57
Anatomy
fultoportulae, except that it has a simpler inner structure. The rimoportula does not have satellite
pores in the inner matrix. However, the rimoportula does have some elaborate outer structures that
bend, have slits, or are capped. Sometimes the valve can outgrow beyond its margin in structures
called setae that help link adjacent cells into linear colonies as in Chaetoceros spp. or possesses
protuberances as in Biddulphia spp., which allow the cells to gather in zigzag chains (Figure 2.13).
In other genera such as Skeletonema, the valve presents a marginal ridge along its periphery consisting of long, straight spines, which make contact between adjacent cells, and unite them into
filaments. Some genera also possess a labiate process, a tube through the valve with internally
thickened sides that may be flat or elevated.
Diatoms are by far the most significant producer of biogenic silica, dominating the marine silicon cycle. It is estimated that over 30 million km 2 of ocean floor are covered with sedimentary
deposits of diatom frustules. The geological and economical importance of these silica coverings as
well as the mechanism of silica deposition will be discussed in Chapter 4.
Cell Wall
A cell wall, defined as a rigid, homogeneous, and often multilayered structure, is present in both
prokaryotic and eukaryotic algae.
In Cyanobacteria, the cell wall lies between the plasma membrane and the mucilaginous
sheath; its fine structure is of Gram-negative type. The innermost layer, the electron-opaque layer
or peptidoglycan layer, overlays the plasma membrane, and in most cyanobacteria its width varies
between 1 and 10 nm, but can reach 200 nm in some Oscillatoria species. Regularly arranged
discontinuities are present in the peptidoglycan layer of many cyanobacteria; pores are located
in single rows on either side of every cross-wall and are also uniformly distributed over the cell
surface. The outer membrane of the cell wall appears as a double track structure tightly connected
FIGURE 2.13 Cells of Biddulphia sp.
Anatomy
fultoportulae, except that it has a simpler inner structure. The rimoportula does not have satellite
pores in the inner matrix. However, the rimoportula does have some elaborate outer structures that
bend, have slits, or are capped. Sometimes the valve can outgrow beyond its margin in structures
called setae that help link adjacent cells into linear colonies as in Chaetoceros spp. or possesses
protuberances as in Biddulphia spp., which allow the cells to gather in zigzag chains (Figure 2.13).
In other genera such as Skeletonema, the valve presents a marginal ridge along its periphery consisting of long, straight spines, which make contact between adjacent cells, and unite them into
filaments. Some genera also possess a labiate process, a tube through the valve with internally
thickened sides that may be flat or elevated.
Diatoms are by far the most significant producer of biogenic silica, dominating the marine silicon cycle. It is estimated that over 30 million km 2 of ocean floor are covered with sedimentary
deposits of diatom frustules. The geological and economical importance of these silica coverings as
well as the mechanism of silica deposition will be discussed in Chapter 4.
Cell Wall
A cell wall, defined as a rigid, homogeneous, and often multilayered structure, is present in both
prokaryotic and eukaryotic algae.
In Cyanobacteria, the cell wall lies between the plasma membrane and the mucilaginous
sheath; its fine structure is of Gram-negative type. The innermost layer, the electron-opaque layer
or peptidoglycan layer, overlays the plasma membrane, and in most cyanobacteria its width varies
between 1 and 10 nm, but can reach 200 nm in some Oscillatoria species. Regularly arranged
discontinuities are present in the peptidoglycan layer of many cyanobacteria; pores are located
in single rows on either side of every cross-wall and are also uniformly distributed over the cell
surface. The outer membrane of the cell wall appears as a double track structure tightly connected
FIGURE 2.13 Cells of Biddulphia sp.