遠古岩石記錄到產生氧氣的光合作用首次發生時留下的痕跡

原文網址：www.sciencedaily.com/releases/2015/10/151006192107.htm

Ancient rocks record first evidence for photosynthesis that made oxygen

遠古岩石記錄到產生氧氣的光合作用首次發生時留下的痕跡

A new study shows that iron-bearing rocks that formed at the ocean floor 3.2 billion years ago carry unmistakable evidence of oxygen. The only logical source for that oxygen is the earliest known example of photosynthesis by living organisms, say University of Wisconsin-Madison geoscientists.

新研究顯示32億年前於海床形成的岩石中，具有斬釘截鐵的證據顯示氧氣已經出現。威斯康辛大學麥迪遜分校的地質學家說，這些氧氣的唯一合理來源即為生物進行光合作用的最早案例。

"Rock from 3.4 billion years ago showed that the ocean contained basically no free oxygen," says Clark Johnson, professor of geoscience at UW-Madison and a member of the NASA Astrobiology Institute. "Recent work has shown a small rise in oxygen at 3 billion years. The rocks we studied are 3.23 billion years old, and quite well preserved, and we believe they show definite signs for oxygen in the oceans much earlier than previous discoveries."

「34億年前的岩石顯示出當時的海洋基本上是毫無丁點自由氧的。」威斯康辛大學麥迪遜分校地質科學系的Clark Johnson教授說。他也是NASA天體生物學研究所的成員。「最近的研究表示氧氣濃度在30億年前有小幅的上升。我們研究的岩石年代為32.3億年且保存相當良好，而我們確信它們展現的訊號明確表示出海洋出現氧氣的時間要比先前的研究早了許多。」

The most reasonable candidate for liberating the oxygen found in the iron oxide is cyanobacteria, primitive photosynthetic organisms that lived in the ancient ocean. The earliest evidence for life now dates back 3.5 billion years, so oxygenic photosynthesis could have evolved relatively soon after life itself.

最有可能釋放發現於這些氧化鐵中的氧氣的候選人便是藍綠菌，這是一種生活在遠古海洋的原始光合生物。生命已經出現的證據目前最早可追溯至35億年前，因此產氧光合作用(oxygenic photosynthesis)相當有可能在生命出現不久之後就演化出來了。

Until recently, the conventional wisdom in geology held that oxygen was rare until the "great oxygenation event," 2.4 to 2.2 billion years ago.

直到最近，地質學界抱持的共識認為氧氣在24億年至22億年後的「大氧化事件」才變得較為普遍。

The rocks under study, called jasper, made of iron oxide and quartz, show regular striations caused by composition changes in the sediment that formed them. To detect oxygen, the UW-Madison scientists measured iron isotopes with a sophisticated mass spectrometer, hoping to determine how much oxygen was needed to form the iron oxides.

他們研究的岩石稱作碧玉(jasper)，由氧化鐵和石英組成，因為形成時沉積物成分的變化而具有規律帶狀構造。為了偵測其中的氧氣，威斯康辛大學麥迪遜分校的科學家利用先進的質譜儀來測量鐵同位素，期望可以定量出形成這些氧化鐵需要多少氧氣。

"Iron oxides contained in the fine-grained, deep sediment that formed below the level of wave disturbance formed in the water with very little oxygen," says first author Aaron Satkoski, an assistant scientist in the Geoscience Department. But the grainier rock that formed from shallow, wave-stirred sediment looks rusty, and contains iron oxide that required much more oxygen to form.

「在波浪影響深度之下的深層細粒沉積物中含有的氧化鐵，為在氧氣相當少的水中形成。」第一作者，地質科學系的助理教授Aaron Satkoski說。但在較淺處，會受波浪攪動的粗顆粒沉積物則帶有鏽色，而含有的氧化鐵需要較多氧氣才能形成。

The visual evidence was supported by measurements of iron isotopes, Satkoski said.

