Optimal preparation of water samples for the examination of colloidal material by transmission electron microscopy

We describe an effective combination of techniques for the analysis of aquatic colloidal material by transmission electron microscopy (TEM). Specimens are prepared either by direct and quantitative ultracentrifugation onto supporting grids (whole mounts approach) or by resin embedding and ultramicrotomy (thin sections approach). Both preparation schemes require fastidious attention to protocol to obtain minimally perturbed images of native colloidal material present in natural samples. In the case of the whole mounts approach, 'optical properties' (turbidity, light scattering, fluorimetry) of the suspension are measured in order to evaluate the optimal specimen preparation according to a dataset of optimal centrifugation conditions. This fast and non-artifacted approach is complementary to the time-consuming thin sections approach, their combination allowing a clear representation of the ultrastructure of colloids and their S-dimensional relationships. The whole mounts permit quantification of relative amounts of different colloid 'species' while the sections permit colloid descriptions according to internal differentiation. The use of this correlative approach is illustrated with lacustrine and marine samples, with a special emphasis on the interactions between bacteria, their fibrillar extra-cellular polymeric substances (fibrils, exopolymers) and other colloids.