(^) Silicon for frustule building must be available in the form of silicic acid; in fact the
entire process of cell division is placed under control of silicic acid availability.
Enzymes requiring it as a cofactor control DNA replication in diatoms. Silicic acid is
taken into the cell in most cases directly before frustule formation, not stored
internally in any great quantity (Darley et al. 1976). Both uptake and deposition
require energy (Lewin 1955). Opal is laid down as “nano” (less than 1 μm) spheres on
a modified protein matrix (Kröger et al. 1999). A backbone of hydroxyl-group-
bearing amino acids has paired, modified lysine residues spaced along it. One of those
carries a chain of 5 to 10 repeats of N-methyl-propylamine, the other is N,N-dimethyl
lysine (Fig. 2.10).
Fig. 2.10 (a) Amino acid sequence of silaffin, the protein promoting opal deposition
from dilute silicic acid solutions and embedded in diatom opal; the side chain of poly-
N-methyl-propylamine is shown expanded; the repeat number is variable. (b) Opal
precipitated on two modifications of silaffin out of dilute silicic acid.
(Courtesy of Nils Kröger, University of Regensburg, with permission from Science.)
ff
(ff)
#1