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

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 Si cycle led to what appears to be inconsistency. Despite a lower biogenic silica (bSiO2) dissolution rate and a diffusion of the silicic acid (dSi) similar in aggregate and in seawater, dSi surprisingly accumulate 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 results on the effective bSiO2 dissolution and transport of dSi out of the aggregate. In the model, dSi transport out of the aggregate is modulated considering alternatively retention -decrease of dSi diffusion constant- and adsorption -reversible chemical bounds between dSi and the aggregate matrix- processes. Modelled bSiO2 dissolution is modulated by the impact of dSi concentration inside aggregate and diatom viability, as enhanced persistence of metabolically active diatom 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 20% of the total silica flux). Moreover, the model states that bSiO2 dissolution is effectively decreased in aggregates mainly due to higher diatom viability but also to other parameters discussed here