圈構成的世界某種程度上可視為「地下的加拉巴哥群島」，其中的成員包含了全部三大生物域：細菌和古菌(兩類微生物皆沒有被膜包裹的細胞核)，以及真核生物(為單細胞或多細胞生物，細胞擁有細胞核以及被膜包覆的胞器)。
• 細菌和古菌這兩種微生物主宰了深層地球。它們有數百萬種獨特的類型，大多還沒被人們發現或記敘下來。這些微生物「暗物質」大幅拓展了我們對生命樹的認知。研究深層生命的科學家表示地球上的細菌和古菌大約70%都是生活在地下。
• 深層微生物跟它們住在地表的表親通常有很大的差異。深層微生物的生命週期幾乎要以地質時間尺度來計算，某些案例僅依靠岩石產生的能量來過活。
• 地下生命的基因多樣性跟地表差不多，甚至還要更加複雜。
• 地下微生物群落隨著環境不同有很大的差異，不過某些屬或者更高的分類群卻相當常見――它們似乎存在於整個地球。
• 微生物群落的豐富程度跟發現這些細胞的海洋沉積物年齡相關，意謂年代較老的沉積物中，食物和能量隨著時間遞減使得微生物群落變少。
• 目前還沒有發現地球上的生命在溫度、壓力與能量需求這些方面有絕對的極限。紀錄不斷地被打破。在自然界中，目前高踞地球最耐熱的生物是Geogemma barossii，這種單細胞生物存活於海底熱泉。其微小的球狀細胞可以在攝氏121度(比水的沸點還高21度)下生長與分裂。
• 在實驗室培養的情況下，微生物可以存活的溫度紀錄高達122°C。(相較之下，地表最高溫的紀錄保持地點是在伊朗一處毫無人煙的沙漠，約為71°C――跟全熟牛排一樣的溫度)
• 發現微生物的最深記錄在陸地大約是地下5公里，海中則是海水表面以下10.5公里。此處的壓力極大，舉例而言，在海水表面以下4000公尺之處的壓力已經比海平面高了400倍左右。
• 科學家對於人類利用地下環境時對生物的影響有更深的瞭解(像是壓裂頁岩、碳捕集與封存)。

DNA定序技術的準確度不斷提升且費用越來越低廉，加上深海鑽探技術的突破使得研究人員首次有機會可以詳細觀察深層生物圈的組成(目前最先進的深海鑽探設備位於日本的研究船地球號，最終目標是要鑽入地球上某些地震最頻繁地區的海床深處)。

在陸地也有類似的研究鑽到更深的位置，並運用可以維持住壓力的採樣設備來保存微生物(科學家認為這些微生物皆對人體健康沒有害處但也無益)。

日本的科學鑽探船「地球號」讓科學家得以取得地底深處的微生物。這艘船設計的最終目標是要在世上某些地震活動最頻繁的地區鑽到海床下方7公里處。DCO的研究人員在2016年前往南海海槽的航次中登船，目的是要找出在120°C以上的環境中，微生物生存的溫度壓力極限。圖片來源：JAMSTEC

為了估計地球陸地下方深層生命的總質量，團隊用的方法像是彙整從全球不同地點得到的細胞密度和微生物多樣性數據。

由紐約熨斗研究院計算生物學中心的Cara Magnabosco領導的科學團隊，將許多層面的因素考量進來，包括全球熱流、表面溫度、深度與岩性(指每個地方岩石的物理性質)，估計出陸地下方的細胞總數有2到6× 10²⁹。

結合海床之下生物數量的估計結果，深層地球的生物質量全球加總起來約含有15至23 Pg(petagram，等於10億噸)的碳。

DCO的深層生命學會總共有超過300個研究人員，來自34個國家。共同主席，美國伍茲霍爾海洋生物學實驗室的Mitch Sogin表示：「探索地球深處就像是去亞馬遜雨林探險一樣。到處都有生命存在。每個地方都有許多種未曾預料到的奇異生物，令人嘆為觀止。」

「分子研究顯示微生物暗物質的多樣性可能比我們現在認為的還超出許多，而生命樹最底層的血脈分支也挑戰了Carl Woese在1977年提出的三域觀。經由研究深層生命，我們或許能把生命最早的可能分支模式連結起來。」

「對於統治地下生物圈的細菌與微生物的生理運作，我們在10年前的瞭解遠遠不如現在。」美國田納西大學諾克斯維爾分校的Karen Lloyd表示，「我們現在知道在許多場所它們投資的大部分能量只是用來讓它們維持活著的狀態，幾乎沒有用來成長。這是一種相當驚人的生活方式。」

