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FIGURE 9.3 Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) micrographs of nanoscale fibrils in aquatic systems. (a) TEM whole mount specimen showing the interconnections between fibrils and nanoscale particles from the Middle Atlantic Bight (courtesy of K. Wilkinson; scale bar = 500 nm). (b) AFM image of fibrils and small nano-colloids from the Middle Atlantic Bight, with an architecture like pearls on a necklace.14 (c) A specimen collected by centrifugation from a freshwater lake, Paul Lake (MI), imaged by TEM, showing fibrils rendered electron dense by the attachment of nanoscale globules of natural iron oxide (scale bar: 500 nm)6; (d) natural hydrous iron oxide aggregates found between 6.5 and 7.5 m in the water column of Paul Lake, where particulate Fe shows a maximum, and below which [Fe2+] is increasing in concentration (scale bar = 1 ^m).6 The TEM micrographs in (d) display intimate mixtures of organic fibrils naturally stained by natural iron oxides. The EPS spectrum shown in (e) of these mixtures shown in (d) displays some Fe-Pb elemental association. The Cu peak originates from the TEM grid.

Copyright 2005 by CRC Press

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FIGURE 9.3 Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) micrographs of nanoscale fibrils in aquatic systems. (a) TEM whole mount specimen showing the interconnections between fibrils and nanoscale particles from the Middle Atlantic Bight (courtesy of K. Wilkinson; scale bar = 500 nm). (b) AFM image of fibrils and small nano-colloids from the Middle Atlantic Bight, with an architecture like pearls on a necklace.14 (c) A specimen collected by centrifugation from a freshwater lake, Paul Lake (MI), imaged by TEM, showing fibrils rendered electron dense by the attachment of nanoscale globules of natural iron oxide (scale bar: 500 nm)6; (d) natural hydrous iron oxide aggregates found between 6.5 and 7.5 m in the water column of Paul Lake, where particulate Fe shows a maximum, and below which [Fe2+] is increasing in concentration (scale bar = 1 ^m).6 The TEM micrographs in (d) display intimate mixtures of organic fibrils naturally stained by natural iron oxides. The EPS spectrum shown in (e) of these mixtures shown in (d) displays some Fe-Pb elemental association. The Cu peak originates from the TEM grid.

Copyright 2005 by CRC Press and atomic force microscopy techniques to document the various forms, shapes, and architectures of marine and freshwater colloids from different environments. For the first time, polysaccharide-rich fibrils of recent (determined by radiocarbon analysis; ref. [14]) origin were documented to make up a significant fraction of all colloidal sized nanoparticles (Figure 9.3). It is also important to realize that these fibrillar extracellular polymeric substances (EPS) molecules are much more abundant in the <0.5 ^m "dissolved" than in the >0.5 ^m particulate fraction. This is due to the approximately two orders of magnitude higher concentration of DOC than POC in the ocean, and the relative abundances of total and acid polysaccharides (APSs) that are similar in the two size fractions of organic carbon.34 Being able to accurately detect these nanoparticles is important because, although they are too small to settle out of the water column at appreciable rates, they do aggregate and are capable of forming the matrix for the formation of larger aggregates that can settle faster.35,36 However, so far no quantitative estimate exists of their number concentration in marine systems.

Transparent exopolymer particles (TEP, Figure 9.2 and Figure 9.3) form an important component of aggregates in natural waters.37-41 These particles are natural exudates from marine algae and bacteria.42 They consist of surface active polysac-charides rich in acidic functional groups43,44 and are formed from the aggregation of nanoscale fibrils.45,46 Recent results, however, indicate that only a small fraction of the total carbohydrate content of marine suspended and sinking matter consists of surface-active acid polysaccharide compounds, with total uronic acids making up about 7% (0.2% to 2% of POC), and total acid polysaccharides about 11% of the total carbohydrate, or about 1% of the POC content.34,47,48 Thus, it appears that, much like small amounts of glue needed to hold man-made materials together, surface-active substances that provide the stickiness of the TEP do not have to be in high abundance to be effective.

TEPs have a high stickiness and their presence has been shown to stimulate aggregation amongst phytoplankton cells.43 As a matter of fact, times of highest particulate organic carbon export from the ocean coincide with times of large phyto-plankton blooms, diatoms in particular,49 which are strong TEP producers as well as providers of "mineral ballast," enhancing density and settling velocity of sinking particle aggregates. This relationship was documented by a close relationship between diatom pigments (fucoxanthin) and 234Th-derived POC flux from the surface ocean,49,50 producing a higher efficiency of the "biological pump" (i.e., ratio of POC flux to primary production). In addition to phytoplankton species, bacteria also produce abundant acidic polysaccharide-rich compounds,31,42,51 especially when attached to particles as a "micro-biofilm." Indeed, significant relationships between APS concentrations and heterotrophic bacterial production (BP), and 234Th/POC ratios and BP were recently demonstrated by Santschi et al.,47 which strongly suggest microbial involvement through production of Th(IV)-binding APS compounds, while their enzymatic activities can produce smaller but more stable filter-passing Th(IV)-binding fragments.

Macromolecular COM, a result of exopolymer formation by algae and bacteria, makes up 30% to 40% of conventionally defined dissolved organic matter.27,52-54 The aggregation of fibrils and other biopolymers, with an architecture like pearls on a necklace (Figure 9.3), into rapidly sinking marine snow provides an important pathway for the removal of DOM and associated metals and radionuclides55'56 from surface waters (Figure 9.1).

This important transport system is not, however, well understood. A promising research direction is suggested by potential gaps in conventional aggregation models. These models predict lower coagulation rates than those observed in nature. It has been suggested by Hill57 that one could reconcile the model results with observations if there existed a background distribution of particles, and by Alldredge and others that this background distribution can be accounted for by TEP (Figure 9.2 and Figure 9.3). Therefore, it would be important to characterize the hitherto neglected nanoscale components of heterogeneous aquatic aggregates and integrate these components into aggregation models, so that the models will be able to account for observed coagulation rates.

It is of great interest to aquatic scientists to better understand the processes by which components of these aggregates scavenge metals and pollutants and thereby endow the assembled aggregates with their pollutant-clearing properties. Suspended particles can scavenge trace metals, providing an efficient mechanism for removing chemicals from solution.5,6,58-61 Colloidal particles dominate the particulate surface area distribution, making them excellent at scavenging chemicals from the bulk water. In particular, metal oxides have been observed to coat fibrils (Figure 9.3c,d). So, to understand the removal of trace metals from solution requires understanding the properties and dynamics of both the dissolved species and the properties of the particles that scavenge them.

Extracellular polymeric substances (EPS) in specific marine or freshwater environments are known to initiate or modify precipitation of MnO2 and FeOOH,62 SiO2,63 CaCO3,64 and uptake of different trace metals.56 Thus, the organic template can be important for mineral formation in the ocean. These exopolymers are part of the marine DOC pool and have a modern radiocarbon age,14 as compared to the bulk of the DOC. Microbially produced APS-rich compounds do not only have chelating properties for trace metals,31 but also emulsifying properties through a protein trace component, with the hydrophilic polysaccharide chains providing protective layers that confer effective steric stabilization over time.65

In activated sludge flocs, EPS have been shown to be important for establishing the floc pore structure,8 whereby their relative composition can govern floc surface properties and bioflocculation.66,67 For example, the ratios of protein to total carbohydrates, hydrophobicity and surface charge are a function of EPS composition at the floc/water interface, and thus are important parameters for predicting the extent of bioflocculation.66-68 Bacterial hydrophobicity appears to be a good overall parameter for predicting the adhesion potential of their EPS to soil particles.69

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