新研究挑戰了目前對於生物如何遷居深海熱泉的理論

原文網址：http://www.mbari.org/new-study-challenges-prevailing-theory-about-how-deep-sea-vents-are-colonized/" 

新研究挑戰了目前對於生物如何遷居深海熱泉的理論

Kim Fulton-Bennett

甫刊登於《英國皇家學會報告B》(Proceedings of the Royal Society B)的論文敘述了在加利福尼亞南部發現兩座截然不同的熱泉區。雖然它們相對而言十分靠近彼此，但兩座熱泉擁有的動物群集卻大不相同。學界一般的假設中認為鄰近的熱泉會擁有類似動物群集，但此篇新論文與其相反，提出區域地質和熱泉噴出流體的化學性質是影響熱泉生物群集的重要因子。

2012年，蒙特利灣水族館附設研究所(MBARI)的科學家利用水下機器人，沿著加利福尼亞灣南端的 Alarcón隆起發現了一處新的熱泉區。2015年他們接續前項成果，在其北方僅距75公里的Pescadero海盆中發現了第二座截然不同的熱泉區。

研究由MBARI的科學家領導，成員包括墨西哥、加拿大、俄羅斯和德國的科學家。在籌備這篇近日刊出的論文時，他們分析了採集到的生物樣品和機器人進行調查時拍的影片，以確立群集的物種組成。他們也呈現了海水樣品的DNA分析結果來鑑定熱泉動物幼體的種類，同時運用穩定同位素分析結果來評估個別噴泉區的食物來源。

科學家比較生存在Alarcón和Pescadero海盆熱泉區的動物種類，以及在北方400公里瓜伊馬斯海盆和南方300公里東太平洋海脊發現的動物種類。研究人員發現儘管Alarcón和Pescadero熱泉區的所在位置相當接近，它們供養的動物群集卻完全不同。在61種動物當中，它們共有的只有7種。

這項發現跟學界一般的假設中，認為鄰近棲地會擁有類似動物群集有所牴觸。反之，研究結果提出區域地質和熱泉噴出流體的化學性質，在熱泉動物群集建立時具有主要地位。這項發現跟評估海底採礦可能帶來的生態衝擊有關――科學家必須要考量到區域地質和化學性質的獨特性，不能預設附近會有一處熱泉提供共有的動物幼體來重新拓殖並修復棲地。

主要作者，MBARI的兼任研究員與美國西方學院的副教授Shana Goffredi解釋：「就像人類的都市一樣，某個區域的群集由什麼動物組成不只要看誰能到達此地點，也要考慮其中蘊含的資源是否適合能讓牠們繁榮生長。生物多樣性對於一個區域的生物群集穩定性來說相當重要，而它有很大一部份取決於該區域擁有的資源種類，不論是在物理或化學方面。」

雖然位於鄰近區域，但Alarcón隆起和Pescadero海盆熱泉區的地質背景卻相當不同。Alarcón隆起的海床被近期產生的新鮮岩漿覆蓋，從噴口冒出的液體溫度相當高(將近360℃)，其中含有的金屬硫化物形成了深黑色且易碎的高塔，稱作「黑煙囪」。生活在Alarcón隆起的動物跟南方許多(幾乎有300公里)的東太平洋隆起的熱泉物種較為類似。

另一方面，Pescadero海盆的熱泉流體則經過了厚重的海底泥層。當高溫熱泉流體經過這層底泥，它會「烹煮」其中的有機物質而產生甲烷(天然氣)和其他油狀的碳氫化合物。Pescadero海盆熱泉含有的硫化物相當少，使得高熱流體產生巨大、淺色、由碳酸鹽組成的煙囪，上方斑駁覆著油膩的碳氫化合物。

在Pescadero噴泉發現的動物大多是蠕蟲，其中有許多物種是科學上的新發現。數量最多的管蟲(Oasisia屬)在灣區其他區域並不常見。令人驚訝的是，Pescadero熱泉的動物有三分之二在其南方和北方的熱泉中都沒有發現。

過去二十年來，海洋生物學家一直在嘗試證明海床的動物是利用什麼方法從一座孤立的熱泉棲地散佈至另一處熱泉。大多生活在熱泉的動物會釋放出能被海流攜帶的微小幼體。如果這些幼體中有些存活得夠久並到達另一座熱泉，它們就有機會附著在海床然後發育為成體，而成功拓殖到新的熱泉。

該拓殖理論使得熱泉生物學家推測鄰近的熱泉區應該會擁有類似動物群集組成。然而，此篇新論文卻顯示從一座熱泉出生的幼體可能無法成功在鄰近的熱泉定居。MBARI的研究人員Shannon Johnson利用高通量DNA定序技術，來鑑定從熱泉附近採得的水體中含有的幼體種類。她的分析結果顯示從其他地方來的生物幼體可以到達Pescadero海盆，但當地的地質和化學條件顯然讓牠們無法在此地定居並生長。

研究人員總結在特定熱泉找到的動物群集組成會受到許多因素影響。水深、海床地質、熱泉流體的溫度和化學組成，以及生物幼體從其他熱泉遷移至此的能力都具有相當影響。由於有越來越多計畫試著在深海熱泉區開採貴金屬，參與此研究的科學家建議保育人員和管理機構在嘗試預測採礦活動對環境的衝擊以及受影響生物群集的韌性時，需要考慮更多因素。

New study challenges prevailing theory about how deep-sea vents are colonize

An article just published in the Proceedings of the Royal Society B describes two remarkably different hydrothermal vent fields discovered in the southern Gulf of California. Despite being relatively close together, these vents host very different animal communities. This finding contradicts a common scientific assumption that neighboring vents will share similar animal communities. Instead, the new paper suggests that local geology and the chemistry of the vent fluids are important factors affecting vent communities.