「我們現在也曉得地下生命是十分常見的。10年前，我們只有在一些地方――預期可以找到生命的環境類型中採樣。現在拜超深採樣技術所賜，我們瞭解到幾乎每個地方都能發現它們，雖然這些樣本顯然只觸及了深層生物圈的極小一部份。」

「從深層生物圈微生物的研究中，我們不只得到了許多新知識，也深刻體悟到我們對地下生命還有很多可以探討的地方。」美國奧勒岡州立大學的Rick Colwell表示，「比方說，科學家仍完全不知道深層生命對地表生命有什麼樣的影響，反之亦然。深層生命的代謝作用特質使得它們能在地球深處對生物來說極為貧瘠且嚴酷的環境中存活下來，對此目前我們能做的只是發出讚嘆而已。」

以下為地球深層生命諸多未解之謎的其中幾個：

移動：深層生命如何散播――也就是如何透過岩石的裂縫往旁邊散佈出去？又是怎麼往上往下傳播？為什麼南非、西雅圖和華盛頓的深層生命如此相似？是因為它們的起源相當類似，但之後受到像是板塊構造作用之類的因素分開？或者是微生物群落本身的遷移造成？在深層生命的移動過程中，大型地質事件，像是板塊構造運動、地震、大型火成岩區域的形成、隕石轟炸等具有什麼樣的腳色？

起源：地球上的生命是出現在地球深處(地殼內部、海底熱泉附近、或隱沒帶內部)，然後再往上前進，朝太陽移動？或者生命起源於地表溫暖的小水池裡，之後才往下遷移？這些地下微生物殭屍如何繁殖？又是如何在數百萬到數千萬年都不分裂的情況下活著？

能量：對深層生命來說，最重要的能量來源是甲烷、氫氣還是天然輻射(來自鈾或其他元素)？在不同的環境中哪種深層能量的來源是最重要的？對於地下微生物的分布情形和多樣性而言，缺乏養分和極端的溫度壓力會造成什麼樣的影響？

Life in Deep Earth totals 15 to 23 billion tonnes of carbon —hundreds of times more than humans

Barely living "zombie" bacteria and other forms of life constitute an immense amount of carbon deep within Earth's subsurface—245 to 385 times greater than the carbon mass of all humans on the surface, according to scientists nearing the end of a 10-year international collaboration to reveal Earth's innermost secrets.

On the eve of the American Geophysical Union’s annual meeting, scientists with the Deep Carbon Observatory today reported several transformational discoveries, including how much and what kinds of life exist in the deep subsurface under the greatest extremes of pressure, temperature, and low energy and nutrient availability.

Drilling 2.5 kilometers into the seafloor, and sampling microbes from continental mines and boreholes more than 5 km deep, the team has used the results to construct models of the ecosystem deep within the planet.

With insights from now hundreds of sites under the continents and seas, they have approximated the size of the deep biosphere—2 to 2.3 billion cubic km (almost twice the volume of all oceans)—as well as the carbon mass of deep life: 15 to 23 billion tonnes (an average of at least 7.5 tonnes of carbon per cu km subsurface).

The work also helps determine types of extraterrestrial environments that could support life.

Among many key discoveries and insights:

• The deep biosphere constitutes a world that can be viewed as a sort of “subterranean Galapagos” and includes members of all three domains of life: bacteria and archaea (microbes with no membrane-bound nucleus), and eukarya (microbes or multicellular organisms with cells that contain a nucleus as well as membrane-bound organelles)
• Two types of microbes—bacteria and archaea—dominate Deep Earth. Among them are millions of distinct types, most yet to be discovered or characterized. This so-called microbial “dark matter” dramatically expands our perspective on the tree of life. Deep Life scientists say about 70% of Earth's bacteria and archaea live in the subsurface
• Deep microbes are often very different from their surface cousins, with life cycles on near-geologic timescales, dining in some cases on nothing more than energy from rocks
• The genetic diversity of life below the surface is comparable to or exceeds that above the surface
• While subsurface microbial communities differ greatly between environments, certain genera and higher taxonomic groups are ubiquitous - they appear planet-wide
• Microbial community richness relates to the age of marine sediments where cells are found—suggesting that in older sediments, food energy has declined over time, reducing the microbial community
• The absolute limits of life on Earth in terms of temperature, pressure, and energy availability have yet to be found. The records continually get broken. A frontrunner for Earth’s hottest organism in the natural world is Geogemma barossii, a single-celled organism thriving in hydrothermal vents on the seafloor. Its cells, tiny microscopic spheres, grow and replicate at 121 degrees Celsius (21 degrees hotter than the boiling point of water)
• Microbial life can survive up to 122°C, the record achieved in a lab culture (by comparison, the record-holding hottest place on Earth’s surface, in an uninhabited Iranian desert, is about 71°C—the temperature of well-done steak)
• The record depth at which life has been found in the continental subsurface is approximately 5 km; the record in marine waters is 10.5 km from the ocean surface, a depth of extreme pressure; at 4000 meters depth, for example, the pressure is approximately 400 times greater than at sea level
• Scientists have a better understanding of the impact on life in subsurface locations manipulated by humans (e.g., fracked shales, carbon capture and storage)

