Silicon nutrient ocean

The committee recommends the development of a seawater-based reference material containing the nutrient elements nitrogen (as N03), phosphorus (as P04) and silicon (as Si(OH)4) at concentrations similar to those in oceanic deep waters (40 pM for N03, 3 pM for P04, and 150 pM for Si(OH)4) and certified for these constituents. 

Over the last three decades, we have moved from the monoHthic view that nitrogen is the primary limiting nutrient for phytoplankton in the world s oceans (e.g., Thomas, 1966 Ryther and Dunstan, 1971), to the recognition that iron, silicon or phosphorus may at times be in shortest supply. Further, it is recognized that separate groups with distinct biogeochemical functions exist within the phytoplankton that may concur-rendy exhibit limitation by different nutrient factors (Dugdale and Wilkerson, 1998). 

Treguer, P., Jacques, G. (1992). Dynamics of nutrients and phytoplankton, and fluxes of carbon, nitrogen and silicon in the Antarctic Ocean (Review). Polar Biology 12, 149—162. 

The upper limit of primary production is set by the supply of nutrients (nitrogen, phosphorus, silicon, iron) to the euphotic zone. Nitrogen inputs to the surface ocean may limit the primary productivity of the whole ocean over short time-scales. Over timescales approaching and exceeding the (l-5)XlO" yr residence time of phosphorus (Ruttenberg, 1993 Filippelli and Delaney, 1996), its inputs limit global ocean primary productivity (Tyrrell, 1999). 

Because of the importance of diatoms in oceanic productivity, silicon is an important algal nutrient in seawater. A transporter of Si(OH)4 has been isolated and sequenced (Hildebrand et al, 1998 Hildebrand et al, 1997) and the physiology of silicon uptake has been well studied (Martin-Jezequel et al, 2000). Nonetheless, the molecular mechanism of Si(OH)4 transport and silica fmstule formation in diatoms are still largely mysterious. From indirect evidence, it appears possible that the Si(OH)4 transporter may contain zinc, coordinated to cysteines, as a metal center in the portion of the protein exposed to the outside of the cell (Hildebrand, 2000 Rueter and Morel, 1981). If true, this would be an unusual example of a transport protein functioning with a metal center. 

The rate of growth of phytoplankton (primary productivity) in the oceans is mainly limited by the availability of light and the rate of supply of limiting nutrients (usually accepted to be nitrogen (N), phosphorus (P), silicon (Si) and iron (Fe)).The need for light confines productivity to the upper layers of oceans. Also, in polar waters there will be no phytoplankton growth during the dark winter months. 

Elements showing nutrient-like distribution often have long oceanic residence times, although shorter than conservative elements. The residence times of NO( silicon and DIP have been estimated to be 57 000,20000 and 69000 years respectively (Table 6.9). The vast reservoirs of nutrients in the deep ocean mean that increases in the concentrations of NO( in riverwaters due to human activity (see... 

Other substances also have well-studied cycles. Water, of com e, is constantly moving into, through, and out of the ocean. Some of the atmospheric gases such as oxygen and carbon dioxide are vitally important to life. Nutrient elements such as nitrogen, phosphorus, and silicon are necessary to the phytoplankton, and form the basis of the oceanic food web. 

Silicon enters the sea by glacial weathering of rocks in Antarctica and through rivers. Silicon concentrations in rivers very much depend on the geological formation of the respective area. The highest concentrations are found in volcanic areas (350-550 /nutrient concentrations of oceanic surface waters apply also to silicate. Deep water concentrations range from about 25 pmol/L (Atlantic) to 170 jumol/L (Pacific). 

Hendry, K.R., Georg, R.B., Rickaby, R.E.M., Robinson, L.F., and Halliday, A.N. (2010) Deep ocean nutrients during the last glacial maximum deduced from sponge silicon isotopic compositions. Earth Planet. Sci. Lett., 292 (3 ), 290-300. 

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