Combined SEM (secondary electrons, backscatter, cathodoluminescence) and atomic force microscope investigation of fracture surfaces in Martian meteorite ALH84001: preliminary results

A variety of microscope techniques have been used to study surficial phenomena on the fracture surfaces of the Martian meteorite ALH84001. The aim of the investigation was to determine the most useful microscopy methods in the search for morphological signs of biogenic activity. Emphasis was placed on scanning electron microscopy (SEM) using secondary, backscatter and cathodoluminescence modes combined with observation of samples at a variety of accelerating voltages. High resolution SEM imaging was compared with atomic force microscopy. These techniques revealed a number of structures of possible abiotic and biotic origin: (1) a large, fibrous-looking carbonaceous structure, (2) fine, flaky films coating pyroxene surfaces, (3) finely granular calcium carbonate deposit is associated with the fine film, and (4) lacy-structured, mineralized polymers on the pyroxene surface. Another sample contains further evidence of water-lain deposits in a cracked, iron oxide coat on a fracture surface

Magnetosome Formation in Prokaryotes

Magnetotactic bacteria were discovered almost 30 years ago, and for many years and many different reasons, the number of researchers working in this field was few and progress was slow. Recently, however, thanks to the isolation of new strains and the development of new techniques for manipulating these strains, researchers from several laboratories have made significant progress in elucidating the molecular, biochemical, chemical and genetic bases of magnetosome formation and understanding how these unique intracellular organelles function. We focus here on this progress

Correlative Electron and Fluorescence Microscopy of Magnetotactic Bacteria in Liquid: Toward In Vivo Imaging

Magnetotactic bacteria biomineralize ordered chains of uniform, membrane-bound magnetite or greigite nanocrystals that exhibit nearly perfect crystal structures and species-specific morphologies. Transmission electron microscopy (TEM) is a critical technique for providing information regarding the organization of cellular and magnetite structures in these microorganisms. However, conventional TEM can only be used to image air-dried or vitrified bacteria removed from their natural environment. Here we present a correlative scanning TEM (STEM) and fluorescence microscopy technique for imaging viable cells of Magnetospirillum magneticum strain AMB-1 in liquid using an in situ fluid cell TEM holder. Fluorescently labeled cells were immobilized on microchip window surfaces and visualized in a fluid cell with STEM, followed by correlative fluorescence imaging to verify their membrane integrity. Notably, the post-STEM fluorescence imaging indicated that the bacterial cell wall membrane did not sustain radiation damage during STEM imaging at low electron dose conditions. We investigated the effects of radiation damage and sample preparation on the bacteria viability and found that approximately 50% of the bacterial membranes remained intact after an hour in the fluid cell, decreasing to ~30% after two hours. These results represent a first step toward in vivo studies of magnetite biomineralization in magnetotactic bacteria