Most of Earth’s prokaryotes live under energy limitation, yet the full breadth of strategies that enable survival under such conditions remain poorly understood. Here we report the isolation of a bacterial strain, IA91, belonging to the candidate phylum Marine Group A (SAR406 or ‘Candidatus Marinimicrobia’) that is unable to synthesize the central cell wall compound peptidoglycan itself. Using cultivation experiments and microscopy, we show that IA91 growth and cell shape depend on other bacteria, deriving peptidoglycan, energy and carbon from exogenous muropeptide cell wall fragments released from growing bacteria. Reliance on exogenous muropeptides is traceable to the phylum’s ancestor, with evidence of vertical inheritance across several classes. This dependency may be widespread across bacteria (16 phyla) based on the absence of key peptidoglycan synthesis genes. These results suggest that uptake of exogenous cell wall components could be a relevant and potentially common survival strategy in energy-limited habitats like the deep biosphere.

地球上的大多数原核生物都生活在能量有限的情况下，但人们对在这种条件下生存的全部策略仍然知之甚少。在此，我们报告分离出一种细菌菌株 IA91，该菌株属于候选海洋 A 组门（SAR406 或“Candidatus Marinimicrobia”），该菌株本身无法合成中央细胞壁化合物肽聚糖。通过培养实验和显微镜观察，我们发现 IA91 的生长和细胞形状依赖于其他细菌，从生长的细菌释放的外源壁肽细胞壁片段中获取肽聚糖、能量和碳。对外源性胞肽的依赖可以追溯到该门的祖先，有跨多个类的垂直遗传的证据。由于缺乏关键肽聚糖合成基因，这种依赖性可能广泛存在于细菌（16 个门）中。这些结果表明，在能量有限的栖息地（如深层生物圈）中，吸收外源细胞壁成分可能是一种相关且潜在常见的生存策略。