Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
Proc Natl Acad Sci U S A. 2012 Jun 26;109(26):E1762-71. doi: 10.1073/pnas.1201731109. Epub 2012 May 31.
Diatoms are responsible for ~40% of marine primary production and are key players in global carbon cycling. There is mounting evidence that diatom growth is influenced by cobalamin (vitamin B(12)) availability. This cobalt-containing micronutrient is only produced by some bacteria and archaea but is required by many diatoms and other eukaryotic phytoplankton. Despite its potential importance, little is known about mechanisms of cobalamin acquisition in diatoms or the impact of cobalamin scarcity on diatom molecular physiology. Proteomic profiling and RNA-sequencing transcriptomic analysis of the cultured diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana revealed three distinct strategies used by diatoms to cope with low cobalamin: increased cobalamin acquisition machinery, decreased cobalamin demand, and management of reduced methionine synthase activity through changes in folate and S-adenosyl methionine metabolism. One previously uncharacterized protein, cobalamin acquisition protein 1 (CBA1), was up to 160-fold more abundant under low cobalamin availability in both diatoms. Autologous overexpression of CBA1 revealed association with the outside of the cell and likely endoplasmic reticulum localization. Cobalamin uptake rates were elevated in strains overexpressing CBA1, directly linking this protein to cobalamin acquisition. CBA1 is unlike characterized cobalamin acquisition proteins and is the only currently identified algal protein known to be implicated in cobalamin uptake. The abundance and widespread distribution of transcripts encoding CBA1 in environmental samples suggests that cobalamin is an important nutritional factor for phytoplankton. Future study of CBA1 and other molecular signatures of cobalamin scarcity identified here will yield insight into the evolution of cobalamin utilization and facilitate monitoring of cobalamin starvation in oceanic diatom communities.
硅藻是海洋初级生产力的 40%左右，是全球碳循环的关键参与者。越来越多的证据表明，硅藻的生长受到钴胺素（维生素 B(12)）可用性的影响。这种含钴的微量营养素仅由一些细菌和古菌产生，但许多硅藻和其他真核浮游植物都需要它。尽管其潜在重要性，但人们对硅藻中钴胺素的获取机制以及钴胺素缺乏对硅藻分子生理学的影响知之甚少。培养的硅藻菱形藻和拟南芥的蛋白质组学和 RNA 测序转录组分析揭示了硅藻应对低钴胺素的三种不同策略：增加钴胺素获取机制、降低钴胺素需求以及通过改变叶酸和 S-腺苷甲硫氨酸代谢来管理减少的甲硫氨酸合酶活性。一种以前未表征的蛋白，钴胺素获取蛋白 1（CBA1），在两种硅藻中，低钴胺素可用性下的丰度高达 160 倍。CBA1 的自体过表达揭示了与细胞外部和可能的内质网定位的关联。CBA1 过表达的菌株中的钴胺素摄取率升高，直接将这种蛋白与钴胺素摄取联系起来。CBA1 不同于已表征的钴胺素获取蛋白，是目前唯一被认为与钴胺素摄取有关的藻类蛋白。在环境样本中编码 CBA1 的转录本的丰度和广泛分布表明，钴胺素是浮游植物的重要营养因素。对 CBA1 和这里确定的其他钴胺素缺乏的分子特征的未来研究将深入了解钴胺素利用的演变，并有助于监测海洋硅藻群落中的钴胺素饥饿。
