MEPS 517:35-49 (2014)  -  DOI:

Biogenic silica dissolution in diatom aggregates: insights from reactive transport modelling

Brivaëla Moriceau1,*, Goulven Gildas Laruelle2,3, Uta Passow4, Philippe Van Cappellen5, Olivier Ragueneau1

1Laboratoire des Sciences de l’Environnement Marin, UMR 6539 CNRS/UBO/IRD, IUEM, Technopôle Brest-Iroise,
4 rue Dumont d’Urville, 29280 Plouzané, France
2Faculty of Geosciences, Department of Earth Sciences—Geochemistry, Utrecht University, PO Box 80021, 3508 TA Utrecht, The Netherlands
3Biogeochemical Modelling of the Earth System, Dépt. des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles, 1050 Bruxelles, Belgium
4Marine Science Institute, University of California Santa Barbara, CA 93106, USA
5Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON N2L 3G1, Canada
*Corresponding author:

ABSTRACT: Diatom aggregates contribute significantly to the vertical sinking flux of particulate matter in the ocean. These fragile structures form a specific microhabitat for the aggregated cells, but their internal chemical and physical characteristics remain largely unknown. Studies on the impact of aggregation on the Si cycle led to apparent inconsistency. Despite a lower biogenic silica (bSiO2) dissolution rate and diffusion of the silicic acid (dSi) being similar in aggregates and in seawater, dSi surprisingly accumulates in aggregates. A reaction-diffusion model helps to clarify this incoherence by reconstructing dSi accumulation measured during batch experiments with aggregated and non-aggregated Skeletonema marinoi and Chaetoceros decipiens. The model calculates the effective bSiO2 dissolution rate as opposed to the experimental apparent bSiO2 dissolution rate, which is the results of the effective dissolution of bSiO2 and transport of dSi out of the aggregate. In the model, dSi transport out of the aggregate is modulated by alternatively considering retention (decrease of the dSi diffusion constant) and adsorption (reversible chemical bonds between dSi and the aggregate matrix) processes. Modelled bSiO2 dissolution is modulated by the impact of dSi concentration inside aggregates and diatom viability, as enhanced persistence of metabolically active diatoms has been observed in aggregates. Adsorption better explains dSi accumulation within and outside aggregates, raising the possible importance of dSi travelling within aggregates to the deep sea (potentially representing 20% of the total silica flux). The model indicates that bSiO2 dissolution is effectively decreased in aggregates mainly due to higher diatom viability but also to other parameters discussed herein.

KEY WORDS: Silicic acid · dSi accumulation · dSi diffusion · dSi adsorption · Transparent exopolymer particles · TEP  · Viability · Si cycle

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Cite this article as: Moriceau B, Laruelle GG, Passow U, Van Cappellen P, Ragueneau O (2014) Biogenic silica dissolution in diatom aggregates: insights from reactive transport modelling. Mar Ecol Prog Ser 517:35-49.

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