SILICON DEPOSITION IN DIATOMS: CONTROL BY THE pH INSIDE THE SILICON DEPOSITION VESICLE

Abstract

To test the hypothesis that silicification occurs under acid conditions in the silicon deposition vesicle (SDV), the acidity of the SDV of the pennate diatoms Navicula pelliculosa (Brébisson et Kützing) Hilse, N. salinarum (Grunow) Hustedt, and Nitzschia sigma (Kützing) Smith was determined during development of new frustule valves. Cells were incubated with the weak base 3-(2,4-dinitroanilino)-3amino-N-methylpropylamine (DAMP) followed by immunocytochemical localization in whole cells and on ultrathin sections. After resupplying silicate to cells synchronized by silicon depletion, the uptake of this nutrient from the medium was the same with or without DAMP; new valves developed without morphological aberrations that could conceivably have been caused by the probe. DAMP was found in cellular compartments known to be acidic, such as vacuoles active as lysosomes, the lumen of thylakoids, and microbodies. In the nucleus and mitochondria, which are circumneutral and basic compartments, the probe did not appear. Besides its presence in acidic compartments, DAMP was specifically accumulated within the SDV during formation of new valves; during the process of valve maturation, the SDV seemed to become increasingly acidic. In control experiments using the ionophores chloroquine, valinomycin, and nigericin, the compartmental location of DAMP was clearly disturbed, resulting in a random intracellular distribution. Accumulation of the fluorescent probe rhodamine 123, which can be translocated over membranes by a reducing potential, confirmed that the SDV can translocate weak bases. The results with DAMP suggest that the pH of the SDV is important in the silicification of diatoms: It facilitates a fast nucleation and aggregation of silica particles, thus increasing the rate of formation of the mature frustules. In addition, the acidic environment might protect the newly formed valves against dissolution before completion and coverage by the organic casing prior to their secretion. 1 Key index words: Bacillariophyceae; 2,7-bis-(2-carboxyethyl)-5(and-6)-carboxyfluoresceinacetoxymethylester (BCECF-AM); 3-(2,4-dinitroanilino)-3-amino-N-methyl propylamine (DAMP); frustule; organellar pH; rhodamine 123; silicon deposition vesicle (SDV) Abbreviations: BCECF-AM, 2,7-bis-(2-carboxyethyl)-5(and-6)-carboxyfluoresceinacetoxymethylester; DAMP, 3-(2,4-dinitroanilino)-3-amino-N-methyl propylamine; SDV, silicon deposition vesicle; STV, silicon transport vesicle

Salinity-dependent diatom biosilicification implies an important role of external ionic strength

“…In the latter case, ions, cotransported by symport (13), also could enter the SDV and therewith directly change the reaction conditions for silica polymerization. The presence of a membrane potential or proton gradient over the SDV membrane has also been demonstrated by molecular probing (14). The importance of ionic strength in silica polymerization has been demonstrated already.…”

Application of AFM in understanding biomineral formation in diatoms

“…Close examination of detailed structure formation in diatoms that make complicated threedimensional structures [41,42] indicate a level of control over all aspects that is not consistent with solely diffusionbased, self-association, or phase separation process. This is not to discount the contribution of chemical or diffusional processes; for example, the lumen of the SDV has been measured at a low pH [50], which would be conducive to silica polymerization [23], and based on in vitro work, chemical influences such as self-association of silaffins and LCPAs, which appear to be essential for polymerization [27,46], are likely to occur in vivo as well. The discovery that LCPAs are amphiphilic and can self-associate led to the suggestion that sequential formation of LCPA droplets, and their subsequent incorporation into silica could, by phase separation, generate a series of smaller and smaller circular silica structures as were seen in the valve of Coscinodiscus species [47].…”

Exploring Bioinorganic Pattern Formation in Diatoms. A Story of Polarized Trafficking

“…The observation that the acidity of the SDV increases with silica deposition (Vrieling et al, 1999) can be paralleled with the maturation hypothesis of mammalian cells, which postulates that late endosomes/lysosomes derive from maturation of early endosomes (Mullins and Bonifacino, 2001). The identification of markers specific for early and late endosomes and lysosomes (e.g.…”

“…During this process, the SDV becomes acidic as a result of the silica polymerization process (Vrieling et al, 1999). Some of the organic components eventually form a coat around the silica framework, whereas others are involved in silica deposition.…”