Chemistry Reference
In-Depth Information
ionized form as well. In marine diatoms, uptake is linked to the transport of sodium ions
in a 1Si:1Na ratio (Bhattacharyya and Volcani 1980). Silicic acid uptake is specifically
induced just prior to cell wall silicification (Sullivan 1977). More recent studies have
defined and sequenced a family of five silicic acid transporters (SIT 1-5) (Hildebrand et
al. 1997, 1998) from one specific diatom,
Cylindotheca fusiformis
. These transporters
have highly conserved hydrophobic regions, a signature amino acid sequence for sodium
symporters and a long hydrophilic carboxy-terminus (Hildebrand et al. 1997; Hildebrand
2000). The carboxy-terminal segment in all five SITs had a very high probability to form
a coiled-coil structure, suggesting that this portion of the protein interacts with other
proteins, It is proposed that this end of the molecule may be involved with control of
activity and localization (Hildebrand et al. 1998). The most significant amount of
variation between the five SIT proteins was in the C-terminus region suggesting that the
different SIT proteins might be found in distinct locations and perform different roles
during the formation of the silicified cell wall. Exact information on specific locations for
the individual SITs is at present unavailable so this hypothesis cannot be validated.
In most diatom species silicic acid transport and cell wall silica deposition are
temporally coupled (Chisholm et al. 1978) and the model that satisfies the experimental
data is that of “internally controlled uptake” that requires a feedback mechanism
operating through saturation or near saturation of intracellular pools (Conway and
Harrison 1977). The rate of silicic acid uptake is largely independent of external silicic
acid concentration, but for exceptions see below, and depends on the rate of utilization or
cell wall deposition. More recently it has been proposed that regulation of uptake may not
be determined by the absolute levels of silicic acid in the intracellular pools but by the
relative ratio of silicic acid to silicic acid binding components (Hildebrand 2000).
According to this model, silicic acid for the cell wall is drawn from internal pools as
needed with flux from the pools connecting the silicification process with uptake. In this
model, uptake does not drive silicification. Although the components of this model are
consistent with one another, there is likely to be some effect of external silicon
concentration on uptake and transport as the overall extent of silicification for a diatom
species can be affected by external silicic levels (Martin-Jezequel et al. 2000) in much the
same way as has been found for sponges.
Internal silicon pools
Intracellular pools of soluble silicon were identified in the mid 1960's by Werner
(Werner 1966). There have been difficulties in measuring the soluble silicon pool due to
problems in the estimation of cell water volume and hence water content and thus widely
differing measures of cell silicon have been reported. Reliable estimates from Hildebrand
(Hildebrand 2000) where the assumption has been made that the soluble silicon,
measured as orthosilicic acid is evenly dispersed throughout the cell, gives a range of 19-
340 mM for a range of diatoms, Figure 22. For an individual diatom these values can also
change in a regular manner during the course of cell wall synthesis (Hildebrand and
Wetherbee 2003). The solubility limit for orthosilicic acid is of the order of 2 mM and
may drop to ca. 0.5 mM (Harrison and Loton 1995) in the presence of specific positively
charged ions including ammonia/amine counterions thus the concentrations measured are
all in vast excess of this and questions arise as to the location of the silicic acid and to
how such high levels can be maintained?
Experiments performed on
Nitzschia alba
and
Cylindrotheca fusiformis
(Mehard et
al. 1974) showed that silicic acid was able to move around the cell freely as experiments
with radiolabelled silicic acid co-purified with a range of cell organelles and subcellular
fractions. Three possibilities for the maintenance of high solution concentrations are,
silicic acid is stabilized as the monomer or as small oligomers by complexation (the use