In 2012, scientists from the Monterey Bay Aquarium Research Institute (MBARI) used undersea robots to discover a new hydrothermal vent field along the Alarcón Rise at the southern end of the Gulf of California. Continuing the effort in 2015, they discovered a second, very different vent field in the Pescadero Basin, just 75 kilometers to the north.

Led by MBARI scientists, the research involved scientists from Mexico, Canada, Russia, and Germany. In preparing the recent paper, they analyzed collected organisms and video surveys to determine community composition. They also performed DNA analyses of water samples to identify larvae of vent animals and stable-isotope analysis to assess food supplies at each vent field.

The scientists compared the animals living at the Alarcón and Pescadero Basin vent fields with those found in the Guaymas Basin, 400 kilometers to the north, and on the East Pacific Rise, about 300 kilometers to the south. The researchers found that, despite their close proximity, the Alarcón and Pescadero vent fields support radically different animal communities, sharing only seven out of 61 animal species.

This finding contradicts a common scientific assumption that neighboring habitats will share similar animal communities. Instead, the results suggest that local geology and chemistry of the vent fluids play dominant roles in structuring the animal communities. The findings are relevant to assessing the possible ecological impacts of seafloor mining—scientists must account for the uniqueness of local geology and chemistry and not assume that a common supply of animal larvae will colonize and restore neighboring habitats.

Lead author Shana Goffredi, an MBARI adjunct and associate professor at Occidental College, explained, “Just like human cities, the community that forms in a particular area depends not only on who arrives at that location, but also whether the underlying resources are suitable for their success. Variation in these resources, whether physical or chemical, contributes greatly to the diversity of the region, which is important for community stability.”

Though neighbors, the Alarcón Rise and Pescadero Basin vent fields are geologically very different. The seafloor along the Alarcón Rise is covered in young, fresh lava, and the fluids spewing out of the vents are very hot (up to 360 degrees Celsius) and rich in metal sulfides that form dark, crumbly chimneys known as “black smokers.” Animals at the Alarcón Rise are similar to locations further south (almost 300 kilometers) on the East Pacific Rise.

In Pescadero Basin, however, hydrothermal-vent fluids pass through thick layers of seafloor mud. As the hot hydrothermal fluid flows through this mud, it “cooks” organic material, forming methane (natural gas) and oil-like hydrocarbons. The Pescadero Basin vents contain very little sulfide, and the superheated fluids produce giant, light-colored, carbonate chimneys streaked with dark, oily hydrocarbons.

Most of the animals found at the Pescadero vents are worms, and many species are new to science. The dominant tubeworms (genus Oasisia) are not common elsewhere in the Gulf. Surprisingly, two thirds of the Pescadero vent animals are not found at vents to the north and south.

For the last two decades, marine biologists have been trying to document how seafloor animals manage to disperse from one discrete hydrothermal vent habitat to another. The majority of vent animals release microscopic larvae that are carried by ocean currents. If some of these larvae survive long enough to reach another hydrothermal vent, they may settle on the seafloor, grow into adults, and colonize a new vent.

This colonization theory led vent biologists to assume that neighboring vent fields should harbor similar animal communities. However, the new paper shows that larvae from one vent may not successfully colonize a neighboring vent. MBARI researcher Shannon Johnson used high-throughput DNA sequencing to identify larvae collected from the water around the vents. Her results showed that larvae from other sites can reach the Pescadero Basin, but prevailing geological and chemical conditions apparently preclude their settlement and growth there.

The researchers conclude that numerous factors affect the composition of the animal communities found at particular vents. Water depth, geology of the seafloor, temperature and chemistry of the vent fluids, and the ability of larvae from other vents to colonize the site all play roles. Given developing efforts to mine deep-sea hydrothermal vent fields for precious metals, the scientists involved in this research suggest that conservationists and management agencies need to consider a broader range of factors in their efforts to predict the environmental impacts and the resiliency of affected communities.

原始論文：Shana K. Goffredi, Shannon Johnson, Verena Tunnicliffe, David Caress, David Clague, Elva Escobar, Lonny Lundsten, Jennifer B. Paduan, Greg Rouse, Diana L. Salcedo, Luis A. Soto, Ronald Spelz-Madero, Robert Zierenberg, Robert Vrijenhoek. Hydrothermal vent fields discovered in the southern Gulf of California clarify role of habitat in augmenting regional diversity. Proceedings of the Royal Society B: Biological Sciences, 2017; 284 (1859): 20170817 DOI: 10.1098/rspb.2017.0817

引用自：Monterey Bay Aquarium Research Institute. " New study challenges prevailing theory about how deep-sea vents are colonized