Abstract
The history of Earth’s carbon cycle reflects trends in atmospheric composition convolved with the evolution of photosynthesis. Fortunately, key parts of the carbon cycle have been recorded in the carbon isotope ratios of sedimentary rocks. The dominant model used to interpret this record as a proxy for ancient atmospheric CO2 is based on carbon isotope fractionations of modern photoautotrophs, and longstanding questions remain about how their evolution might have impacted the record. We interrogated the intersection of environment and evolution by measuring both biomass (εp) and enzymatic (εRubisco) carbon isotope fractionations of a cyanobacterial strain (Synechococcus elongatus PCC 7942) solely expressing a putative ancestral Form 1B rubisco dating to >>1 Ga. This strain, nicknamed ANC, grows in ambient pCO2 and displays larger εp values than WT, despite having a much smaller εRubisco (17.23 ± 0.61‰ vs. 25.18 ± 0.31‰, respectively). Measuring both enzymatic and biomass fractionation revealed a surprising result—ANC εp exceeded ANC εRubisco in all conditions tested, violating prevailing models of cyanobacterial carbon isotope fractionation. However, these models were corrected by accounting for cyanobacterial physiology, notably the CO2 concentrating mechanism (CCM). Our modified model indicated that powered inorganic carbon uptake systems contribute to εp, and this effect is exacerbated in ANC. These data suggested that understanding the evolution of both the CCM and rubisco is critical for interpreting the carbon isotope record, and that large fluctuations in the record may reflect the evolving efficiency of carbon fixing metabolisms as well as changes in atmospheric CO2.
Significance Statement Fossils record the past, but so too do modern organisms via comparative biology. Rubisco is the most abundant protein on the planet, and is a keystone enzyme in photosynthesis. To understand how this process has co-evolved with changes in the abundance of atmospheric carbon dioxide, we reconstructed an ancestral rubisco (>one billion years old), and generated a mutant Cyanobacteria strain that must rely on this ancient protein for growth. By measuring the carbon isotope fractionation in vitro and in vivo we found that prevailing models of carbon flow in Cyanobacteria could be corrected by accounting for known aspects of cyanobacterial physiology. This highlighted the value of considering both evolution and physiology for comparative biological approaches to understanding Earth history.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Competing Interest Statement: Authors have no competing interests.