Direct measurement of diffusivity within diatom aggregates containing transparent exopolymer particles

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(-1) could explain the observation that concentrations of silicic acid are significantly higher within marine snow compared to that of the ambient water.