Scenarios for driving the Krebs cycle in reverse in the absence of enzymatic networks have received much attention in the literature of prebiotic evolution because repeated cycling provides a core mechanism for the synthesis of useful biomolecules from carbon dioxide. Here, we show that three of the five reductive steps in the cycle can be driven photochemically at high yield by a ZnS colloid. The results show the conversion of oxaloacetate to malate, fumarate to succinate, and oxoglutarate to oxalosuccinate. The experimental conditions of circumneutral pH and 288 K, as well as the ZnS semiconductor photocatalyst particles (sphalerite), are believed to have been prevalent in the waters of early Earth. These findings therefore both establish and constrain the plausibility of the occurrence of heretofore difficult chemical conversions in prebiotic metabolic systems.

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Photoelectrochemistry on mineral surfaces has the potential to play a central role in the prebiotic syntheses of building blocks for biomolecules. In this study, photoreduction of C(+IV) as bicarbonate is used as a probe to investigate the photoelectrochemical properties of alabandite (MnS) colloidal particles. Our experimental results show that photoreduction occurs and that formate is the initial photoproduct. A quantum efficiency of 4.2% is obtained (pH = 7.5). The quantum efficiency is temperature-independent from 298 to 328 K. In addition to formate, longer chain carbon products are also produced. Ion chromatography shows the presence of acetate and propionate. Infrared spectroscopy and mass spectrometry indicate the formation of longer chain organic molecules that contain oxygenated functional groups. Our results suggest that some prebiotic syntheses could have occurred via photoelectrochemical reactions on semiconducting minerals.