發現與人類睫毛一樣長的創紀錄細菌

3個月前
發現與人類睫毛一樣長的創紀錄細菌

"這些細菌大約比大多數細菌大5000倍。美國勞倫斯伯克利國家實驗室聯合基因組研究所的讓-馬里-沃蘭德說:"從這個角度來看,它相當於我們人類遇到了另一個像珠穆朗瑪峰一樣高的人。

幾厘米長的T. magnifica並不是地球上最大的單細胞生物體。那可能是一種叫做Caulerpa taxifolia的水生藻類,它仍然是10倍長。但是當你考慮到地球上有許多複雜得多的生命形式需要某种放大鏡來觀察時,這種細菌絕對令人印象深刻。想想外面那些非常小的蠕蟲和蒼蠅。

T. magnifica最早於2009年在小安的列斯群島的瓜德羅普島被發現。但它最初被擱置在一邊。直到最近,Volland博士及其同事才開始詳細研究它。

這是一個重要的啟示,因為直到現在,DNA在一個有膜的隔間內的包裝被認為是所謂的真核細胞的專利,它是人類、其他動物和植物等高等生物的組成部分。

而T. magnifica攜帶有大量的DNA。如果你計算它的生命密碼或基因組中的所有 "字母",或鹼基,大約有1200萬。但是在每個細胞中,可能有50萬個基因組的副本。

"如果你現在把1200萬個鹼基的基因組大小,乘以基因組副本的數量--因此,50萬--你最終有大約6000個千兆,或十億個鹼基的DNA。作為比較,一個二倍體人類基因組的大小約為6千兆鹼基。因此,這意味着與人類細胞相比,我們的Thiomargarita本身儲存了幾個數量級的DNA,"同樣來自勞倫斯伯克利的Tanja Woyke博士解釋道。

她補充說,在所有這些DNA中,有一些關於該生物體巨大尺寸的驅動因素的線索。一些與伸長有關的基因似乎被重複了,而一些通常參與分裂的基因似乎被遺漏了。

T. magnifica是一種化學合成細菌。它通過氧化紅樹林沼澤沉積物中腐爛的有機物所產生的硫化合物來製造它所需的糖類燃料。它所需要的只是一些堅固的東西來掛在上面。

"它們只需要一些堅硬的基質與硫化物接觸,並與海水接觸以獲得氧氣和二氧化碳。我發現的最高濃度的Thiomargarita是在一個塑料袋上--很不幸。"


"These bacteria are about 5,000 times larger than most bacteria. And to put things into perspective, it is the equivalent for us humans to encounter another human who would be as tall as Mount Everest," said Jean-Marie Volland from the Joint Genome Institute at the Lawrence Berkeley National Laboratory, in the US.

Centimetre-long T. magnifica is not the largest single-celled organism on Earth. That's probably a type of aquatic alga called Caulerpa taxifolia which is 10 times longer still. But the bacterium is definitely impressive when you consider there are many, much more complex life forms on Earth that require some sort of magnification to be observed. Think of those really teeny worms and flies out there.

T. magnifica was first identified back in 2009 in Guadeloupe, in the Lesser Antilles. But it was initially put to one side. Only recently have Dr Volland and colleagues got around to studying it in detail.

It's a significant revelation because until now, the packing of DNA inside a membrane-bound compartment was considered the preserve of so-called eukaryotic cells, which are the building blocks of higher organisms such as humans, other animals and plants.

And T. magnifica carries a lot of DNA. If you count all the "letters", or bases, in its life code, or genome, there are some 12 million. But in each cell, there may be half a million copies of the genome.

"If you now take the genome size of 12 million bases, multiply that by the number of genome copies - so, half a million - you end up with approximately 6,000 giga, or billion, bases, of DNA. For comparison, a diploid human genome is approximately six giga bases in size. So this means that our Thiomargarita stores several orders of magnitude more DNA in itself as compared to a human cell," explained Dr Tanja Woyke, also from Lawrence Berkeley.

In all that DNA, there are clues to the drivers of the organism's great size, she added. Some genes associated with elongation seem to be duplicated and some genes ordinarily involved in division appear to be missing.

T. magnifica is a chemosynthetic bacterium. It makes the sugars it needs to fuel itself by oxidising the sulphur compounds produced by the rotting organic matter in the sediments of the mangrove swamp. All it needs is something solid to hang on to.

"They just need some hard substrate to be in contact with the sulphides and in contact with the seawater to get oxygen and CO2. The highest concentration of Thiomargarita I found was on a plastic bag - unfortunately."

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