地球歷史上死傷最慘重的時期也是最難聞的

 原文網址：https://news.ucr.edu/articles/2021/12/20/deadliest-period-earths-history-was-also-stinkiest

微生物打嗝造成了大滅絕

By Jules Bernstein

一項新研究提出微生物打嗝產生的有毒氣體不但是地球歷史上最嚴重大滅絕事件的成因之一，也讓這段時期持續更久。

位在今日西伯利亞的火山造成了世界上最嚴重的大滅絕事件。

科學家一般相信2億5000萬年前二疊紀結束時的大滅絕事件，最主要的成因是西伯利亞的火山爆發所排出的溫室氣體。這些氣體造成氣候極度暖化，使得海洋裡80%的物種以及許多陸棲生物邁入滅亡。

不過目前為止，科學家還沒有辦法解釋高熱造成生物死亡的確切機制。由加州大學河濱分校主持，發表在《自然―地球科學》(Nature Geoscience)的新研究指出高熱加速了微生物的代謝作用，創造出相當致命的環境條件。

「有機物分解使得海洋中的氧氣耗盡之後，微生物開始『呼吸』硫酸鹽並產生硫化氫，這種氣體聞起來就像臭雞蛋，對動物來說具有毒性，」加州大學河濱分校的地球系統模擬專家Dominik Hülse表示。

海洋中的光合生物，也就是某些微生物與植物構成了食物鏈的基礎。當它們逐漸腐爛，其他微生物便會快速消耗氧氣，使得較大的生物只剩下一些氧氣能用。在沒有氧氣的狀況下，微生物還會消耗硫酸鹽，接著排出有毒、惡臭的硫化氫(H₂S)，創造出極為致命的環境，稱為euxinia。生物分解過程中產生的養分使得euxinia可以保持下去，促使更多有機物形成，有助於維持這種惡臭、有毒的循環。

「我們的研究顯示並非整個海洋都處於euxinic的狀態。這種情況開始於較深的水層，」Hülse表示。「隨著溫度逐漸上升，euxinic的區域也跟著變大、毒性更強，並且開始移動到水層上方大多數海洋動物所居住的陸棚環境，使得牠們遭到毒害。」

透過沉積物標本中的化學訊號可以偵測出euxinic區域的擴大現象。

缺氧在今日是一種持續存在的問題，而且在未來氣候變遷的影響之下勢必會變得更糟。洛杉磯16英里長的多明格斯水道便出現這種euxinic的水體。2021年9月一座倉庫起火而把乙醇釋放到這條水道，使得其中的植物死亡。接著微生物把它們分解並消耗掉之後，產生了濃度足以造成毒性的硫化氫。在發臭的河水氣味可及的範圍之內，數千人通報他們出現了嘔吐、腹瀉、頭暈、失眠、頭痛、打噴嚏與其他症狀。

這種作用會對今日的海洋與水道造成危害，若要加以了解，從古代學到的教訓也許是相當重要的。

「把古代的大滅絕事件投射到今日的地球不過是純屬臆測，」Hülse表示。「然而，這項研究確實顯示出我們可能低估了二氧化碳濃度提高時，海洋會出現的反應。」

 

Deadliest period in Earth’s history was also the stinkiest

Toxic microbe burps caused mass extinction

Tiny microbes belching toxic gas helped cause — and prolong — the biggest mass extinction in Earth’s history, a new study suggests.

Generally, scientists believe Siberian volcanos spitting greenhouse gases primarily drove the mass extinction event about 250 million years ago, at the end of the Permian period. The gases caused extreme warming, which in turn led 80% of all marine species, as well as many land species, to go extinct.

Until now, scientists could not explain exactly how the heat caused those deaths. A new UC Riverside-led study in Nature Geoscience shows that the heat accelerated microbes’ metabolisms, creating deadly conditions.

“After oxygen in the ocean was used up to decompose organic material, microbes started to ‘breathe’ sulfate and produced hydrogen sulfide, a gas that smells like rotten eggs and is poisonous to animals,” said UC Riverside Earth system modeler Dominik Hülse.

As ocean photosynthesizers — the microbes and plants that form the base of the food chain — rotted, other microbes quickly consumed the oxygen and left little of it for larger organisms. In the absence of oxygen, microbes consumed sulfate then expelled toxic, reeking hydrogen sulfide, or H₂S, creating an even more extreme condition called euxinia. These conditions were sustained by the release of nutrients during decomposition, promoting the production of more organic material which helped to maintain this stinky, toxic cycle.

“Our research shows the entire ocean wasn’t euxinic. These conditions began in the deeper parts of the water column,” Hülse said. “As temperatures increased, the euxinic zones got larger, more toxic, and moved up the water column into the shelf environment where most marine animals lived, poisoning them.”

The expanding euxinic zones can be detected through chemical signatures in sediment samples.

Oxygen depletion is a problem that persists today and is bound to get worse under future climate change. Euxinic waters can be found in places like Los Angeles County’s 16-mile-long Dominguez Channel, into which a September 2021 warehouse fire released ethanol. The ethanol killed vegetation in the channel, which decayed and got consumed by microbes. They then produced hydrogen sulfide at toxic levels. Thousands in breathing range of the reeking river reported vomiting, diarrhea, dizziness, insomnia, headaches, sneezing, and other symptoms.

Lessons from the ancient world may be important for understanding the processes that are challenging our modern oceans and waterways.

“It would be speculative to superimpose the ancient mass extinction event on today’s planet,” Hülse said. “However, the study does show us that the ocean’s response to higher carbon dioxide concentrations in the atmosphere may be underestimated.”

原始論文：Dominik Hülse, Kimberly V. Lau, Sebastiaan J. van de Velde, Sandra Arndt, Katja M. Meyer, Andy Ridgwell. End-Permian marine extinction due to temperature-driven nutrient recycling and euxinia. Nature Geoscience, 2021; 14 (11): 862 DOI: 10.1038/s41561-021-00829-7

引用自：University of California - Riverside. "Deadliest period in Earth’s history was also the stinkiest.”