A review of solubility and dissolution rates of biogenic silica: Present and future direction

Abstract

In the marine ecosystem, diatoms, which utilize silicon essentially to construct their cell walls, are the most important primary producer and play a major role in the downward export of organic carbon from the euphotic zone. Finally their debris buried in the sediments contributes to the disclosure of they past environment and climate change. An understanding of the factors that control the dissolution rates and mechanisms is important for reconstructing the modeling of the biogeochemical cycle of silicon and also for modeling the impact of carbon on global climate change.

From these aspects, we summarize and discuss the solubility and the dissolution kinetics of biogenic silica in seawater. Several factors reviewed herein that control biogenic silica dissolution are specific surface area, Al content, hydration state, aging, and the rate of degradation of organic matter encasing silica walls. Furthermore, the rate-limiting step in the dissolution processes is still poorly understood: the greatest ambiguity would seem to arise from uncertainties regarding the nature of the surface active site on the reactant and the extent to which the effective surface area changes with reaction progress. Approaches that incorporate the surface reduction appear promising, but extensive parameterization will require substantially more verification. To overcome this point, a technique such as atomic force microscopy, which makes possible the direct observation of processes occurring in the solid/solution interface, seems to promise a better understanding of the reaction mechanisms on the surface of the silica skeleton.

Concurrent measures of the kinetic dissolution of the siliceous skeleton are available for only a limited number of marine diatom species. Solubility and dissolution rates of biogenic silica should be examined by a standardized method using various and representative diatom species selected on the basis of their morphological characteristics.

Our review extends further to the biological effects on the silica dissolution in the ocean interior, such as bacterial activity, that have been shown to enhance the dissolution of biogenic silica by cleaning the frustule surfaces from the protective organic coating. Kinetic data on the biologically mediated acceleration of biogenic silica dissolution under in situ conditions as well as the fate of silica frustules in fecal pellets produced by mesozooplankton are lacking. An understanding of how these biological processes interact with other physicochemical factors regulating the dissolution of biogenic silica in seawater, provides a substantial future challenge.