Introduction

Particles are the vehicles of vertical transport of material in aquatic systems. Large, heterogeneous aggregates can sink through the water column at rates of 10 to 100 m per day carrying with them carbon, nutrients, and trace metals.1 In the open ocean, sinking particles carry carbon (e.g., in the form of phytoplankton, detritus, and mucilage) from the surface waters to the sediments, thereby playing an important role in the global

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carbon cycle.2 These particles also carry nutrients, which help support food webs in the mid-depths and benthos.3 In estuarine and coastal systems, terrigenous particles settle out of the water column removing clays and a large and variable amount of trace elements. In rivers, large quantities of suspended material are transported in the form of nanoparticles.4 Nanoscale particles of Fe and Mn are also formed at oxic/anoxic transitions in aquatic systems.5-7 Aggregation and subsequent settling of particulate material is a crucial step in many industrial processes such as those used in water treatment plants.8 This removal process, whose efficiency depends on the presence of some principal components, that is, fibrillar microbial exudates, humic-type material, and mineral matter,9,10 as well as environmental conditions, that is, pH and ionic strength, is depicted in Figure 9.1.

Particulate material in aquatic systems covers a range of sizes greater than a million-fold, from nanoscale colloidal particles to millimeter-sized flocs.1,9,11-14 Particle size distributions in marine environments tend to follow a power-law distribution.15-19 Large particles (>100 ¡m) are relatively rare and represent the dominant agent of sedimentation. For example, for aggregates with equivalent spherical diameters >1.5 mm, numbers of 4 to 40 aggregates/l, peaking at the euphotic zone and in mid-depth and near-bottom nepheloid layers, have been reported for the Middle Atlantic Bight.20 Aggregate peak concentration regions coincided with strong 234Th deficiencies in the water column, demonstrating their high efficiency for scavenging particles and particle-reactive elements.20 Sediment trap data and in situ camera observations21-23 indicate that marine particles settle as large, heterogeneous aggregates, such as marine snow (Figure 9.2). The sinking rate of an aggregate is a function of its size, composition, and structure. Dense, compact particles (e.g., fecal pellets) sink faster than larger, porous marine snow particles. Differences in the timing between peaks in surface particle concentrations and peaks detected by sediment traps throughout the water column indicate that these aggregates can have settling velocities of 50 to 100 m per day or more.24-26

Colloidal particles (operationally defined in environmental aquatic chemistry as microparticles and macromolecules with sizes between about 1 ¡m and 1 nm)

Fibrils (TEP)

Trace metal/pollutant

Aggregation J '

Inorganic colloids

Sinking

Sinking

FIGURE 9.1 Diagram representing the major routes of the formation of large-scale aggregates from the aggregation of fibrils and colloidal particles.

FIGURE 9.2 Marine snow. Clear organic matrix that enmeshes fecal pellets and smaller biomolecules.11

dominate the particle number density and surface area. Ultrafiltration measurements27 revealed typical concentrations of colloidal organic carbon (COC) in oceanic surface waters with sizes between about 1 nm (1 kDa) and <0.2 ^.m, of about 30 to 40 ^M-C (about 1 mg, organic matter/l), COC >3 kDa about 11 ^.M, and COC > 10 kDa about 3 ^M. If marine colloids are present as spherical particles, the average molecular weight of COC > 1 kDa in marine environments would be about 2 to 3 kDa. This should give an average particle number density in surface ocean water of 1014 to 1015 nanoparticles per milliliter. However, Wells and Goldberg28,29 reported number densities of at most 109 per milliliter of spherical nanoparticles they called "Koike" particles, a concentration that is similar to that in ground water where colloid concentrations are in the range of a few micrograms per liter.30 This large discrepancy between expected and measured colloids concentration in marine environments indicates that (1) the majority of the colloidal fraction was undetected by Wells and Goldberg,12,28,29 which is likely, since the colloids were not stained for transmission electron microscopy (TEM); (2) the assumption of spherical shape for calculating the average molecular weight is incorrect; this is likely, since many biomolecules are not spherical but fibrillar; (3) colloids are present as aggregates.

Colloids are indeed present as aggregates, since recent observations of colloidal particles using TEM31 and atomic force microscopy (AFM)14,32 have revealed that an important fraction of colloidal organic matter (COM) in aquatic systems is present as nanoscale fibrils that also contain smaller molecules assembled like pearls on a necklace (Figure 9.3). These fibrils are acid-polysaccharide rich, have diameters of 1 to 3 nm and can be missed by standard fractionation techniques.14,31 Fibrils have estimated molecular weights between 105 and 106 kDa and yet, because of their shape, they are able to pass through a 10 kDa filter.14 Wells and Goldberg12 did not use state-of-the-art preparatory and staining techniques for electron microscopy imaging and, therefore, were not able to document existing colloids in a representative manner. Santschi et al.,14 Leppard et al.,33 and Wilkinson et al.32 used state-of-the-art electron

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