Cyanobacteria and Eukaryotic Algae Use Different Chemical Variants of Vitamin B12.
Cyanobacteria and Eukaryotic Algae Use Different Chemical Variants of Vitamin B12.
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DOI：
10.1016/j.cub.2016.02.041
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发表时间：
2016-04-25
期刊：
影响因子：
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通讯作者：
Smith AG
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文献类型：
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作者：
Helliwell KE;Lawrence AD;Holzer A;Kudahl UJ;Sasso S;Kräutler B;Scanlan DJ;Warren MJ;Smith AG
真核微藻和原核蓝藻是浮游植物的主要组成部分。因此，确定控制这些初级生产者生长的因素以及它们如何相互作用，对于了解水生生态系统生产力至关重要。代表海洋和淡水栖息地的一半以上的微藻物种需要类角质辅因子B12来生长，而这种辅因子仅由某些原核生物（包括大多数蓝藻）重新合成。B12有几种化学变体，它们在功能上不一定可以互换。钴胺素是人类生物可利用的形式，其下轴配体为5,6-二甲基苯并咪唑（DMB）。在这里，我们证明了丰富的海洋蓝藻聚藻球菌只合成假钴胺素，其中下轴配体是腺嘌呤。此外，对超过100个已测序的蓝藻基因组中B12生物合成基因的生物信息学搜索，包括那些参与核苷酸环组装的基因，表明这是蓝藻更广泛合成的形式。我们进一步证明，对于代表不同谱系的几种b12依赖性微藻来说，假钴胺素的生物利用度比钴胺素低几个数量级。这表明两个主要的浮游植物群使用不同的B12货币。然而，在一个有趣的转折中，如果提供DMB，一些微藻物种可以使用假钴胺素，这表明它们能够重塑辅因子，而聚球菌却不能。这种物种特有的属性意味着藻类重塑者是B12循环的新型关键参与者，改变了我们对水生生态系统中这种营养物质流动的动态和复杂性的看法。优势海洋蓝藻只合成假钴胺素假钴胺素对真核藻类的生物可利用性要低几个数量级。某些藻类可以将假钴胺素重塑为生物可利用的形式。这意味着在光带微生物之间存在复杂的B12循环。Helliwell等人证明，光带的两大光合微生物群蓝藻和微藻使用不同形式的维生素B12。这些发现挑战了B12从生产者到消费者的线性流动的过于简化的假设，支持更复杂的B12生产、吸收和循环网络。
Eukaryotic microalgae and prokaryotic cyanobacteria are the major components of the phytoplankton. Determining factors that govern growth of these primary producers, and how they interact, is therefore essential to understanding aquatic ecosystem productivity. Over half of microalgal species representing marine and freshwater habitats require for growth the corrinoid cofactor B12, which is synthesized de novo only by certain prokaryotes, including the majority of cyanobacteria. There are several chemical variants of B12, which are not necessarily functionally interchangeable. Cobalamin, the form bioavailable to humans, has as its lower axial ligand 5,6-dimethylbenzimidazole (DMB). Here, we show that the abundant marine cyanobacterium Synechococcus synthesizes only pseudocobalamin, in which the lower axial ligand is adenine. Moreover, bioinformatic searches of over 100 sequenced cyanobacterial genomes for B12 biosynthesis genes, including those involved in nucleotide loop assembly, suggest this is the form synthesized by cyanobacteria more broadly. We further demonstrate that pseudocobalamin is several orders of magnitude less bioavailable than cobalamin to several B12-dependent microalgae representing diverse lineages. This indicates that the two major phytoplankton groups use a different B12 currency. However, in an intriguing twist, some microalgal species can use pseudocobalamin if DMB is provided, suggesting that they are able to remodel the cofactor, whereas Synechococcus cannot. This species-specific attribute implicates algal remodelers as novel and keystone players of the B12 cycle, transforming our perception of the dynamics and complexity of the flux of this nutrient in aquatic ecosystems. Dominant marine cyanobacteria synthesize only pseudocobalamin Pseudocobalamin is orders of magnitude less bioavailable to eukaryotic algae Certain algae can remodel pseudocobalamin to a bioavailable form This implies a complex B12 cycle between microbes in the photic zone Helliwell et al. demonstrate that the two major groups of photosynthetic microbes in the photic zone, cyanobacteria and microalgae, use different forms of vitamin B12. These findings challenge the oversimplified assumption of a linear flux of B12 from producers to consumers, in favor of a more complex network of B12 production, uptake, and cycling.