Marchetti Adrian, Parker Micaela S, Moccia Lauren P, Lin Ellen O, Arrieta Angele L, Ribalet Francois, Murphy Michael E P, Maldonado Maria T, Armbrust E Virginia
School of Oceanography, University of Washington, Box 357940, Seattle, Washington 98195, USA.
Nature. 2009 Jan 22;457(7228):467-70. doi: 10.1038/nature07539. Epub 2008 Nov 26.
Primary productivity in 30-40% of the world's oceans is limited by availability of the micronutrient iron. Regions with chronically low iron concentrations are sporadically pulsed with new iron inputs by way of dust or lateral advection from continental margins. Addition of iron to surface waters in these areas induces massive phytoplankton blooms dominated primarily by pennate diatoms. Here we provide evidence that the bloom-forming pennate diatoms Pseudo-nitzschia and Fragilariopsis use the iron-concentrating protein, ferritin, to safely store iron. Ferritin has not been reported previously in any member of the Stramenopiles, a diverse eukaryotic lineage that includes unicellular algae, macroalgae and plant parasites. Phylogenetic analyses suggest that ferritin may have arisen in this small subset of diatoms through a lateral gene transfer. The crystal structure and functional assays of recombinant ferritin derived from Pseudo-nitzschia multiseries reveal a maxi-ferritin that exhibits ferroxidase activity and binds iron. The protein is predicted to be targeted to the chloroplast to control the distribution and storage of iron for proper functioning of the photosynthetic machinery. Abundance of Pseudo-nitzschia ferritin transcripts is regulated by iron nutritional status, and is closely tied to the loss and recovery of photosynthetic competence. Enhanced iron storage with ferritin allows the oceanic diatom Pseudo-nitzschia granii to undergo several more cell divisions in the absence of iron than the comparably sized, oceanic centric diatom Thalassiosira oceanica. Ferritin in pennate diatoms probably contributes to their success in chronically low-iron regions that receive intermittent iron inputs, and provides an explanation for the importance of these organisms in regulating oceanic CO(2) over geological timescales.
世界上30%-40%的海洋初级生产力受微量营养元素铁的可利用性限制。铁浓度长期较低的区域会偶尔因沙尘或来自大陆边缘的侧向平流带来新的铁输入而出现脉冲现象。在这些区域向表层水体添加铁会引发主要由羽纹硅藻主导的大规模浮游植物水华。在此，我们提供证据表明，形成水华的羽纹硅藻拟菱形藻属和脆杆藻属利用铁浓缩蛋白铁蛋白来安全储存铁。此前在不等鞭毛类的任何成员中都未报道过铁蛋白，不等鞭毛类是一个多样的真核生物谱系，包括单细胞藻类、大型藻类和植物寄生虫。系统发育分析表明，铁蛋白可能通过横向基因转移出现在这一小部分硅藻中。源自多列拟菱形藻的重组铁蛋白的晶体结构和功能分析揭示了一种具有铁氧化酶活性并能结合铁的大型铁蛋白。该蛋白预计靶向叶绿体以控制铁的分布和储存，从而使光合机构正常运作。拟菱形藻铁蛋白转录本的丰度受铁营养状况调节，并与光合能力的丧失和恢复密切相关。与同等大小的海洋中心硅藻海洋海链藻相比，利用铁蛋白增强铁储存使海洋硅藻颗粒拟菱形藻在缺铁情况下能多进行几次细胞分裂。羽纹硅藻中的铁蛋白可能有助于它们在长期缺铁但会间歇性获得铁输入的区域取得成功，并解释了这些生物在地质时间尺度上调节海洋二氧化碳方面的重要性。
