Carboxylation of pyruvate with CO₂ into the reverse tricarboxylic acid cycle promoted by water microdroplets

Pyruvate, serving as a bridge between the linear acetyl coenzyme A (acetyl-CoA) and the reductive tricarboxylic acid (rTCA) cycle, is of prime importance for life evolution. However, the carboxylation of pyruvate with carbon dioxide (CO₂) followed by coupling into the rTCA cycle remains challenging in prebiotic chemistry. Here, we present that pyruvate spontaneously undergoes carboxylation with CO_2, giving oxaloacetate, malate, and fumarate of the rTCA cycle in water microdroplets at room temperature without catalysts. The current pathway that pyruvate directly uses CO₂ as a carbon source depends on the microdroplet surface, distinct from the previously reported pathway using glyoxylate. These findings suggested that pyruvate may have participated in the cycle of enzyme-free carbon fixation on a prebiotic Earth. Furthermore, the widespread microdroplets in the atmosphere may also have acted as a potential synthesis setting for CO₂ fixation and utilization. This offers a promising strategy for climate change control and the promotion of sustainable carbon cycling in the future.