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Most of the silicon (Si) in marine coastal systems is thought to recirculate under the biological control of planktonic diatoms. We challenge this view after comparing the biogenic silica (bSi) standing stocks contributed by communities of planktonic diatoms and benthic sponges in five habitats of an extensive continental shelf area of the Mesoamerican Caribbean. In most habitats (outer reefs, patch reefs, sea grass beds, and mangroves), the sponge bSi stocks surpassed those of diatoms. Collectively, bSi in sponge communities was about 88.6% of the total Si pool. Diatoms represented 4.2% and ambient silicate about 7.2%. Consequently, when constructing future regional Si budgets in coastal areas, the Si standing stocks in sponge populations should be empirically examined before deciding that their contribution to the total is negligible. In order to understand Si fluxes in coastal areas where sponges are relevant, we need additional empirical approaches to set the timescale of sponge bSi turnover, which appears to be substantially slower than that of diatom bSi. doi: 10.4319/lo.2010.55.5.2001

The possible mechanisms of uptake of silicic acid from the aquatic environment, its transport across the plasmalemma, and intracellular transport and deposition of silica inside the specialized Silica Deposition Vesicle (SDV) are discussed.

Silicon and carbon uptake rates were studied over a 24 h light/dark cycle in a synchronised culture of the marine diatom Cylindrotheca fusiformis (Reimann et Lewin) using [.sup.32.Si] and [.sup.14.C]. The silicic acid uptake rate per cell ([rho][.sup.c.Si]) varied between 1.2 and 20.0 fmol Si [cell.sup.-1] [h.sup.-1] and was closely correlated to the G2 + M phase of the cell cycle. A linear and significant relationship was determined between the percentage of cells present in G2 + M and [rho][.sup.c.Si]. Evolution of the soluble free-silicon pool was studied simultaneously. The concentration of the total soluble free pool of silicon ([Q.sup.p]Si) varied from 1% to 7% of the total silicon content. A significant difference of 1.5 fmol Si [cell.sup.-1] between [Q.sup.p]Si and the labelled free pool ([Q.sup.np]Si) was measured, indicating the presence of an unlabelled fraction of the pool. The concentration of [Q.sup.np]Si was around 1.0 fmol Si [cell.sup.-1] prior to cell division and did not change as a function of [rho][.sup.c.Si], which indicated a feedback mechanism coupling uptake into the free pool and incorporation into the frustule. In parallel, [.sup.14.C] uptake variation ([rho][.sup.c.C]) was measured during the division of the population. The value of [rho][.sup.c.C] varied between 0.44 and 0.78 pmol C [cell.sup.-1] [h.sup.-1] and appeared to be maximal when cells were in the G1 phase. This variation of [rho][.sup.c.C] marginally affected the total carbon content of the cells ([Q.sup.T]C) in comparison with the light/dark cycle. The variations in the Si/C ratio, from 0.021 to 0.046, demonstrated the different control mechanisms of Si and C metabolisms during the course of the cell- and photocycle., Introduction Diatoms are unicellular algae that have the capacity to form a silicified cell wall called a frustule, which essentially consists of hydrated amorphous silica (Round et al. 1990). Silicon [...]

The ecological success of marine diatoms comes despite their unusual additional requirement for silicon, a nutrient that often limits their growth in the ocean. There may be, however, some physiological and ecological advantages to silicon limitation. A model of nitrogen and silicon metabolism in diatoms (Flynn et al. 1997; Flynn and Martin-Jezequel 2000) suggests that silicon-starved diatoms will recover more quickly than nitrogen-starved (but otherwise identical) diatoms on the resupply of nutrients. In culture, following 24 h of stationary phase induced by nutrient exhaustion, silicon-starved Thalassiosira weissflogii displayed maximum growth rates within a few hours of the readdition of silicon. But 37 h elapsed before nitrate-starved cultures began dividing again and nitrate-starved cultures did not reattain maximum growth rates over the period investigated. The model suggests that, in terms of number, cells descended from the nitrate starved will never catch up to cells descended from the silicon starved if both populations are simultaneously supplied with a pulse of nutrients. The longer the lag between nutrient supply and the resumption of growth in the nitrate-starved cells, the greater the proportion of the supplied nutrients that ends up in the descendants of the silicon starved. In the model run, the silicon-starved cells acquired 75% of the added nutrients and the nitrate starved acquired only 25%. Silicon limitation may thus be a mechanism by which diatoms ensure they respond more quickly than other phytoplankton to the upwelling of nutrients into the euphotic zone.

