Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA.
School of Oceanography, Shanghai Jiao Tong University, Shanghai, China.
Nat Ecol Evol. 2023 Sep;7(9):1398-1407. doi: 10.1038/s41559-023-02147-0. Epub 2023 Aug 3.
The reverse tricarboxylic acid (rTCA) cycle is touted as a primordial mode of carbon fixation due to its autocatalytic propensity and oxygen intolerance. Despite this inferred antiquity, however, the earliest rock record affords scant supporting evidence. In fact, based on the chimeric inheritance of rTCA cycle steps within the Chlorobiaceae, even the use of the chemical fossil record of this group is now subject to question. While the 1.64-billion-year-old Barney Creek Formation contains chemical fossils of the earliest known putative Chlorobiaceae-derived carotenoids, interferences from the accompanying hydrocarbon matrix have hitherto precluded the carbon isotope measurements necessary to establish the physiology of the organisms that produced them. Overcoming this obstacle, here we report a suite of compound-specific carbon isotope measurements identifying a cyanobacterially dominated ecosystem featuring heterotrophic bacteria. We demonstrate chlorobactane is C-depleted when compared to contemporary equivalents, showing only slight C-enrichment over co-existing cyanobacterial carotenoids. The absence of this diagnostic isotopic fingerprint, in turn, confirms phylogenomic hypotheses that call for the late assembly of the rTCA cycle and, thus, the delayed acquisition of autotrophy within the Chlorobiaceae. We suggest that progressive oxygenation of the Earth System caused an increase in the marine sulfate inventory thereby providing the selective pressure to fuel the Neoproterozoic shift towards energy-efficient photoautotrophy within the Chlorobiaceae.
反向三羧酸 (rTCA) 循环因其自动催化倾向和对氧气的不耐受性而被誉为原始的碳固定模式。然而，尽管有这种推断的古老性，但最早的岩石记录提供的证据很少。事实上，根据 Chlorobiaceae 内 rTCA 循环步骤的嵌合遗传，即使使用该组的化学化石记录也受到质疑。虽然 16.4 亿年前的 Barney Creek 地层含有最早已知的假定 Chlorobiaceae 衍生类胡萝卜素的化学化石，但伴随的碳氢化合物基质的干扰迄今为止阻止了确定产生它们的生物体生理学所需的碳同位素测量。克服这一障碍，我们在这里报告了一系列特定于化合物的碳同位素测量结果，这些结果确定了一个以蓝细菌为主导的生态系统，其中存在异养细菌。我们证明与当代等同物相比，chlorobactane 是 C 耗尽的，与共存的蓝细菌类胡萝卜素相比仅略有 C 富集。这种诊断性同位素指纹的缺失反过来证实了系统发育基因组学假说，即需要 rTCA 循环的后期组装，从而导致 Chlorobiaceae 中自养作用的延迟获得。我们认为，地球系统的逐渐氧化导致海洋硫酸盐库存增加，从而提供了选择性压力，推动了新元古代在 Chlorobiaceae 中向节能光合作用的转变。
