Biogeochemical significance of silicon in the world oceans

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Biogeochemical significance of silicon in the world oceans

Diatoms are the major autotrophic communities in the euphotic zone in most regions of the world oceans. One of the most essential nutrients for frustule formation (cell wall or, theca) in the diatoms is silicate (SiO3). Higher the availability of SiO3 faster and better is the growth of diatoms. While emphasizing on the pivotal role of Si in the biology of diatoms and radiolarians in the oceanic realms, Dr Ittekkot, in his seminar at NIO, cautioned that anthropogenic activities, in particular construction of dams on major rivers in the world, are causing reduced Si discharges into the oceans and affecting the growth and abundance of diatoms over there. Based on the available literature on diatom composition, abundance and pre- and post-obstructions of the reverine discharges he indicated that the diatoms are adversely affected and giving alarming signals of depleting phytoplankton assemblages post-obstruction. Evidences point to catastrophic effects at least in the ocean-margins on diatoms, the major, innocuous autotrophs in the oceans. With Si depletion but nitrate and phosphate in abundance, the dinoflagellates - many species of which are toxic to many marine and terrestrial life forms - bloom and bring in unwanted changes in the marine ecosystem including mass mortalities of aquatic fauna and reduced fish harvests. He also highlighted the role of Si in regulating C fluxes [higher C fluxes when Si is adequately available for phytoplankton] and suggested that a great deal of new science needs to be done to address the consequences of reduced Si reaching the oceans due to dam constructions world over. Dr Ittekkot also brought out that 2 micromoles of SiO3 in the euphotic zone is adequate for the normal growth and activities of diatoms. While the deep-sea SiO3 concentrations are much higher, the time taken to reach up in the surface layers is very long and, often may not make it to the surface layers owing to physical barriers such as upper layer stratification. He summed up the seminar by appealing scientific community to address the Si issue from both the proposed river basin connectivity and the long term consequences of ocean productivity and biogeochemistry in an altered Si cycling scenario. Dr Venu Ittekkot works at ZMT, Bremen, Germany. (Reported by: N. Ramaiah)