Sulfate- and Sulfur-Reducing Bacteria — Possible Analogs for Microbial Life on Jupiter's Satellite Io

by

Elena V. Pikuta
17 July 2001

Observations from the Voyager and Galileo spacecraft have shown Jupiter's moon Io to be the most volcanically active body of our Solar System. The Galileo Near Infrared Imaging Spectrometer (NIMS) revealed extensive deposits of sulfur compounds, elemental sulfur and SO₅ frost on the surface of Io. The surface of Io has extreme temperature variations — ranging from -143C to >2000C at the Pillan Patera volcanic vent. The presence of active volcanoes, fumaroles, calderas, and lava lakes and vast sulfur deposits on this frozen moon has lead us to consider the significance of Io to Astrobiology. In this paper we discuss the Sulfur- and Sulfate-reducing microbiota of Earth as possible analogs for the types of life forms that may be capable of thriving on this intriguing volcanic moon of Jupiter.

Earth's life forms that depend on sulfur are members of two domains- Bacteriaand Archaea. Two basic links of the biogeochemical sulfur cycle have been studied: a. the sulfur oxidizing process (occurring at aerobic conditions) and b. the process of sulfur-reduction to hydrogen sulfide (anaerobic conditions). Sulfate-reducing bacteria (StRB) and sulfur-reducing bacteria (SrRB) are responsible for anaerobic reducing processes. At present time the systematics of StRB include over 111 species distributed into 35 genera of Bacteriaand Archaea. Moderately thermophilic and mesophilic SrRB belong to the Bacteria and the hyperthermophilic SrRB predominately belong to the domain Archaea. Hyperthermophilic SrRB are included in the Archaea genera: Pyrodictium, Thermoproteus, Pyrobaculum, Thermophilum, Desulfurooccus, Thermodiscus. The StRB and SrRB use a wide spectrum of substrates as electron donors (e.g. H₂ + CO₂, H₂ + acetate + CO₂, alcohols, ketones, carbonic acids, volatile acids, fatty acids, saturated hydrocarbons, amino acids, proteins, carbohydrates, Na₂S₂O₃ + acetate, Na₂SO₃ + acetate, S^o + CO₂. The electron acceptors for the StRB include sulfate, thiosulfate, sulfite, sulfur, dithionite, tetrathionate, sulfur monoxide, iron, nitrite, selenite, fumarate, oxygen, and chlorine-containing phenol compounds. The Sulfate- and Sulfur-reducing bacteria are widely distributed in anaerobic ecosystems, including extreme environments like hot springs, deep-sea hydrothermal vents, soda and high salinity lakes, and cryo-environments. The StRB and SrRB are of Astrobiological significance as these anaerobic extremophiles may represent relic life forms that were dominant associated with extensive volcanic activity of Earth's early evolutionary period.