Thermothrix thioparus gen. et sp. nov. a facultatively anaerobic facultative chemolithotroph living at neutral pH and high temperature

Abstract

Thermothrix thioparus gen. et sp. nov. occurs naturally in a New Mexico hot spring at a temperature of 74 °C, a pH of 7.0, and a HS− concentration of 1 mg/litre. The organism is gram-negative, non-motile, 0.5–1.0 × 3–20 μm, and forms cell chains up to 1 cm in length. The resulting filaments do not possess a sheath. Sulfur is deposited extracellularly. The organism was isolated using an autotrophic medium with HS− as the energy source and NO3− as the terminal electron acceptor. Anaerobically either NO2− or NO3− is required, NO2− is formed from NO3−, and no observable gas is evolved. Oxygen can also be used as the terminal electron acceptor, but growth is poor because of the decreased solubility of O2 at temperatures required for growth. Alternate energy sources used aerobically and anaerobically include hexose, HS− SO3−, and S2O3=. The temperature optimum is 70–73 °C and growth occurs from 62 to 77 °C. The organism's thermal and physiological characteristics are compared to those of Bacillus stearothermophilus, Methanobacterium thermoautotrophicum, Sulfolobus acidocalderius, Thermus aquaticus, Thermits flavus, as well as Thiobacillus denitrificans, the latter being the only other facultatively anaerobic chemolithotroph which has been isolated and described.