Alternative Pathways of Carbon Dioxide Fixation: Insights into the Early Evolution of Life?

The fixation of inorganic carbon into organic material (autotrophy) is a prerequisite for life and sets the starting point of biological evolution. In the extant biosphere the reductive pentose phosphate (Calvin-Benson) cycle is the predominant mechanism by which many prokaryotes and all plants fix CO2 into biomass. However, the fact that five alternative autotrophic pathways exist in prokaryotes is often neglected. This bias may lead to serious misjudgments in models of the global carbon cycle, in hypotheses on the evolution of metabolism, and in interpretations of geological records. Here, I review these alternative pathways that differ fundamentally from the Calvin-Benson cycle. Revealingly, these five alternative pathways pivot on acetyl-coenzyme A, the turntable of metabolism, demanding a gluconeogenic pathway starting from acetyl-coenzyme A and CO2. It appears that the formation of an activated acetic acid from inorganic carbon represents the initial step toward metabolism. Consequently, biosyntheses likely started from activated acetic acid and gluconeogenesis preceded glycolysis.