International audienceSilicon (Si), in the form of dissolved silicate (DSi), is a key nutrient in marine and continental ecosystems. DSi is taken up by organisms to produce structural elements (e.g., shells and phytoliths) composed of amorphous biogenic silica (bSiO(2)). A global mass balance model of the biologically active part of the modern Si cycle is derived on the basis of a systematic review of existing data regarding terrestrial and oceanic production fluxes, reservoir sizes, and residence times for DSi and bSiO(2). The model demonstrates the high sensitivity of biogeochemical Si cycling in the coastal zone to anthropogenic pressures, such as river damming and global temperature rise. As a result, further significant changes in the production and recycling of bSiO(2) in the coastal zone are to be expected over the course of this century

Silicic-acid uptake kinetics for a field population of benthic diatoms were examined using a radioisotope tracer over a concentration range of 0-300 mmol L 21 . The microphytobenthos half saturation constant (54 mmol L 21 ) and specific uptake rates (0.096 h 21 ) for silicic acid were well above those usually found in the pelagic environment. Silicic-acid kinetics were hyperbolic only at low concentrations (,60 mmol L 21 ). At higher concentrations, a second mechanism that did not suggest saturation was activated. Many benthic diatoms are motile and migrate vertically in the upper few centimeters of sediment where there are strong gradients of nutrient concentrations. The multiphasic uptake may allow them to take maximum advantage of the high silicic-acid concentrations found at depth.

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