Department of Microbiology, Oregon State University, Corvallis, Oregon, United States of America.
PLoS One. 2011 May 9;6(5):e19725. doi: 10.1371/journal.pone.0019725.
Previous studies have demonstrated that Candidatus Pelagibacter ubique, a member of the SAR11 clade, constitutively expresses proteorhodopsin (PR) proteins that can function as light-dependent proton pumps. However, exposure to light did not significantly improve the growth rate or final cell densities of SAR11 isolates in a wide range of conditions. Thus, the ecophysiological role of PR in SAR11 remained unresolved. We investigated a range of cellular properties and here show that light causes dramatic changes in physiology and gene expression in Cand. P. ubique cells that are starved for carbon, but provides little or no advantage during active growth on organic carbon substrates. During logarithmic growth there was no difference in oxygen consumption by cells in light versus dark. Energy starved cells respired endogenous carbon in the dark, becoming spheres that approached the minimum predicted size for cells, and produced abundant pili. In the light, energy starved cells maintained size, ATP content, and higher substrate transport rates, and differentially expressed nearly 10% of their genome. These findings show that PR is a vital adaptation that supports Cand. P. ubique metabolism during carbon starvation, a condition that is likely to occur in the extreme conditions of ocean environments.
先前的研究表明，SAR11 类群的成员假交替单胞菌属（Candidatus Pelagibacter ubique）持续表达能够作为光依赖性质子泵起作用的视蛋白（PR）蛋白。然而，在广泛的条件下，光照并没有显著提高 SAR11 分离物的生长速度或最终细胞密度。因此，PR 在 SAR11 中的生态生理作用仍未解决。我们研究了一系列细胞特性，结果表明，光照会导致饥饿碳源的假交替单胞菌属细胞在生理和基因表达上发生剧烈变化，但在有机碳底物上的活性生长过程中几乎没有或没有优势。在对数生长期，光照和黑暗下细胞的耗氧量没有差异。在黑暗中，饥饿的细胞呼吸内源碳，变成接近细胞最小预测大小的球体，并产生大量的菌毛。在光照下，饥饿的细胞保持大小、ATP 含量和更高的底物转运速率，并差异表达近 10%的基因组。这些发现表明，PR 是一种重要的适应机制，支持假交替单胞菌属在碳饥饿期间的代谢，这种情况很可能发生在海洋环境的极端条件下。
