Chemistry Reference
In-Depth Information
biopolymers etc. in this “animal” system to be understood. Figure 18 contains a postulated
mechanism for the formation of a functional skeleton in siliceous sponges including the
stage that is most likely to be affected by germanium (Krasko et al. 2000)
Diatoms: introduction and structural information
Diatoms are single celled algae of the class Bacillariophyceae and there are more
than 100,000 species known today. They are found in both sea water and fresh water and
are a dominant component of the phytoplankton. They did not exist before the Permian-
Triassic extinction of ~250 Ma ago and are implicated in the change of pattern of location
of many sponge types as their blooms drastically reduced the silicon levels of the oceans
where sponges had previously dominated. They have been studied since the advent of
microscopy in the early 18
th
century and the studies performed on them have largely been
dictated by the techniques available. As time has gone on the process of formation has
been explored progressively by more advanced microscopic techniques (e.g., electron
microscopy, atomic force microscopy) and by chemical, biochemical and cell biology
techniques. The latter has been as important in the study of these organisms as in the
study of silicified sponges discussed above.
The diatom cell wall, or frustule has two halves with the upper half called the epitheca
and the lower half the hypotheca. The upper and lower surfaces of the frustule are known
as valves and the structures extending on the sides and overlapping the two halves of the
cell are the girdle bands (Fig. 19). The valves may be perforated with numbers of pores,
known as areolae and both the pore system and the number of girdle bands is species-
specific. Diatoms also vary in the extent of their silicification. There are two general classes
of diatoms, the centrics and the pennates, thus defined according to their general symmetry
(the centrics are radially symmetric, the pennates have an axis of symmetry) with the
pennates commonly exhibiting a central slit known as the raphe fissure through which
Functional skeleton
Functional skeleton
spicules
spicules
Organic sheath
Organic sheath
Inhibited by
germanium
Inhibited by
germanium
Spongin
Collagen
Spongin
Collagen
Silica deposition
Silica deposition
Silicatein
Silicatein
Myotrophin
Myotrophin
exapinacocytes
exapinacocytes
gene
expression
gene
expression
Silicate
Silicate
sclerocytes
sclerocytes
Figure 18
. A possible mechanism for the formation of a functional skeleton in a siliceous sponge such
as
Suberites domuncula
. Silicate is assumed to cause the expression of silicatein and myotrophin in the
sclerocytes, the cells involved in silicification. N.B. Silicified spicules and collagen are cemented
together to form the functioning skeleton (Krasko et al. 2000).
