A consensus is now emerging that the structure of organic macromolecules will determine their function in aquatic systems. Transmission electron microscopy (TEM) is a widely used technique to study the morphology of biomolecules that, under the proper conditions, allows for accurate determination of the shapes and horizontal dimensions of macromolecules. Atomic force microscopy (AFM) is a newe: technique that has better overall resolution, particularly on the vertical scale. Although careful controls must be in place for both techniques to avoid the introduction of artifacts, they are highly complementary techniques for the study of natural colloids and can, when used together, reveal complementary information about the relative abundance and structures of aquatic macromolecules and colloids. Although AFM techniques have not previously been applied to the study of oceanic macromolecules and colloids, TEM has been used, albeit often under nonoptimal staining and embedding conditions. In this study, colloid samples from the Gulf of Mexico and Middle Atlantic Bight of nominal sizes l-200 nm were collected by cross-flow ultrafiltration, diafiltered, and freeze-dried. Rehydrated colloids were analyzed in parallel by AFM and TEM using standardized techniques. Results from estuarine-, !;urface-, and deep-water samples show that an important fraction of colloidal organic matter (COM) consists of fibrillar material, which is rich in polysaccharides and "fresher" (i.e., has a younger radiocarbon age) than the bulk COM. This result is important because COM makes up 30-70% of oceanic and estuarine nominally "dissolved" organic matter. Other microparticles appear to be quasi-spherical, often attached to the fibrils like pearls. In the surface waters of the Gulf of Mexico, Middle Atlantic Bight, and Trinity River, fibrils with diameters of l-3 nm and lengtils of lOO-2,000 nm were predominant. Although fibrils were also observed in samples from the benthic nepheloid layer in the Gulf of Mexico (1,600 m) and Middle Atlantic Bight (2,600 m), a much greater heterogeneity of colloid and macromolecule shapes and sizes was observed in these deeper waters.

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Tapping mode atomic force microscopy was employed to determine the shape, size, and properties of the Suwannee River humic acid and a U.K. Geological Survey peat humic acid sorbed to mica. Humic acid macromolecules are shown to have a globular shape with an adsorbed height between 0.5 and 2 nm for a concentration of 10 mg L-1, pH > 5, and I < 500 mM NaCl. Aggregation of the humic macromolecules was not systematically observed at low pH and ionic strength. Instead aggregation may have been to be related to the hydrophobicity of the sample: no aggregates were observed for the relatively hydrophilic Suwannee River humic acid between pH 3 and pH 10 (I = 5−50 mM), while aggregates of peat humic acid were observed at low pH (pH < 5) and high ionic strength (I ∼ 500 mM). Calculated and measured hydrodynamic diameters determined under the same conditions were found to be systematically slightly larger than AFM heights, in good agreement with expectations given the different nature of the two measurements.

This paper describes a direct and quantitative study of the size and shape of Laponite clay colloids by atomic force microscopy. Atomic force microscopy (AFM) images of Laponite particles deposited on mica, obtained under ambient conditions, reveal that the particles are mainly present as individual entities. Morphological quantitative analysis of the AFM images indicates that the individual particles have an anisotropic shape with a height of 1.2 nm and mean lateral dimensions of 24.0 ( 6.9 nm and 16.8 ( 4.9 nm. Furthermore, the height histogram indicates that 20% of the particles on the mica are dimers conferring to the system a height polydispersity index of 1.18. Thereafter, AFM results were used to simulate a small-angle neutron scattering (SANS) curve and good agreement was obtained with an experimental SANS curve. This work demonstrates the efficiency of the AFM technique for the characterization of Laponite colloids and nanoparticles in general and its complementarity with neutron scattering techniques.