Ever-increasing accuracy and the declining cost of DNA sequencing, coupled with breakthroughs in deep ocean drilling technologies (pioneered on the Japanese scientific vessel Chikyu, designed to ultimately drill far beneath the seabed in some of the planet’s most seismically-active regions) made it possible for researchers to take their first detailed look at the composition of the deep biosphere.

There are comparable efforts to drill ever deeper beneath continental environments, using sampling devices that maintain pressure to preserve microbial life (none thought to pose any threat or benefit to human health).

To estimate the total mass of Earth’s subcontinental deep life, for example, the team compiled data on cell concentration and microbial diversity from locations around the globe.

Led by Cara Magnabosco of the Flatiron Institute Center for Computational Biology, New York, the scientists factored in a suite of considerations, including global heat flow, surface temperature, depth and lithology—the physical characteristics of rocks in each location—to estimate that the continental subsurface hosts 2 to 6 × 10²⁹ cells.

Combined with estimates of subsurface life under the oceans, total global Deep Earth biomass is approximately 15 to 23 petagrams (15 to 23 billion tonnes) of carbon.

Says Mitch Sogin of the Marine Biological Laboratory Woods Hole, USA, co-chair of DCO’s Deep Life community of more than 300 researchers in 34 countries: “Exploring the deep subsurface is akin to exploring the Amazon rainforest. There is life everywhere, and everywhere there’s an awe-inspiring abundance of unexpected and unusual organisms.

“Molecular studies raise the likelihood that microbial dark matter is much more diverse than what we currently know it to be, and the deepest branching lineages challenge the three-domain concept introduced by Carl Woese in 1977. Perhaps we are approaching a nexus where the earliest possible branching patterns might be accessible through deep life investigation.”

“Ten years ago, we knew far less about the physiologies of the bacteria and microbes that dominate the subsurface biosphere,” says Karen Lloyd, University of Tennessee at Knoxville, USA. “Today, we know that, in many places, they invest most of their energy to simply maintaining their existence and little into growth, which is a fascinating way to live.

“Today too, we know that subsurface life is common. Ten years ago, we had sampled only a few sites—the kinds of places we'd expect to find life. Now, thanks to ultra-deep sampling, we know we can find them pretty much everywhere, albeit the sampling has obviously reached only an infinitesimally tiny part of the deep biosphere.”

“Our studies of deep biosphere microbes have produced much new knowledge, but also a realization and far greater appreciation of how much we have yet to learn about subsurface life,” says Rick Colwell, Oregon State University, USA. “For example, scientists do not yet know all the ways in which deep subsurface life affects surface life and vice versa. And, for now, we can only marvel at the nature of the metabolisms that allow life to survive under the extremely impoverished and forbidding conditions for life in deep Earth.”

Among the many remaining enigmas of deep life on Earth:

Movement: How does deep life spread—laterally through cracks in rocks? Up, down? How can deep life be so similar in South Africa and Seattle, Washington? Did they have similar origins and were separated by plate tectonics, for example? Or do the communities themselves move? What roles do big geological events (such as plate tectonics, earthquakes; creation of large igneous provinces; meteoritic bombardments) play in deep life movements?

Origins: Did life start deep in Earth (either within the crust, near hydrothermal vents, or in subduction zones) then migrate up, toward the sun? Or did life start in a warm little surface pond and migrate down? How do subsurface microbial zombies reproduce, or live without dividing for millions to tens of millions of years?

Energy: Is methane, hydrogen, or natural radiation (from uranium and other elements) the most important energy source for deep life? Which sources of deep energy are most important in different settings? How do the absence of nutrients, and extreme temperatures and pressure, impact microbial distribution and diversity in the subsurface?

Deep Carbon Observatory. "Life in Deep Earth totals 15 to 23 billion tons of carbon -- hundreds of times more than humans."