Direct measurement of diffusivity within diatom aggregates containing transparent exopolymer particles

Abstract

We present the first direct measurements of apparent diffusivity within diatom aggregates using a diffusivity microsensor. Transparent exopolymer particles (TEP) and aggregate dry mass (TEP and cells) were determined in the same aggregates after diffusivity measurements. Carbon in TEP comprised 8-12% of aggregate dry mass. The (wet) volume fraction of TEP in aggregates, however, was on average 7.2-fold larger than that of cells, and it decreased with increasing aggregate size similar to that of cells. The exchangeable pore-water content occupied 87-98% of the aggregate volume. The average apparent diffusivities of gases within aggregates ranged between 0.90 and 0.95 times the free diffusion coefficient in seawater. Using a diffusion-reaction model, we analyzed silicic acid concentrations within marine snow. An apparent diffusivity of silicic acid within marine snow being 0.9 times its free diffusion coefficient in seawater, and a specific net silica dissolution rate of 0.002 h 21 could explain the observation that concentrations of silicic acid are significantly higher within marine snow compared to that of the ambient water.Organic aggregates .0.5 mm comprise a significant component of suspended and sinking organic matter in lakes, rivers, and in the sea (Alldredge and Silver 1988;Simon et al. 2002). These aggregates, also known as marine snow in the sea, form mainly through coagulation of phytoplankton, detritus, and transparent exopolymer particles (TEP) in the euphotic zone (Kiørboe et al. 1994;Passow et al. 2001). Marine snow is characterized by high amounts of TEP that glue the particles together (Alldredge et al. 1993). The physical and chemical microenvironments, and thus the microbial growth conditions, within marine snow are significantly different from those of the bulk. Hydrolysis and respiration rates are high on marine snow (Smith et al. 1992;Ploug et al. 1999). Concentrations of dissolved organic carbon (DOC) (Alldredge 2000), inorganic nutrients (Brzezinski et al. 1997; Shanks and Trent 1979), and oxygen (Alldredge and Cohen 1987;Ploug et al. 1999) within marine snow are significantly different from those of the surrounding water. Hence, small-scale flow and diffusion within and around marine snow appear to be key processes determining coagulation and remineralization of particles in the ocean.The observed concentration gradients of solutes associated with marine snow have led to speculations that diffusion may be slow within these aggregates (Shanks and Reeder 1993;Brzezinski et al. 1997;Alldredge 2000). The diffusive flux in a spherical aggregate is described by Fick's first law of diffusion:where J is the flux, w is the porosity, D s is the effective diffusion coefficient of the solute, and dC/dr is the radial concentration gradient of the solute. The combined 1 To whom correspondence should be addressed. Present address: Alfred-Wegener-Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany. AcknowledgmentsWe are grateful to Karolin Schreiber for m...

Ballast minerals and the sinking carbon flux in the ocean: carbon-specific respiration rates and sinking velocity of marine snow aggregates

“…During aging, the excess density and sinking velocity of diatom aggregates increase (Ploug et al, 2008a). This may be caused by the observed decrease in organic carbon to dry mass ratio in aging aggregates likely due to microbial degradation of TEP (Ploug and Passow, 2007). Hence, the sinking velocities of aggregates in the field might depend on source, density, and age rather than aggregate size (Ploug et al, 2008a).…”

“…TEPs occupy a significant fraction of aggregate volume but contribute little to DW in diatom aggregates (Ploug and Passow, 2007). TEP densities can be lower than that of seawater and decrease aggregate sinking velocities (Engel and Schartau, 1999;Azetsu-Scott and Passow, 2004).…”

Evidence for reduced biogenic silica dissolution rates in diatom aggregates

“…Possibly, chemically binding of dSi to TEP (Welch & Vandevivere 1994, Welch & Ullman 1993, charge restrictions, a lengthening of the diffusion pathway due to the fractal nature of diatom aggregates (Dachs & Bayona 1998), or size-dependant permeability are responsible for the accumulation of dSi in aggregates. Alternatively, TEP may simply allow elevated dSi concentrations to build up by restricting advective flow, without impacting diffusion rates appreciably (Ploug & Passow 2007). In all cases, TEP would contribute to the reduction of the apparent dissolution of bSiO 2 by retaining dSi within aggregates.…”

Diatom-associated bacteria are required for aggregation of Thalassiosira weissflogii

“…The individual impact of attaching versus free-living bacteria as well as the metabolic state of diatom cells on various characteristics of aggregates were studied in rolling tank experiments, which mimic continuous sinking of particles by rotating the liquid against gravity and thus allow the analysis of the formation of aggregates (Shanks and Edmondson, 1989;Ploug and Passow, 2007). It was hypothesized that formation rates, size-dependent densities and settling velocities of diatom aggregates differ in dependence of the physiological state of T. weissflogii and are influenced by presence of specific bacterial strains.…”