Geochemical roots of autotrophic carbon fixation: Hydrothermal experimentsin the system citric acid, H2O-(+/- FeS)-(+/- NiS)

Recent theories have proposed that life arose from primitive hydrothermal e nvironments employing chemical reactions analogous to the reductive citrate cycle (RCC) as the primary pathway for carbon fixation. This chemistry is presumed to have developed as a natural consequence of the intrinsic geoche mistry of the young, prebiotic, Earth. There has been no experimental evide nce, however, demonstrating that there exists a natural pathway into such a cycle. Toward this end, the results of hydrothermal experiments involving citric acid are used as a method of deducing such a pathway. Homocatalytic reactions observed in the citric acid-H2O experiments encompass many of the reactions found in modem metabolic systems, i.e., hydration-dehydration, r etro-Aldol, decarboxylation, hydrogenation, and isomerization reactions. Th ree principal decomposition pathways operate to degrade citric acid under t hermal and aquathermal conditions. It is concluded that the acid catalyzed beta gamma decarboxylation pathway, leading ultimately to propene and CO2, may provide the most promise for reaction network reversal under natural hy drothermal conditions. Increased pressure is shown to accelerate the princi pal decarboxylation reactions under strictly hydrothermal conditions. The e ffect of forcing the pH via the addition of NaOH reveals that the decarboxy lation pathway operates even up to intermediate pH levels. The potential fo r network reversal (the conversion of propene and CO2 up to a tricarboxylic acid) is demonstrated via the Koch (hydrocarboxylation) reaction promoted heterocatalytically with NiS in the presence of a source of CO. Specificall y, an olefin (1-nonene) is converted to a monocarboxylic acid; methacrylic acid is converted to the dicarboxylic acid, methylsuccinic acid; and the di carboxylic acid, itaconic acid, is converted into the tricarboxylic acid, h ydroaconitic acid. A number of interesting sulfur-containing products are a lso formed that may provide for additional reaction. The intrinsic catalyti c qualities of FeS and NiS are also explored in the absence of CO. It was s hown that the addition of NiS has a minimal effect in the product distribut ion, whereas the addition of FeS leads to the formation of hydrogenated and sulfur-containing products (thioethers). These results point to a simple h ydrothermal redox pathway for citric acid synthesis that may have provided a geochemical ignition point for the reductive citrate cycle. Copyright (C) 2001 Elsevier Science Ltd.