Evaluation of electron microscopic sample preparation methods and imaging techniques for characterization of cell-mineral aggregates

Scanning Electron Microscopy (SEM) is used to image geomicrobiological samples, typically containing interfaces between hard and soft materials such as minerals and cells, which represent challenges for artifact-free preparation for high-resolution imaging. We used cell-mineral aggregates produced during microbial Fe(II) oxidation and Fe(III) reduction to evaluate different sample preparation and imaging techniques. Both rapid freezing and standard critical point drying (CPD) preserve structures of geomicrobiological samples, at least the ones obtained for Fe(II)-oxidizing and Fe(III)-reducing bacteria, without artifacts. We recommend a SEM sample preparation scheme for geomicrobiological specimens and discuss critical parameters like fixation, dehydration, coating, and acceleration voltages.