经济学人下载:海洋生物 跳蚤市场

Science and technology
科学技术

Marine biology
海洋生物

Flea market
跳蚤市场

A newly discovered virus may be the most abundant organism on the planet
一种新发现的病毒可能是地球上最丰富的物种。

WHAT is the commonest living thing on Earth?
地球上最常见的生物是什么？

Until now, those in the know would probably have answered Pelagibacter ubique, the most successful member of a group of bacteria, called SAR11, that jointly constitute about a third of the single-celled organisms in the ocean.
直到现在，那些很专业的人可能会说是遍在远洋杆菌—细菌群中最成功的细菌，称为大洋微小细菌,占了海洋单细胞有机体的三分之一。

But this is not P. ubique's only claim to fame, for unlike almost every other known cellular creature, it and its relatives have seemed to be untroubled by viruses.
但是，这不是它成名的原因，因为它和它的亲戚不像其它大部分已知的细胞生物，它们似乎是些不会给你造成麻烦的一族。

As Jonathan Swift put it in a much-misquoted poem, So, naturalists observe, a flea/Hath smaller fleas that on him prey.
就像乔纳森-斯威福特的一首诗表达的一样—虽然这里引用很不贴切，所以，博物学家们观察，正在捕食的跳蚤/小跳蚤们。

Parasites, in other words, are everywhere.
换句话说，寄生虫无所不在。

They are also, usually, more abundant than their hosts.
通常，它们的数量也远远多于它们的寄主。

An astute observer might therefore have suspected that the actual most-common species on Earth would be a flea that parasitised P. ubique, rather than the bacterium itself.
精明的观察者可能因此会猜测，地球上真正最常见的特种是一种跳蚤—寄生的遍生远洋杆菌，而不是细菌本身。

The absence of such fleas has puzzled virologists since 1990, when the SAR11 group was identified.
自从1990年研究SAR11组群以来，病毒学家对没有发现这种跳蚤—即以所谓攻击细菌的噬菌体的形式存在—而感到很困惑。

Some thought the advantage this absence conferred explained the group's abundance. But no.
一些人认为没有发现这种组群的好处解释了它们之所以大量存在的原因。但是，事实不是这样的。

As they report in this week's Nature, Stephen Giovannoni of Oregon State University and his colleagues have discovered the elusive phages.
正如本周自然杂志上刊登的一样，俄勒冈州立大学的斯蒂芬尼-乔凡诺尼和他的同事们发现了这种难以发现的噬菌体。

Swift's wisdom, it seems, still holds good.
看起来，斯威福特的智慧仍旧闪闪发光。

Tracking down a particular virus in the ocean makes finding a needle in a haystack look a trivial task.
与在海洋中跟踪一种特定的细菌相比，大海捞针简直是小菜一碟了。

A litre of seawater has billions of viruses in it.
一公升的海水中有数十亿的细菌。

Modern genetic techniques can obtain DNA sequences from these viruses, but that cannot tie a particular virus to a particular host.
现代基因技术可以猎取这些细菌的DAN序列，但是却不能把一种特定的细菌固定在一个特定的寄主身上。

To do so, Dr Giovannoni borrowed a technique from homeopathy: he diluted some seawater to such an extent that, statistically speaking, he expected a 100-microlitre-sized aliquot to contain only one or two viruses.
为了实现这个目标，乔凡诺尼博士借用了顺势医疗论的一项技术：他把一些海水的浓度稀释到，从统计学角度上讲，即稀释后的100微升海水中只含有一种或两种病毒。

The difference between his approach and a homeopath's was that what homeopathy dilutes almost to nothing are chemicals, and thus cannot breed. A virus can, given a suitable host.
他的方法和顺势疗法的区别是，后者将药液稀释后几乎没有什么合成物了，因此也无法培育繁殖。而提供一个合适的寄主的话，病菌就可以继续繁殖。

So he mixed each of several hundred aliquots into tubes of water containing P. ubique.
因此，他把稀释后的海水分成几百份，再把每一份混合到含有遍在海洋杆菌的水里。

Then he waited.
然后他就等待着。

After 60 hours, he looked to see what had happened.
六十小时后，他看到了结果。

In most cases the bacteria had thrived.
在大部分试验品中，病菌都繁殖得很好。

In a few, though, they had been killed by what looked like viral infection.
但是，在少数的试验品中，病菌似乎由于某种病毒感染而消失了。

It was these samples that he ran through the DNA-sequencing machine, in the knowledge that the only viral DNA present would be from whatever it was had killed the bacteria.
这些少数试验品就是他用DNA测序仪检测，想知道仅有的病毒DNA，无论是不是它杀了病菌，为什么会出现的原因。

His reward was to find not one, but four viruses that parasitise P. ubique.
他最终发现了不只一种病菌，而是四种寄生的遍海洋杆菌。

He then compared their DNA with databases of DNA found in seawater from around the world, to find out how abundant each is.
后来，他把发现病菌的DNA与世界海洋的DNA数据库相对比，发现：

The upshot was that a virus dubbed HTVC010P was the commonest.
四种中的每一个的存在数量都非常多。

It thus displaces its host as the likely winner of the most-common-living-thing prize.
结果是，一种称为HTVC010P人病菌是最常见的。因此，它便取代了它的寄主，成为可能最常见的生物。

That does depend, of course, on your definition of living thing.
当然，这不是你们所定义的那种生物。

Some biologists count viruses as organisms.
一些生物学家认为病菌是微生物。

Some do not.
一些则不这么认为。

The reason is that a virus relies for its growth and reproduction on the metabolic processes of the cell it infects.
原因是，病菌依靠它所感染细胞的新陈代谢来生长和繁殖。

This means viruses themselves are hard to parasitise, since they do no work on which another organism can free-ride.
这就意味着病毒本身很难寄生，因为在其它可以免费寄生的微生物上它们无法正常生存。

Which is why the next two lines of Swift's poem, And these have smaller fleas to bite 'em/And so proceed ad infinitum, are wrong—and why, because HTVC010P itself can have no parasites, it probably really is the commonest organism on the planet.
这也解释了为什么斯威失特接下来的两句这些更小的跳蚤在咬它们，因此这些进程循环往复着是错误的，也是HTVC010P为什么是地球上最常见的微生物，因为它本身没有寄生虫。