FIBRILLAR POLYSACCHARIDES IN MARINE MACROMOLECULAR ORGANIC-MATTER AS IMAGED BY ATOMIC-FORCE MICROSCOPY AND TRANSMISSION ELECTRON-MICROSCOPY

A consensus is now emerging that the structure of organic macromolecul es will determine their function in aquatic systems. Transmission elec tron microscopy (TEM) is a widely used technique to study the morpholo gy of biomolecules that, under the proper conditions, allows for accur ate determination of the shapes and horizontal dimensions of macromole cules. Atomic force microscopy (AFM) is a newer technique that has bet ter overall resolution, particularly on the vertical scale. Although c areful controls must be in place for both techniques to avoid the intr oduction of artifacts, they are highly complementary techniques for th e study of natural colloids and can, when used together, reveal comple mentary information about the relative abundance and structures of aqu atic macromolecules and colloids. Although AFM techniques have not pre viously been applied to the study of oceanic macromolecules and colloi ds, TEM has been used, albeit often under nonoptimal staining and embe dding conditions. In this study, colloid samples from the Gulf of Mexi co and Middle Atlantic Eight of nominal sizes 1-200 nm were collected by cross-flow ultrafiltration, diafiltered, and freeze-dried. Rehydrat ed colloids were analyzed in parallel by AFM and TEM using standardize d techniques. Results from estuarine-, surface-, and deep-water sample s show that an important fraction of colloidal organic matter (COM) co nsists of fibrillar material, which is rich in polysaccharides and ''f resher'' (i.e., has a younger radiocarbon age) than the bulk COM. This result is important because COM makes up 30-70% of oceanic and estuar ine nominally ''dissolved'' organic matter. Other microparticles appea r to be quasi-spherical, often attached to the fibrils like pearls. In the surface waters of the Gulf of Mexico, Middle Atlantic Eight, and Trinity River, fibrils with diameters of 1-3 nm and lengths of 100-2,0 00 nn 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 Eight (2,600 m), a much greater heterogeneity of collo id and macromolecule shapes and sizes was observed in these deeper wat ers.