Mutualistic interactions between vitamin B12-dependent algae and heterotrophic bacteria exhibit regulation
Mutualistic interactions between vitamin B12-dependent algae and heterotrophic bacteria exhibit regulation
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DOI：
10.1111/j.1462-2920.2012.02733.x
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发表时间：
2012-06-01
影响因子：
5.1
通讯作者：
Smith, Alison Gail
中科院分区：
文献类型：
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作者：
Kazamia, Elena;Czesnick, Hjoerdis;Smith, Alison Gail
许多藻类是维生素B12（钴胺素）的营养缺陷型，它们需要维生素B12作为B12依赖性甲硫氨酸合酶（METH）的辅因子。因为只有原核生物才能合成钴胺素，它们一定是维生素的最终来源。在实验室中，藻类和异养细菌之间的直接相互作用已经被证明，细菌提供钴胺素以换取固定碳。在这里，我们建立了一个系统，在分子水平上研究这种相互作用。在B12依赖的绿色雪腐衣原体的培养物中，我们发现了一种污染细菌，通过16S rRNA分析鉴定为中慢生根瘤菌。loti（MAFF 303099），我们发现它能够支持另一种绿色植物，B12依赖的长喙叶单胞菌（Lobomonasrostrata）的生长，以换取固定碳。这两种生物在种群数量方面形成稳定的平衡，在半连续培养中保持了许多代，表明了一定程度的调节。然而，添加维生素B12或碳源的细菌扰乱了平衡，表明共生是互利和兼性的。莱茵衣藻的生长不需要B12，因为它编码一种不依赖B12的甲硫氨酸合酶，METE，该基因通过添加外源B12而被抑制。共培养C. reinhardtii与M. loti还导致METE表达的减少，表明细菌可以将维生素递送到这种B12非依赖性细胞。我们讨论了钴胺素营养缺陷型在藻类王国的广泛分布的影响。
Many algae are auxotrophs for vitamin B12 (cobalamin), which they need as a cofactor for B12-dependent methionine synthase (METH). Because only prokaryotes can synthesize the cobalamin, they must be the ultimate source of the vitamin. In the laboratory, a direct interaction between algae and heterotrophic bacteria has been shown, with bacteria supplying cobalamin in exchange for fixed carbon. Here we establish a system to study this interaction at the molecular level. In a culture of a B12-dependent green alga Chlamydomonas nivalis, we found a contaminating bacterium, identified by 16S rRNA analysis as Mesorhizobium sp. Using the sequenced strain of M. loti (MAFF303099), we found that it was able to support the growth of B12-dependent Lobomonas rostrata, another green alga, in return for fixed carbon. The two organisms form a stable equilibrium in terms of population numbers, which is maintained over many generations in semi-continuous culture, indicating a degree of regulation. However, addition of either vitamin B12 or a carbon source for the bacteria perturbs the equilibrium, demonstrating that the symbiosis is mutualistic and facultative. Chlamydomonas reinhardtii does not require B12 for growth because it encodes a B12-independent methionine synthase, METE, the gene for which is suppressed by addition of exogenous B12. Co-culturing C. reinhardtii with M. loti also results in reduction of METE expression, demonstrating that the bacterium can deliver the vitamin to this B12-independent alga. We discuss the implications of this for the widespread distribution of cobalamin auxotrophy in the algal kingdom.