HALOMETHANOCOCCUS DOII GEN. NOV., SP. NOV.: AN OBLIGATELY HALOPHILIC METHANOGENIC BACTERIUM FROM SOLAR SALT PONDS

Abstract

A new obligately halophilic methanogenic bacterium, strain IY-1, was isolated from a salt water pond near San Francisco Bay. The bacterial cells are pleomorphic cocci, 0.3-1.5μm in diameter and require a minimum of 10% salt (1.8M NaCl) for growth. Optimum growth occurs at 17.5% salt (3.0M NaCl). Methanogenic substrates included methylamines and methanol, but not formate, H₂/CO₂, or acetate. Acetate and rumen fluid were required for cell growth. Growth occurred throughout the temperature range 5 to 45°C with an optimum at 35°C. The pH optimum for growth was 6.8 and cell growth occurred throughout the pH range 5.8 to 7.6. Growth in complex medium had a doubling time of 9hr at 37°C. The cells were not easily ruptured by mechanical means and were not sensitive to penicillin G, cycloserine, or rifampicin. Bacterial colonies formed on solid medium were white and circular to oval in shape, with irregular margins. Electron micrographs of ultrathin sections revealed homogeneous granular ribosome-like particles and a thin cell wall, but the membrane could not be observed by the standard Kellenberger's fixation method. The DNA base composition of the isolate was 43.2mol% guanine plus cytosine. We propose Halomethanococcus gen. nov. as the genus and Halomethanococcus doii sp. nov. as the type species for this novel bacterium. Strain IY-1 is the type strain (ATCC 43619).