Muddy microbes retrieved from the abyss

Michael Hopkin

Submarine mission finds life in the deepest trench on Earth.

Their home lies further beneath sea level than Everest's peak rises above it. And yet tiny organisms have been found living at the very bottom of the Pacific Ocean's deepest trench, the remotest spot on the globe.

The microscopic organisms, called foraminifera, live in mud at the bottom of Challenger Deep in the Mariana Trench, almost 11 kilometres beneath the waves of the western Pacific Ocean. The pressure at this depth is a crushing 1,090 times that at the surface.

Researchers led by Hiroshi Kitazato of the Japan Agency for Marine-Earth Science and Technology in Yokosuka collected samples of sediment from the trench using the Kaiko submarine. This remote-controlled deep-sea vessel made its first trip to the Mariana Trench in 1995; it was lost at sea on a mission in May 2003.

From the samples, Kitazato's team recovered numerous bacteria and 432 living foraminifera. The latter measure a few dozen micrometres across; hundreds could crowd on to a pinhead. Most of the foraminifera were soft-walled, either spherical or needle-shaped, and coloured brown.They report their discovery in Science1.

Shell shortage

Foraminifera are perhaps the most abundant ocean life after bacteria, says Alan Hughes, who studies the creatures at the Southampton Oceanography Centre, UK. Most have intricate shells made of calcium carbonate.

But these shells are absent from the foraminifera found in Challenger Deep. Instead, 85% of the researchers' haul belongs to a soft-walled group called allogromiids, which make up only 5-20% of other foraminiferan communities nearer the surface.

This is probably because the deepest ocean has little calcium carbonate, meaning that microbes may not be able to build a shell. "As you go deeper and deeper you get to a depth called the calcium compensation point," Hughes explains. "Below that you tend to get soft-walled foraminifera."

Kitazato's group speculates that soft foraminifera thrive in the Mariana mud because they are among the few creatures that can withstand the huge pressures there.    Geologists think that the trench was created over the past 6 million to 9 million years, during which time the ecosystem's less hardy species may literally have been crushed to death.

取自深海的泥泞微生物

迈克尔·霍普金

海底任务在地球上最深的海沟里发现了生命

它们的家园在大海的最深处，那个深度可以浸没整个珠穆朗玛峰。但是恰恰是在太平洋最深处的海沟底部——这个地球上最偏僻的角落，还是发现有微生物在此定居。

这些借助显微镜才能看见的生物，叫做有孔虫。它们生活在马里亚纳海沟的查林杰海渊(又译：挑战者海渊)底部的泥浆中。查林杰海渊底部距离波涛汹涌的西太平洋海面有11公里之遥，那个深度下的压力是海洋表面压力的1090倍，如此高压几乎可以压碎一切！

日本的海洋与地球科学技术研究社坐落于横须贺。该社的研究者们在北里洋·土屋正史的带领下，用海沟号潜水艇收集了一些海沟底部的沉积物的样品。这艘遥控深海船在1995年首航马里亚纳海沟。该船于2003年5月在海中执行任务时失踪。

从这些样品中，北里洋的研究小组发现了无数的细菌和432条活的有孔虫。后者长几十个微米，一个针尖上可以放下几百条。绝大多数的有孔虫是软壳的，软壳为球状或针状，呈棕色。他们在《科学》杂志上报告了这一发现。

缺少甲壳

艾伦·休斯在英国的南安普顿海洋学中心研究生物，他认为有孔虫也许是除了细菌以外，海洋中的第二大生物。有孔虫大多具有错综复杂的碳酸钙甲壳。

但是在查林杰海渊发现的这些有孔虫却没有这种甲壳。实际上，捕捞上来的这些虫子中有85%属于软壳的allogromiid属。而在靠近海洋表面的生物群落中，该属生物只占其它有孔虫目生物的5~20%。

这或许是因为在海洋的最深处几乎没有碳酸钙，也就意味着微生物可能无法建造甲壳。“当你越潜越深的时候，你就会到达一个叫做钙补偿点的深度。再往下，你就能遇到软壳的有孔虫。”休斯解释道。

北里洋的研究小组推测软壳有孔虫之所以在马里亚纳海沟的泥里能够兴旺发达是因为它们是为数不多的能够抵挡那儿如此巨大压力的生物。地质学家认为，马里亚纳海沟是在过去的六百万到九百万年之间形成的，当时生态系统中那些不那么耐压的生物种类差不多都是被活活压死了。

(文章摘自《natrue》，由浙江大学博士石冼凡翻译)