Thermophilic biomethanation of acetic acid: morphology and ultrastructure of a granular consortium

The ultrastructure of a thermophilic methanogenic consortium appearing as morphologically distinct bacterial granules (up to approximately 3 mm in diameter) has been studied. The consortium was enriched and maintained at 60 °C in continuous culture in a defined mineral – vitamin – acetic acid medium at a hydraulic retention time of 44 h. Thin-section electron microscopy showed three morphologically distinct layers of the granules. (1) The outer division zone bears resemblance to a pseudoparenchyma and consists of "macrocysts" and coccoid cells, resembling Methanosarcina cells. (2) The inner zone is built up of loosely packed ovoid cells and (3) internal cavities containing rods.The cavities were the exclusive site of gas formation, as shown with a specially designed minifermenter allowing direct light microscopic observation of gassing granules. Based on labeling studies with cationic ferritin and scanning electron microscopy a working model for methane formation from acetate by the consortium is presented.