Phenotypic plasticity allows individuals to respond to the selective forces of a new environment, followed by adaptive evolution. We do not know to what extent phenotypic plasticity allows thermal tolerance evolution in bacteria at the border of their physiological limits. We analyzed growth and reaction norms to temperature of strains of two bacterial lineages, Bacillus cereus sensu lato and Bacillus subtilis sensu lato, that evolved in two contrasting environments, a temperate lagoon (T) and a hot spring (H). Our results showed that despite the co-occurrence of members of both lineages in the two contrasting environments, norms of reactions to temperature exhibited a similar pattern only in strains within the lineages, suggesting fixed phenotypic plasticity. Additionally, strains from the H environment showed only two to three degrees centigrade more heat tolerance than strains from the T environment. Their viability decreased at temperatures above their optimal for growth, particularly for the B. cereus lineage. However, sporulation occurred at all temperatures, consistent with the known cell population heterogeneity that allows the Bacillus to anticipate adversity. We suggest that these mesophilic strains survive in the hot-spring as spores and complete their life cycle of germination and growth during intermittent opportunities of moderate temperatures. The limited evolutionary changes towards an increase in heat tolerance in bacteria should alert us of the negative impact of climate change on all biological cycles in the planet, which at its most basic level depends on microorganisms.

中文翻译：

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温带和温泉环境细菌的表型可塑性和耐热性演变

表型可塑性允许个体对新环境的选择性力量做出反应，然后进行适应性进化。我们不知道表型可塑性在多大程度上允许细菌在其生理极限边缘进行热耐受性进化。我们分析了两种细菌谱系（蜡样芽孢杆菌和感觉枯草芽孢杆菌）菌株的生长和反应规范，它们在两个截然不同的环境中进化，温带泻湖 (T) 和温泉 (H)。我们的结果表明，尽管两个谱系的成员在两种对比环境中同时出现，但对温度的反应规范仅在谱系内的菌株中表现出相似的模式，表明具有固定的表型可塑性。此外，来自 H 环境的菌株的耐热性仅比来自 T 环境的菌株高 2 到 3 摄氏度。它们的生存力在高于其最佳生长温度时会降低，尤其是对于蜡状芽孢杆菌谱系而言。然而，孢子形成在所有温度下都会发生，这与已知的细胞群异质性一致，这使得芽孢杆菌能够预测逆境。我们建议这些嗜温菌株在温泉中以孢子的形式存活，并在间歇性中等温度的机会下完成其发芽和生长的生命周期。细菌耐热性增加的有限进化变化应该提醒我们气候变化对地球上所​​有生物循环的负面影响，这在最基本的层面上取决于微生物。