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硅藻基因组
硅藻充满了海洋和湖泊，它们在碳循环中扮演着重要的角色，硅藻每年生产5百多亿吨的有机碳，也是许多动物的重要粮食。自2002年开始，美国能源部附属基因组研究所的Daniel Rokhsar和西雅图华盛顿大学的Virginia Armbrust等人，就开始解读海洋硅藻Thalassiosira pseudonana的基因组。该基因组草图共有3千4百万个碱基，和约1万1千5百多个基因。
分析了硅藻的基因和蛋白质，显示它们有一段复杂的过去。和其它微生物一样，它们靠吞噬其它微生物邻居的DNA而获取新基因。它们从其它藻类细胞中得到进行光合作用的装置。有些科学家认为硅藻在动植物分家前就自成一家了。
这个基因组也让科学家能够研究硅藻如何建构复杂精巧的硅壳子。他们也发现了好些和硅沉积有关的蛋白质，而且也预期还有更多。Armbrust指出，以往海洋学家以为我们了解了硅藻如何利用氮，可是他们却很惊呀地发现原来硅藻也有尿酸循环，这个动物的氮代谢路径在行光合作用的生物中从未发现过。
原学术论文：
Armbrust, E. V. et al. The Genome of the diatom Thalassiosira pseudonana: ecology, evolution, and metabolism. Science 306, 79-86 (2004). Scientists sequence genome of kind of organism central to biosphere's carbon cycle
The first ever genomic map of a diatom, part of a family of microscopic ocean algae that are among the Earth's most important inhabitants, has yielded surprising insights about the way they may be using nitrogen, fats and silica in order to thrive.
Diatoms, most of which are far too tiny to see without magnification, are nevertheless thought to absorb carbon dioxide, a major greenhouse gas, in amounts comparable to all the world's tropical rain forests combined.
"These organisms are incredibly important in the global carbon cycle," says Virginia Armbrust, a University of Washington associate professor of oceanography and lead author of a research article in the Oct. 1 issue of Science. Together, these single-celled organisms generate as much as 40 percent of the 50 billion to 55 billion tons of organic carbon produced each year in the sea, and in the process use carbon dioxide and produce oxygen. And they are an important food source for many other marine organisms.
The genome work, funded by the Department of Energy and conducted at its Joint Genome Institute in California, gives insight into how the diatom species Thalassiosira pseudonana prospers in the marine environment, Armbrust says. It's important to understand because diatoms like Thalassiosira pseudonana and other phytoplankton are vital components of the biosphere's role in mediating global warming.
"Now that we have a glimpse at the inner workings of diatoms, we're better positioned to understand how changes in the environment will translate into increases or decreases in diatom abundance," says Dan Rokhsar, who heads computational genomics at the Joint Genome Institute and one of the co-authors on the article.
Scientists would like to better understand how these organisms react to changes in sea temperatures, the amount of light penetrating the oceans and nutrients.
"Oceanographers thought we understood how diatoms use nitrogen, but we discovered they have a urea cycle, something no one ever suspected," Armbrust says. A urea cycle is a nitrogen waste pathway found in animals and has never before been seen in a photosynthetic eukaryote like a diatom, she says. Nitrogen is crucial for diatom growth and is often in short supply in sea water, depending on ocean conditions. The genome work revealed that diatom Thalassiosira pseudonana has the genes to produce urea-cycle enzymes that may help to reduce its dependence on nitrogen from the surrounding waters.
The genome work also shed additional light on how this diatom species uses fats, or lipids, that it is known to store in huge amounts.
"Learning the actual pathways they use to metabolize their fats helps explain the ability of diatoms to withstand long periods with little sunlight ?even to overwinter ?and then start growing really rapidly once they return to sunlight," she says.
Three or four microns in width ?as many as 70 could fit in the width of a human hair ?Thalassiosira pseudonana is among the smallest diatoms. Like its brethren, it is encased by a frustule, a rigid cell wall delicately marked with pores in patterns distinctive enough for scientists to tell the species apart. Another new finding reported in Science concerns the unusual way the diatom metabolizes silicon to form its characteristically ornate silica frustule.
"Diatoms can manipulate silica in ways that nanotechnologists can only dream about. If we understood how they can design and build their patterned frustule as part of their biology, perhaps this could be adapted by humans," Rokhsar says.
Scientists on the project, which includes 46 researchers from 26 institutions, also considered the evolutionary implications revealed by the genomic work. The research provided direct genetic confirmation of a hypothesis that diatoms evolved when a heterotroph, a single-cell microbe, engulfed what scientists say was likely a kind of red alga. The two became one organism, an arrangement called endosymbiosis, and swapped some genetic material to create a new hybrid genome.
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作者:admin@医学,生命科学 2011-09-18 17:15
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