Abstract：
Most autotrophic organisms possess a single carbon fixation pathway. The chemoautotrophic symbionts of the hydrothermal vent tubeworm Riftia pachyptila, however, possess two functional pathways: the Calvin-Benson-Bassham (CBB) and the reductive tricarboxylic acid (rTCA) cycles. How these two pathways are coordinated is unknown. Here we measured net carbon fixation rates, transcriptional/metabolic responses and transcriptional co-expression patterns of Riftia pachyptila endosymbionts by incubating tubeworms collected from the East Pacific Rise at environmental pressures, temperature and geochemistry. Results showed that rTCA and CBB transcriptional patterns varied in response to different geochemical regimes and that each pathway is allied to specific metabolic processes; the rTCA is allied to hydrogenases and dissimilatory nitrate reduction, whereas the CBB is allied to sulfide oxidation and assimilatory nitrate reduction, suggesting distinctive yet complementary roles in metabolic function. Furthermore, our network analysis implicates the rTCA and a group 1e hydrogenase as key players in the physiological response to limitation of sulfide and oxygen. Net carbon fixation rates were also exemplary, and accordingly, we propose that co-activity of CBB and rTCA may be an adaptation for maintaining high carbon fixation rates, conferring a fitness advantage in dynamic vent environments.
摘要：
大多数自养生物具有单一的碳固定途径。热液喷口管虫Riftiapachyptila的化学自养共生体，然而,具有两种功能途径：Calvin-Benson-Bassham（CBB）和还原性三羧酸（rTCA）循环。这两种途径如何协调是未知的。在这里，我们测量了净碳固定率，通过在环境压力下孵化从东太平洋上升收集的管虫，Riftiapachyptia内共生体的转录/代谢反应和转录共表达模式，温度和地球化学。结果表明，rTCA和CBB转录模式随不同的地球化学机制而变化，并且每种途径与特定的代谢过程有关；rTCA与氢化酶和异化硝酸盐还原有关，而CBB与硫化物氧化和同化硝酸盐还原有关，提示在代谢功能中具有独特但互补的作用。此外，我们的网络分析暗示rTCA和一组1e氢化酶是对硫化物和氧气限制的生理反应的关键参与者。净碳固定率也是示例性的，因此，我们建议CBB和rTCA的共活性可能是维持高碳固定率的一种适应方法，在动态通风环境中赋予健身优势。
