[ti:Researchers Discover World’s Largest Bacterium]
[by:www.21voa.com]
[00:00.00]更多听力请访问21VOA.COM
[00:00.04]The largest-known bacterium is changing researchers' ideas
[00:05.60]about what is possible for bacteria, Earth's most ancient life form.
[00:11.56]The organism was discovered on the Caribbean island of Guadeloupe
[00:16.96]and is big enough to be seen with the human eye.
[00:21.20]Scientists said recently that the bacterium,
[00:25.60]called Thiomargarita magnifica, is unusual for its size
[00:31.00]-- up to about two centimeters long.
[00:34.92]It is also notable because its structure is unlike other bacteria.
[00:41.12]The DNA, an organism's genetic information,
[00:45.20]is not free-moving inside the cell like in most bacteria.
[00:50.28]Instead, the DNA is contained within many small membrane-bound sacs.
[00:57.24]Membrane-bound structures in cells are called organelles.
[01:02.40]Jean-Marie Volland was a co-leader of the study
[01:06.44]that appeared in the publication Science.
[01:09.60]Volland said of the bacteria,
[01:12.64]"It is thousands of times larger than regular-sized bacteria.
[01:17.72]Discovering this bacterium is like encountering
[01:21.12]a human being as tall as Mount Everest."
[01:25.56]Researchers have found the bacterium in several places in Guadeloupe,
[01:30.52]a French island group in the Caribbean.
[01:34.12]Study co-leader Olivier Gros first discovered the bacterium
[01:39.24]in the sulfur-rich seawater of a Guadeloupe swamp
[01:43.40]- land that is wet and partly covered with water.
[01:48.08]Gros described finding long white materials
[01:51.24]attached to a leaf of a mangrove tree.
[01:54.76]He brought the materials back to the lab to study.
[01:59.20]"Big surprise for me to have so huge a bacterium
[02:02.92]living in the mangroves of Guadeloupe," he said.
[02:07.32]A normal bacteria measures one to five micrometers long.
[02:12.36]A micrometer is one millionth of a meter.
[02:16.24]This species averages 10,000 micrometers or one cm long.
[02:23.28]Some Thiomargarita magnifica are double that length.
[02:28.92]Volland said the bacterium is bigger "than what we thought
[02:32.84]was the maximum possible size for a single bacterium."
[02:37.72]He added, "They are about the same size and shape of an eyelash."
[02:44.40]The largest-known bacterium until now
[02:47.48]had a maximum length of around 750 micrometers.
[02:53.84]Bacteria are single-celled organisms that live nearly everywhere on Earth.
[02:59.72]Bacteria are thought to have been the first organisms
[03:03.60]and remain very simple in structure billions of years later.
[03:08.36]The bodies of humans are filled with bacteria.
[03:12.56]Only a small number of these cause disease.
[03:17.48]Caribbean mangrove swamps are filled with organic material.
[03:22.24]Microbes break down this material and produce sulfur.
[03:27.04]The sulfur-rich environment offers an energy source
[03:31.12]for bacteria like Thiomargarita magnifica.
[03:36.00]The researchers named its DNA-holding organelles "pepins"
[03:40.52]after a French word for small seeds inside fruits.
[03:45.32]"Aside from two exceptions, there were no other bacteria
[03:50.12]known to keep their DNA inside a membrane-bound organelle.
[03:55.28]That is, in fact, a characteristic of more complex cells
[04:00.40]which have a membrane-bound nucleus, such as human cells,
[04:04.76]or animal and plant cells," Volland said.
[04:08.64]A genome map showed that Thiomargarita magnifica
[04:13.24]has lost some genes needed for cell division
[04:17.00]and has more than the usual number of copies
[04:20.32]of genes responsible for cell lengthening.
[04:25.36]"The genome is also very big
[04:27.60]and contains three times the average number of genes
[04:31.44]that are usually found in bacteria. Half of those genes,
[04:36.40]we have no idea what are they for," Volland said.
[04:40.16]This bacterium shows how life on Earth
[04:43.72]still has surprises awaiting discovery, he said.
[04:48.68]"Life is fascinating, very diverse and very complex," Volland added.
[04:54.88]"It's important to stay curious and have an open mind."
[05:00.40]I'm John Russell. 更多听力请访问21VOA.COM
END OF TRACK. "END OF TRACK." The two men bowed. "Whoever was that person you were talking to?" she enquired, as soon as they stood together. The took of triumph faded from her eyes, she had grown worn and weary. The roses were wilting on the walls, the lights were mostly down now. Hetty, looking in to see if anything was wanted, found herself driven away almost fiercely. I only saw Master Jervie once when he called at tea time, The year 1747 was opened by measures of restriction. The House of Lords, offended at the publication of the proceedings of the trial of Lord Lovat, summoned the parties to their bar, committed them to prison, and refused to liberate them till they had pledged themselves not to repeat the offence, and had paid very heavy fees. The consequence of this was that the transactions of the Peers were almost entirely suppressed for nearly thirty years from this time, and we draw our knowledge of them chiefly from notes taken by Horace Walpole and Lord Chancellor Hardwicke. What is still more remarkable, the reports of the House of Commons, being taken by stealth, and on the merest sufferance, are of the most meagre kind, sometimes altogether wanting, and the speeches are given uniformly under fictitious names; for to have attributed to Pitt or Pelham their[112] speeches by name would have brought down on the printers the summary vengeance of the House. Many of the members complained bitterly of this breach of the privileges of Parliament, and of "being put into print by low fellows"; but Pelham had the sense to tolerate them, saying, "Let them alone; they make better speeches for us than we can make for ourselves." Altogether, the House of Commons exhibited the most deplorable aspect that can be conceived. The Ministry had pursued Walpole's system of buying up opponents by place, or pension, or secret service money, till there was no life left in the House. Ministers passed their measures without troubling themselves to say much in their behalf; and the opposition dwindled to Sir John Hinde Cotton, now dismissed from office, and a feeble remnant of Jacobites raised but miserable resistance. In vain the Prince of Wales and the secret instigations of Bolingbroke and Doddington stimulated the spirit of discontent; both Houses had degenerated into most silent and insignificant arenas of very commonplace business. "It certainly will be. Miss Widgeon," answered Maria, with strictly "company manners." "One who has never had a brother exposed to the constant dangers of army life can hardly understand how glad we all feel to have Si snatched from the very jaws of death and brung back to us." "Just plug at 'em as you would at a crow, and then go on your way whistlin'?" persisted Harry. "Hurroo!" echoed Hennessey; "that's the ticket." "Come forward, keeper," continued the baron, "and state how these arrows came into your hands!" "Yes." HoMEJULIA京香2018下载
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