SEM, SEM-images, SEM-images of diatoms, diatoms

Diatoms display a fascinating variety of forms and delicate structures when investigated with a microscope, but often the spatial arrangements of structural elements still remain enigmatic, since at higher magnifications the in this case very restricted depth of field makes interpretation difficult. Moreover, often disturbing image overlays with deeper structures occur, especially when inspecting diatoms from the side ( "girdle band view").

When studying diatoms the SEM shows its advantages, not only because of its higher resolution and depth of field, but because the SEM is an electronic "reflected light microscope": You see the diatom either from the outside or from the inside, and this literally "one-sided approach" makes the analysis of the spatial structure of the frustules an easier task. But there are also disadvantages, because you cannot see the outer side and the inner side of the frustule at the same time. So the internal septa remain concealed when you look at the diatom from the outside. This makes identification difficult.

At least three different types of shell-constructions can be distinguished, they are here referred to by the letters A, B and C:

Type A: The wall of the frustules is very thin and perforated with fine pores which allowe diffusion. Often very small species belong to this type, such as Cyclotella and Navicula, but also larger species like Pleurosigma and Gyrosigma.

Type B: The wall of the frustules is composed of coarse parallel or intersecting ribs, which are covered from inside with a very thin perforated silica wall whose pores allow diffusion. The spaces between the ribs appear as slots or areolae, the fine pores of the inner wall usually remain invisible because they are below the maximum resolution of the normal microscope. This is typical of many Centrales, such as Triceratium.

Type C: Like type B, but the thin perforated silica wall is positioned outside. This is typical of many Pennales, such as Epithemia.

Probably there is one more type: Frustules with an inner and an outer perforated wall ( "sandwich structure"). It is noteworthy that HUSTEDT has described all these types more than fifty years ago!

The upper row of images (Fig.1 to 5) shows species which probably all belong to type A.

Species of the genus Triceratium (Fig. 9 to 14) show the thin inner wall (type B). This wall can be seen even with a microscope.

The genus Arachnoidiscus (Fig.15 and 16) provides an unusual image in the SEM: When investigated with a microscope the coarse radial ribs are predominant, but using the SEM you can see them only when the inner surface is inspected (fig.15) - investigated from outside the resulting image is very surprising.

Actinoptychus (the "folded star", Fig.17 and 18) is the natural enemy of the micro-photographer, because the deeply corrugated frustules cannot be brought into focus satisfactorily even when moderately magnified. Using a common microscope one seems to see two different coarse pore systems masking each other. The true conditions are shown in figure 18: There is only one single pore system with smaller pores located in larger ones - assigning them to the shell types defined above is impossible.

The interior of Epithemia corresponds to the optical image, because the coarse septa can be seen from the outside and the inside as well, they are predominant when inspected with a microscope, but the subtle pore structure is nearly invisible. In particular we cannot detect the intricately shaped apertures (pores which are closed by a leaf-shaped structure).


21	22	23	24 Biddulphia	25 Biddulphia

26 Mastogloia	27 Mastogloia	28 Mastogloia	29 Biddulphia	30 Biddulphia

31 Ardissonia	32 Cocconeis	33 Cocconeis	34 Cocconeis	35 Actinoptychus

36 Actinoptychus	37 Actinoptychus	38 Triceratium	39 Triceratium	40 Cymbella

41 Cymbella	42 Navicula	43 Navicula	44	45


1 Aulacodiscus	2 Biddulphia	3 Navicula	4 Diploneis	5 Cymbella

6 Navicula	7 Pinnularia	8 Coscinodiscus (?)	9 Triceratium	10 Triceratium

11 Triceratium	12 Triceratium	13 Triceratium, innen	14 Triceratium, innen	15 Arachnoidiscus, innen

16 Arachnoidiscus	17 Actinoptychus	18 Actinoptychus	19 Epithemia, innen	20 Epithemia, außen

DIATOMS SEEN WITH THE SEM