Although chlorites are an important source of Mg and micronutrients in soils, little is known about their weathering behavior in acidic environments. The effect of organic (acetic, oxalic, citric) and inorganic (hydrochloric, nitric, sulfuric) acids on the dissolution of ripidolite was studied at 25[degrees]C over an acid concentration range of 0.03 to 10 mM. The increasing ripidolite dissolution with increasing acid concentration can be described by the same empirical equation for all six acids used in this study. Because proton and ligand promoted dissolution mechanisms are additive, the influence of ligands can be calculated by subtracting the influence of protons from the total dissolution of ripidolite. We assume that the influence of [Cl.sup.-] in HCl systems can be neglected. Greater dissolution of ripidolite in the presence of the other acids at the same pH is attributed to the accompanying anion. At pH 3.5 and an anion concentration of [10.sup.-3.5] mol [L.sup.-1], the relative effectiveness of the acids used in this experiment in promoting dissolution was nitric (enhancement factor: 0.97) [congruent to] hydrochloric (1.00) [congruent to] acetic (1.01) < sulfuric (1.19) < citric (2.70) < oxalic acid (3.27). The dissolution of ripidolite was nonstoichiometric with a preferential release of Si relative to Al, Fe, and, in some cases, Mg at low proton and ligand concentrations for all six acids. At higher acid concentrations the dissolution becomes almost stoichiometric in the presence of inorganic acids, whereas in the presence of oxalic acid Al, Fe, and Mg were released preferentially relative to Si. In the presence of citric acid, Fe is released preferentially relative to Si. The alteration of chlorites in soils and the amount of released elements into the soil solution therefore depends on the composition of acidifying agents in soils.

Evidence is presented that the silicon uptake rate by diatoms is limited by the equatorial Pacific Ocean's ambient silicic acid concentration. However, low diatom growth rates may also be influenced by other factors, including interaction between the iron and silicon limitations.

The addition of water during the photocatalytic oxidation of liquid 3-octanol and 3-octanone made the hydrophylic photocatalyst stay at the liquid-liquid interface and reduced the rate of CO2 evolution. NaCl, on the other hand, reduced the rate of CO2 evolution further. The photocatalyst is forced to remain dispersed in the organic phase when the TiO2 was overcoated with hydrophobic silicone.

Carbonates are one of the most important group of minerals that fix phosphates in soils. Magnesium and silicon are known to reduce the P fixation of calcite. The effect of Mg and Si addition on P sorption-desorption of a mixture of 90% clay and 10% calcite was studied. The mixtures were montmorillonite-calcite (Mont-Ca[CO.sub.3]), palygorskite-calcite (Pal-Ca[CO.sub.3]), and sepiolite-calcite (Sep-Ca[CO.sub.3]). Concentrations of 0.0, 5.0, and 10.0 mg [L.sup.-1] of dissolved Mg or Si were introduced into P sorbing systems. The chemical speciation of P sorption solutions was predicted using the SOILCHEM computer program. Desorption of sorbed P on different samples was studied in successive extractions with 0.01 M KCl. The P sorption of Mont-Ca[CO.sub.3], Pal-Ca[CO.sub.3], and Ca[CO.sub.3] were similar ([approximately equals] 10 cmol [kg.sup.-1]). Addition of 10.0 mg [L.sup.-1] J Mg or Si sharply reduced the P sorption of calcite to [is less than] 1 cmol [kg.sup.-1], while it induced only a 20% reduction in P sorption of the mixtures. Magnesium and silicon treatments increased the desorption of sorbed P. The Sep-Ca[CO.sub.3] mixture showed a P sorption 60% lower than that of pure Ca[CO.sub.3]. The desorption of sorbed P by Sep-Ca[CO.sub.3] was far above that of other clay-Ca[CO.sub.3] combinations. Addition of Mg and Si inhibited the formation of Ca-phosphates, which would otherwise make major contributions to P retention in all samples used in this study. Phosphate sorbed by silicate clays was the second major fraction of retained P, limiting the effects of Mg and Si treatments. As a slow release source of Mg and Si, sepiolite increased the desorption of P sorbed by Ca[CO.sub.3]. This has an important implication for P availability in arid soils containing sepiolite. This mineral may be mixed thoroughly with commercial P fertilizers before P application into soil.