Nature，Science两篇海洋生物学研究

　　

生物通报道：海洋生物学研究是一门了解生命的本质，海洋生物的特点和习性的科学，近些年来海洋生物资源的开发和利用已成为各海洋大国竞争的焦点之一，其中基因资源的研究和利用是重点。本期的《Nature》杂志就公布了一种海洋硅藻的基因组测序结果，另《Science》杂志则对海洋中的蓝菌进行了探讨。

在第一篇文章中，来自法国，意大利，美国，比利时，德国等多国组成的研究小组发布了三角褐指藻（Phaeodactylum tricornutum）的完整基因组序列，这是第二个被测序的硅藻。

硅藻是一类最重要的浮游生物，分布极其广泛。在世界大洋中，只要有水的地方，一般都有硅藻的踪迹，尤其是在温带和热带海区。因为硅藻种类多、数量大，因而被称为海洋的"草原"。 硅藻是一类具有色素体的单细胞植物，常由几个或很多细胞个体连结成各式各样的群体。

此次测序的硅藻与第一个被测序的硅藻——假微型海链藻（Thalassiosira pseudonana）所做的对比表明，通过硅藻与细菌之间的双向基因转移，硅藻从细菌获得了数百个基因。基因转移在硅藻演化中似乎是普遍存在的，产生了基因的非正规组合（其中包括一些来自植物和动物的基因），很可能在营养管理和环境信号作用中起重要作用。

在第二篇文章中，来自加州大学圣克鲁兹分校的研究人员报道说一组新近发现的蓝菌与之前发现的蓝菌不相同，只专注于固氮，而无需生产氧气和固定碳所需的遗传学机器。

海洋中的蓝菌通常被认作是能够完成多种任务的细菌，它通过光合作用产生了大量的地球上的氧气，它能“固定”碳和氮，将这2种元素转化为在生物学上有用的形式。而这组新近发现的蓝菌则反其道而行之，它只专注于固氮，而无需生产氧气和固定碳所需的遗传学机器。这些微生物在海洋中含量丰富，而生物固氮在控制海洋中的生物生产能力及碳通量上非常重要。因此，现在可能有必要对目前的地球上氮气和碳循环的模型进行重新评估。Jonathan Zehr及其同僚分离出了这种所谓的“UCYN-A”蓝菌并分析了该种生物的基因组。他们发现，这种蓝菌的基因型是从前在非共生蓝菌中所未知的，它们在基因上无法进行产生氧气的光合作用。一个始料未及的情况是，氧气对固氮酶（即催化固氮反应的酶）实际上是有毒的。其它已知固氮蓝菌是多细胞的生物并演化出了一系列的方略来避免使其固氮酶被氧气毒化。另一方面，UCYN-A是一种单细胞的生物，它看来用只以氮气作为标靶已经解决了这一问题。

（生物通：万纹）

原文摘要：

The Phaeodactylum genome reveals the evolutionary history of diatom genomes

Diatoms are photosynthetic secondary endosymbionts found throughout marine and freshwater environments, and are believed to be responsible for around one-fifth of the primary productivity on Earth1, 2. The genome sequence of the marine centric diatom Thalassiosira pseudonana was recently reported, revealing a wealth of information about diatom biology3, 4, 5. Here we report the complete genome sequence of the pennate diatom Phaeodactylum tricornutum and compare it with that of T. pseudonana to clarify evolutionary origins, functional significance and ubiquity of these features throughout diatoms. In spite of the fact that the pennate and centric lineages have only been diverging for 90 million years, their genome structures are dramatically different and a substantial fraction of genes (40%) are not shared by these representatives of the two lineages. Analysis of molecular divergence compared with yeasts and metazoans reveals rapid rates of gene diversification in diatoms. Contributing factors include selective gene family expansions, differential losses and gains of genes and introns, and differential mobilization of transposable elements. Most significantly, we document the presence of hundreds of genes from bacteria. More than 300 of these gene transfers are found in both diatoms, attesting to their ancient origins, and many are likely to provide novel possibilities for metabolite management and for perception of environmental signals. These findings go a long way towards explaining the incredible diversity and success of the diatoms in contemporary oceans.

Globally Distributed Uncultivated Oceanic N2-Fixing Cyanobacteria Lack Oxygenic Photosystem II

Biological nitrogen (N2) fixation is important in controlling biological productivity and carbon flux in the oceans. Unicellular N2-fixing cyanobacteria have only recently been discovered and are widely distributed in tropical and subtropical seas. Metagenomic analysis of flow cytometry–sorted cells shows that unicellular N2-fixing cyanobacteria in "group A" (UCYN-A) lack genes for the oxygen-evolving photosystem II and for carbon fixation, which has implications for oceanic carbon and nitrogen cycling and raises questions regarding the evolution of photosynthesis and N2 fixation on Earth.