Satkoski說鐵同位素的測量結果支持了這些外觀上的證據。

The study was funded by NASA and published in Earth and Planetary Science Letters.

此研究由NASA支助並刊登於《地球和行星科學通訊》。

The samples, provided by University of Johannesburg collaborator Nicolas Beukes, were native to a geologically stable region in eastern South Africa.

樣品由共同作者，約翰尼斯堡大學的Nicolas Beukes提供，來源為南非東部的地質穩定區。

Because the samples came from a single drill core, the scientists cannot prove that photosynthesis was widespread at the time, but once it evolved, it probably spread. "There was evolutionary pressure to develop oxygenic photosynthesis," says Johnson. "Once you make cellular machinery that is complicated enough to do that, your energy supply is inexhaustible. You only need sun, water and carbon dioxide to live."

由於這些樣品來自單一鑽孔採出的岩心，因此科學家無法證明當時光合作用是否已經廣泛分佈；然而，一旦生物演化出光合作用，它們就相當有可能會往外散佈。「演化壓力自然會讓生物發展出產氧光合作用，」Johnson說。「只要你使你的細胞工廠複雜到足以進行此作用，便能夠擁有取之不盡、耗之不竭的能源。你僅需要陽光、水和二氧化碳便能存活。」

Other organisms developed forms of photosynthesis that did not liberate oxygen, but they relied on minerals dissolved in hot groundwater -- a far less abundant source than ocean water, Johnson adds. And although oxygen was definitely present in the shallow ocean 3.2 billion years ago, the concentration was only estimated at about 0.1 percent of that found in today's oceans.

Johnson附加，雖然也有其他生物發展出不會釋放氧氣的光合作用，但它們必須仰賴溶解於高熱地下水中的礦物，而這種資源跟海水相比實在是少太多了。縱使可以肯定32億年前的淺海已經含有氧氣，但濃度估計不過是現今海水氧氣濃度的0.1%左右而已。

Confirmation of the iron results came from studies of uranium and its decay products in the samples, says co-author Brian Beard, a senior scientist at UW-Madison. "Uranium is only soluble in the oxidized form, so the uranium in the sediment had to contain oxygen when the rock solidified."

共同作者，威斯康辛大學麥迪遜分校的資深研究員Brian Beard說，他們經由測定樣品中的鈾與其衰變產物，來確認鐵的分析結果是否可信。「鈾只有在氧化態時才能溶於水，因此在岩石固結過程中，含鈾的沉積物也必然含有氧氣。」

Measurements of lead formed from the radioactive decay of uranium showed that the uranium entered the rock sample 3.2 billion years ago. "This was an independent check that the uranium wasn't added recently. It's as old as the rock; it's original material," Beard says.

經由測量鈾放射線衰變所形成的鉛，他們得出鈾在32億年前進入到這些岩石樣品中。「這項獨立檢驗顯示出鈾並非近期才跑進去的。它跟岩石同樣的古老；它本來就存在於此。」Beard說。

"We are trying to define the age when oxygenic photosynthesis by bacteria started happening," he says. "Cyanobacteria could live in shallow water, doing photosynthesis, generating oxygen, but oxygen was not necessarily in the atmosphere or the deep ocean."

「我們試著去確立細菌何時開始進行產氧光合作用，」他說。「藍綠菌生活在淺水地區，進行光合作用並產生氧氣，但氧氣不盡然會擴散到大氣或者深海地帶。」

However, photosynthesis was a nifty trick, and sooner or later it started to spread, Johnson says. "Once life gets oxygenic photosynthesis, the sky is the limit. There is no reason to expect that it would not go everywhere."

然而，光合作用確實是個傑作，而它遲早會開始散播出去，Johnson說。「生命一得到產氧光合作用，就有了無限可能。沒有理由去認為有生命到達不了之處。」

引用自：University of Wisconsin-Madison. "Ancient rocks record first evidence for photosynthesis that made oxygen." ScienceDaily. ScienceDaily, 6 October 2015.