Fibrillar polysaccharides in marine macromolecular organic matter as imaged by atomic force microscopy and transmission electron microscopy

Abstract

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.

Seqestration of dissolved organic carbon in the deep sea

“…All previous measurements made on operationally defined "carbohydrate-like" fractions of HMWDOC show this fraction to be significantly depleted relative to DIC Δ 14 C, consistent with slow cycling and turnover times of several decades (Santschi et al, 1998, Loh et al 2004. Using the compound specific approach we find carbohydrates to have radiocarbon values equal to or at most only slightly depleted from DIC, consistent with rapid cycling on annual rather than decadal time scales (see next section).…”

“…Therefore, in order to obtain an estimate of semi-reactive DOC residence time without interference from the recalcitrant DOC fraction, several recent studies have reported radiocarbon measurements of carbohydrate fractions isolated from high molecular weight dissolved organic matter (HMWDOM). Santschi et al (1998) carbohydrates (> several decades). However, as carbohydrates were not directly identified or quantified in fractions that were analyzed for radiocarbon, the relative depletion in Δ 14 C observed in both studies could be due to the inclusion of radiocarbon depleted, non-carbohydrate impurities in the carbohydrate-like fraction.…”

Biogeochemical Cycling and Environmental Stability of Pu Relevant to Long-Term Stewardship of DOE Sites

“…Confirmation of such a colloid trap model come from the AFM and TEM results (Santschi et al, 1998, Wilkinson et al, 1999, which show abundant fibrils covered by smaller spherical colloids. Colloid trapping by mineral particles appears to be responsible also for the observed enhancement of the Kd value for Pu(IV) onto mineral particles such as silica, as compared to the linear sum of the individual Kd values (e.g., calculated from silica and EPS as end-members).…”

Contrasting Roles Of Natural Organic Matter On Colloidal Stabilization And Flocculation In Freshwaters

“…They can constitute a significant proportion of freshwater NOM, varying seasonally between ca. 5-30% in the surface waters of lakes (Figure 4; Leenheer 2000)) and often accounting for an even larger proportion of the NOM in the surface waters of marine systems (Aluwihare et al 1997;McCarthy et al 1998;Santschi et al 1998). In marine systems, fibrillar polysaccharides have been shown to be refractory enough to be found in the deep ocean where they may have lifetimes of hundreds of years (Guo and Santschi 1997) while peptidoglycans appear to be degraded much faster (turnover time of weeks to months; (Nagata et al 2003)).…”

“…Fig. 4 ; (Santschi et al 1998;Wilkinson et al 1999)) show that their total length may be larger than 1 µm, whereas their thickness is often only a few nm. Furthermore, because they are often charged molecules (i.e.…”