Reduction of humic substances and Fe(III) by hyperthermophilic microorganisms

The ability of hyperthermophilic microorganisms to transfer electrons to hu mic substances (humics) and other extracellular quinones was evaluated. Whe n H-2 was provided as the electron donor, the hyperthermophile, Pyrobaculum islandicum, transferred electrons to highly purified humics and the humics analog, anthraquinone-2,6-disulfonate (AQDS). A diversity of other hyperth ermophilic Archaea including: Pyrodictium abyssi, Pyrococcus furiosus, Arch aeoglobus fulgidus, Thermococcus celer, Methanopyrus kandleri, as well as t he thermophiles Methanococcus thermolithitrophicus and Methanobacterium the rmoautotrophicum, exhibited H-2-dependent AQDS reduction as did the hyperth ermophilic bacterium Thermotoga maritima. AQDS acted as an electron shuttle between P. islandicum and poorly crystalline Fe(III) oxide and greatly acc elerated rates of Fe(III) reduction. Electron shuttling by AQDS also promot ed the reduction of the crystalline Fe(III) oxide forms, goethite and hemat ite. These results have implications for the potential mechanisms of Fe(III ) reduction in various hot Fe(III)-containing environments such as near hyd rothermal marine vents, terrestrial hot springs, and the deep terrestrial s ubsurface. The finding that the ability to reduce extracellular quinones is a characteristic of ail of the hyperthermophiles evaluated and the fact th at these hyperthermophiles are the organisms most closely related to the la st common ancestor of extant organisms suggests that the last common ancest or had the ability to reduce humics. In combination with plausible geochemi cal scenarios, these results suggest that electron transfer to extracellula r quinones and Fe(III) were initial steps in the eventual evolution of intr acellular electron transport chains that employ quinones and iron-containin g proteins. (C) 2000 Elsevier science B.V. All rights reserved.