Thermophilic microorganisms are expected to have smaller cells and genomes compared with mesophiles, a higher proportion of horizontally acquired genes, and distinct nucleotide and amino acid composition signatures. Here, we took an integrative approach to investigate these apparent correlates of thermophily for Synechococcus A/B cyanobacteria, which include the most heat-tolerant phototrophs on the planet. Phylogenomics confirmed a unique origin of different thermotolerance ecotypes, with low levels of continued gene flow between ecologically divergent but overlapping populations, which has shaped the distribution of phenotypic traits along these geothermal gradients. More thermotolerant strains do have smaller genomes, but genome reduction is associated with a decrease in community richness and metabolic diversity, rather than with cell size. Horizontal gene transfer played only a limited role during Synechococcus evolution, but, the most thermotolerant strains have acquired a Thermus tRNA modification enzyme that may stabilize translation at high temperatures. Although nucleotide base composition was not associated with thermotolerance, we found a general replacement of aspartate with glutamate, as well as a dramatic remodeling of amino acid composition at the highest temperatures that substantially differed from previous predictions. We conclude that Synechococcus A/B genome diversification largely does not conform to the standard view of temperature adaptation. In addition, carbon fixation was more thermolabile than photosynthetic oxygen evolution for the most thermotolerant strains compared with less tolerant lineages. This suggests that increased flow of reducing power generated during the light reactions to an electron sink(s) beyond carbon dioxide has emerged during temperature adaptation of these bacteria.

中文翻译：

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嗜热蓝藻 Synechococcus 在光养极限下的特殊基因组进化


与嗜温微生物相比，嗜热微生物预计具有更小的细胞和基因组、更高比例的水平获得基因以及不同的核苷酸和氨基酸组成特征。在这里，我们采取了一种综合方法来研究聚球菌 A/B 蓝细菌的嗜热性这些明显的相关性，其中包括地球上最耐热的光养生物。系统发育基因组学证实了不同耐热生态型的独特起源，生态差异但重叠的种群之间的持续基因流动水平较低，这塑造了沿这些地热梯度的表型性状的分布。更耐热的菌株确实具有较小的基因组，但基因组减少与群落丰富度和代谢多样性的降低有关，而不是与细胞大小有关。水平基因转移在 Synechococcus 进化过程中起的作用有限，但是，最耐热的菌株已经获得了一种 Thermus tRNA 修饰酶，该酶可以在高温下稳定翻译。尽管核苷酸碱基组成与耐热性无关，但我们发现谷氨酸普遍取代了天冬氨酸，并且在最高温度下氨基酸组成发生了戏剧性的重塑，这与以前的预测大相径庭。我们得出结论，聚球菌 A/B 基因组多样化在很大程度上不符合温度适应的标准观点。此外，与耐受性较差的谱系相比，对于耐热性最强的菌株，碳固定比光合析氧更不耐热。 这表明在这些细菌的温度适应过程中，出现了光反应过程中产生的还原功率流向二氧化碳以外的电子汇。