The picocyanobacteria Prochlorococcus and Synechococcus are found throughout the ocean's euphotic zone, where the daily light:dark cycle drives their physiology. Periodic deep mixing events can, however, move cells below this region, depriving them of light for extended periods of time. Here, we demonstrate that members of these genera can adapt to tolerate repeated periods of light energy deprivation. Strains kept in the dark for 3 d and then returned to the light initially required 18–26 d to resume growth, but after multiple rounds of dark exposure they began to regrow after only 1–2 d. This dark-tolerant phenotype was stable and heritable; some cultures retained the trait for over 132 generations even when grown in a standard 13:11 light:dark cycle. We found no genetic differences between the dark-tolerant and parental strains of Prochlorococcus NATL2A, indicating that an epigenetic change is likely responsible for the adaptation. To begin to explore this possibility, we asked whether DNA methylation—one potential mechanism mediating epigenetic inheritance in bacteria—occurs in Prochlorococcus. LC–MS/MS analysis showed that while DNA methylations, including 6 mA and 5 mC, are found in some other Prochlorococcus strains, there were no methylations detected in either the parental or dark-tolerant NATL2A strains. These findings suggest that Prochlorococcus utilizes a yet-to-be-determined epigenetic mechanism to adapt to the stress of extended light energy deprivation, and highlights phenotypic heterogeneity as an additional dimension of Prochlorococcus diversity.

中文翻译：

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应对黑暗：海洋微微蓝细菌对反复光能剥夺的适应性反应


微微蓝藻原绿球藻和聚球藻遍布海洋的透光带，每日的光暗循环驱动着它们的生理机能。然而，周期性的深度混合事件可以将细胞移动到该区域下方，从而使它们长时间失去光照。在这里，我们证明这些属的成员可以适应忍受反复的光能剥夺。菌株在黑暗中保存3天，然后恢复光照，最初需要18-26天才能恢复生长，但经过多轮黑暗暴露后，它们仅在1-2天后就开始重新生长。这种耐暗表型是稳定且可遗传的。即使在标准的 13:11 光:暗循环中生长，一些培养物仍保留了超过 132 代的特性。我们发现原绿球藻NATL2A 的耐暗菌株和亲本菌株之间没有遗传差异，表明表观遗传变化可能是适应的原因。为了开始探索这种可能性，我们询问原绿球藻中是否存在 DNA 甲基化（一种介导细菌表观遗传的潜在机制）。 LC-MS/MS 分析表明，虽然在其他一些原绿球藻菌株中发现了 DNA 甲基化（包括 6 mA 和 5 mC），但在亲本或耐暗 NATL2A 菌株中均未检测到甲基化。这些发现表明，原绿球藻利用尚未确定的表观遗传机制来适应长期光能剥夺的压力，并强调表型异质性是原绿球藻多样性的另一个维度。