The Life Evolution on the Sulfur Cycle: From Ancient Elemental Sulfur Reduction and Sulfide Oxidation to the Contemporary Thiol-Redox Challenges

Organismal evolution led to innovations in metabolic pathways, many of which certainly modified the surface chemistry of the Earth. Volcanic activity introduced inorganic com pounds (H 2 , CO 2 , CH 4 , SO 2 , and H 2 S) driving the metabolism of early organisms of the domains archaea and bacteria. In the absence of light, H 2 S and Fe 2+ would have been the major electron donors and the electron acceptors could be either oxidized species such as the sulfurs, sulfate, and elemental sulfur, or carbon dioxide by the fermentation of acetate (forming methane). Elemental sulfur was produced by the reaction between H 2 S and SO 2 , while anoxygenic photosynthesis may have provided the sulfate which removed oceanic ferrous iron by its precipitation as sulfide into sediments. Hence, the sulfur cycle par ticipation in life evolution comes from ancient anoxygenic elemental sulfur reduction generating environmental sulfide incorporated as mitochondrial Fe-S for the electron-transport chains. Anoxygenic photosynthesis may have provided the necessary sulfate to promote the evolution of sulfate-reducing bacteria. The evolution of oxygenic photosyn thesis provided for diverse metabolic possibilities including non-photosynthetic sulfide oxidation, nitrification, and methanotrophy. An increase in badly insulated wires in the presence of oxygen (with reduced respiratory complexes) and there is leakage of electrons on to molecular oxygen. The electron leakage results in the formation of superoxide anion (SO) that remains within the mitochondrial matrix. If not promptly detoxified by anti-oxidative defenses, SO and its derived-oxidative species can alter cell signaling or attack cell structures leading to cell apoptosis. Sulfur-containing compounds participate either in oxidative stress generation (at endoplasmatic reticulum) or in (thiol) antioxidant defenses (mainly glutathione), thus functioning as redox sensing for enzyme activity and gene expression. Sulfur compounds that contributed for electron leakage and oxidative stress have counteractions by thiol participation either as antioxi - dant defensors and/or as redox-modulators or cell functions, influencing life evolution and contemporary diseases.