Department of Microbiology, Oregon State University, Corvallis, OR, USA.
Environ Microbiol. 2019 Jul;21(7):2559-2575. doi: 10.1111/1462-2920.14649. Epub 2019 May 21.
Marine bacterioplankton face stiff competition for limited nutrient resources. SAR11, a ubiquitous clade of very small and highly abundant Alphaproteobacteria, are known to devote much of their energy to synthesizing ATP-binding cassette periplasmic proteins that bind substrates. We hypothesized that their small size and relatively large periplasmic space might enable them to outcompete other bacterioplankton for nutrients. Using uptake experiments with C-glycine betaine, we discovered that two strains of SAR11, Candidatus Pelagibacter sp. HTCC7211 and Cand. P. ubique HTCC1062, have extraordinarily high affinity for glycine betaine (GBT), with half-saturation (K ) values around 1 nM and specific affinity values between 8 and 14 L mg cell h . Competitive inhibition studies indicated that the GBT transporters in these strains are multifunctional, transporting multiple substrates in addition to GBT. Both strains could use most of the transported compounds for metabolism and ATP production. Our findings indicate that Pelagibacter cells are primarily responsible for the high affinity and multifunctional GBT uptake systems observed in seawater. Maximization of whole-cell affinities may enable these organisms to compete effectively for nutrients during periods when the gross transport capacity of the heterotrophic plankton community exceeds the supply, depressing ambient concentrations.
海洋细菌浮游生物在有限的营养资源方面面临激烈的竞争。SAR11 是一种普遍存在的非常小且高度丰富的α变形菌门，已知它们将大量能量用于合成结合底物的 ATP 结合盒周质蛋白。我们假设它们的小尺寸和相对较大的周质空间可能使它们能够与其他细菌浮游生物竞争营养物质。我们使用 C-甘氨酸甜菜碱摄取实验发现，两种 SAR11 菌株，Candidatus Pelagibacter sp. HTCC7211 和 Cand. P. ubique HTCC1062，对甘氨酸甜菜碱（GBT）具有极高的亲和力，半饱和值（K ）约为 1 nM，比活值在 8 到 14 L/mg 细胞 h 之间。竞争抑制研究表明，这些菌株中的 GBT 转运蛋白是多功能的，除了 GBT 之外还可以转运多种底物。两种菌株都可以利用大部分转运的化合物进行代谢和 ATP 产生。我们的研究结果表明，Pelagibacter 细胞主要负责海水中观察到的高亲和力和多功能 GBT 摄取系统。全细胞亲和力的最大化可能使这些生物在异养浮游生物群落的总转运能力超过供应时，有效地竞争营养物质，从而降低环境浓度。
