During fixation of biological specimens for electron microscopy they are exposed to the osmotic effect of the vehicle of the fixative, the osmotic effect of the fixative and the fixative effect. The osmotic effect of the vehicle plays a dominant role in determining the quality of the specimen preservation during primary fixation with glutaraldehyde. Incorrect matching of the osmolarities can lead to devastating effects on the quality of the preservation. In the case of marine specimens this matching can be difficult and can vary greatly with different specimens, even with closely related organisms. While it would be assumed that most marine specimens are isotonic to seawater this is, in fact, not the case.
As it can be difficult to match the osmolarity of common buffers like cacodylate and phosphate to marine specimens, seawater is sometimes used as the fixative vehicle. Montanaro et al (2016) suggested the use of a marine PHEM buffer for a wide range of marine specimens.
The use of microwave radiation has been shown to greatly speed up and improve specimen processing by the action of two major effects: the ability to increase the rate of diffusion of solutions through tissues and the ability to enhance reactions and so promote processes.
Microwave assisted fixation of marine specimens gives excellent preservation of ultrastructure. Even using the same fixative, the quality of structural preservation can be quite different when assisted by the use of microwaves, so while a particular fixative can give poor preservation by itself, when used with microwave radiation the preservation can be of high quality. This applies across a wide range of different marine samples. Possible reasons for this avoidance of the osmotic effects of the fixative will be explained.