Freezing the natural moment
A new ‘field emission’ scanning electron microscope (FESEM) is providing Australian Antarctic Division scientists with a clearer and more natural view of the microscopic world.
The FESEM, which images in three dimensions the surface of cells, microbes and other small structures using a high-energy electron beam, has a special sample preparation attachment (a ‘cryo’ preparation chamber and stage) that effectively snap-freezes specimens and liquids without altering their natural structure.
Traditional SEM preparation techniques involve dehydrating biological specimens using a series of increasing alcohol solutions and liquid carbon dioxide (called ‘critical point drying’). As well as taking hours or days to prepare the specimen, this technique may shrink cells and may damage or alter cell structures.
The cryo attachment on the new FESEM, however, uses super-cooled liquid nitrogen to snap freeze a specimen at −210ºC. Preparation time varies from 10 minutes to one hour, depending on the specimen. This rapid freezing process retains the moisture content within the specimen, leaving it in its natural state.
The Antarctic Division’s FESEM can magnify specimens up to 650 000× and reveal structures as small as one nanometre (1 billionth of a metre). It also uses a lower energy electron beam that is cooler than previous scanning electron microscopes. These provide a gentler analysis and a clearer image of the surface detail of the specimen. When combined with the cryo preparation technique, the microscope provides scientists with a never-before-seen view of specimens vulnerable to dehydration (such as delicate seaweeds, mosses, fungi and liquids) in their natural state.
The microscope also allows scientists to examine the chemical elements (such as calcium, magnesium and nitrogen) that are present in a specimen. Different elements give off characteristic X-rays when exposed to the microscope’s electron beam and these are used to analyse the chemical composition of a specimen. This capability will allow scientists to study, for example, the effects of ocean acidification on shell-forming organisms. As the oceans absorb more anthropogenic carbon dioxide from the atmosphere, shell-forming organisms such as zooplankton and certain phytoplankton species will find it increasingly difficult to incorporate calcium carbonate into their shells. Using the FESEM, scientists can look at changes in the calcium content of these shells formed under different carbon dioxide concentrations observed in the Southern Ocean. Of course, the ability to visualise any changes in the physical structure of the shells will also be possible.
This latest model FESEM is the only one of its kind in Tasmania and one of three in Australia with the new cryo attachment. With this new instrumentation, Antarctic Division electron microscopist Rick van den Enden expects that collaborative opportunities with other research organisations, nationally and internationally, will become more frequent.
Corporate Communications, AAD