Creation of artifactual internal membranes during fixation of Methanobacterium thermoautotrophicum

Methanogenic bacteria chemically fixed for electron microscopy contain internal membranes in the form of mesosomes and nucleoid vesicles, and the function of these membranes in methanogenesis and ATP synthesis has been speculated upon. Cells of Methanobacterium thermoautotrophicum fixed in phosphate-buffered 2.5% glutaraldehyde, 0.1% osmium tetroxide, or 0.5% glutaraldehyde – 2.5% formaldehyde always contain internal membranes. Cells freeze-fractured, freeze-substituted, or fixed in cacodylate-buffered 1% osmium tetroxide, 2.5% glutaraldehyde, or simultaneous glutaraldehyde–osmium lack internal membranes. We conclude that internal membranes in this species are artifacts arising during suboptimal fixation conditions.