K+, Na+, and Mg2+ content and permeability of Methanospirillum hungatei and Methanobacterium thermoautotrophicum

Abstract

The K+, Na+, and Mg2+ contents of Methanospirillum hungatei and of the thermophile Methanobacterium thermoautotrophicum were determined at various phases of growth. The intracellular K+ content of exponential phase cells of M. thermoautotrophicum (approximately 780 mM) was 5.4-fold higher than in M. hungatei, and decreased gradually as the culture entered the stationary phase. Both methanogens concentrated Mg2+, exhibiting an increased content as the cultures aged. Comparisons among extraction methods showed that most of the internal K+ was readily released, but a minimum of half of the Mg2+ in M. hungatei, and most of the Mg2+ in M. thermoautotrophicum, was in a bound form. Exponential phase cells of M. hungatei established an intracellular level of Na+ lower than the outside medium, but the thermophile concentrated Na+.Dextran, inulin, sucrose, and glucose penetrated cell pellets to varying degrees and could be used to measure the space corresponding to cytoplasm and to cell wall permeability barriers. L-Phenylalanine penetrated fully and acetate accumulated in both methanogens. Acetate uptake in cell suspensions of M. hungatei was fully inhibited by oxygen, N-ethylmaleimide, or N,N′-dicyclohexylcarbodiimide, but was not affected by the proton conductor carbonylcyanide p-trifluoromethoxyphenylhydrazone. L-Malate, which penetrated M. hungatei cells poorly, was metabolized to glutamate, indicating the presence of an incomplete reductive carboxylic acid cycle.