1. Division:Chrysophyta Class: Bacillariophyceae
Diatoms
Kingdom: Protista
Division:Chrysophyta
Class: Bacillariophyceae
(Unicellular algae)
It has two valves composed of silica (SiO2) with average size 10 to 100 µm in length.
Images for diatoms

2. Introduction
Diatoms are one of the most important and abundant components of marine phytoplankton with two valves (frustules) composed mainly of of silica (SiO2).
Very important for palaeoclimatic reconstruction (environmental conditions), marine biology and ecology, and industry.
Inhabit all aquatic environments. They occur in the body of the water as plankton or on the bottom where they may be attached to plants or rocks or sand particles, or may be free living and able to move between particles of the substratum. In the open ocean, they live only in upper 200 m of the surface water.

3. Colony of pennate diatoms (Asterionella sp.)
LIVING DIATOM
They occur living in a single cell or colonial attached together by gelatinous filaments. When observed as larger and larger masses, the color takes on a darker color, often deep brown or black. Each cell possesses yellow, olive or golden-brown photosynthetic granules, a central vacuole and a nucleus, although it lacks flagella and pseudopodia.
Colony of pennate diatoms (Asterionella sp.)
Diatoms colonies, long chains

4. Cell contents of living diatom:
Protoplasm: occurs within the silicified frustule, containing a nucleus, mitochondria, plastids, and other various organelles.
Cytoplasm: a colorless plasma on the inner side of the frustule and cell wall.
Nucleus: it is at the center of the diatom. It is often spherical or lenticular and encloses the chromosomes and nucleoli.
Plastids: they are organelles similar to chlorophyll in their photosynthetic abilities.
Volutin: Volutin molecules are nitrogen reserves.
Oil Drops: they are actually sugar and starch in globular form. They are located in the cytoplasm.

5. Structure and morphology of a diatom
Diatom frustule is similar to pillbox or a petri dish. It has two interlocking halves , one larger than the other (called "epitheca" and "hypotheca", respectively). It can vary greatly in shape, ranging from box-shaped to cylindrical, symmetrical as well as asymmetrical.

6. Diatom frustule consists of:
Epitheca: Larger = older valve of the frustule with the girdle elements (epicingulum) connected with it.
Hypotheca: Smaller = younger valve of the frustule with the girdle elements (hypocingulum) connected with it.
Connective Zone: This zone is composed of the overlapping girdle elements of the diatom (the epicingulum and hypocingulum). It acts to connect the valves, forming sutures that still allow the two valves to move apart or towards one another.
Septa: Partitions that are formed within the valves. It can be used to characterize diatoms.
Raphe: occurs in pennate diatoms as an elongated fissure in a valve. The raphe is used by diatom for movement. It is important for identification.

7. Raphes in the Pennate diatoms

8. DIATOM SYMMETRY (PLANES)
Bilaterally symmetrical around the apical and transapical axes (Fig. 3, Fig. 4A).
Bilaterally symmetrical, around the transapical (Fig. 4B) or the apical (Fig. 4C) axis.
Rotational symmetry occurs in some bipolar genera that have no planes of symmetry (Fig. 4D).
Circular symmetry occurs in large group of genera (Fig. 4E).

9. DIATOM ORNAMENTATION
Ornamentation is important for diatom identification.
Valve surface is usually covered with striations, pores, spines, punctations or raphes (an elongated fissure through the valve wall, involved in cell motility).
Centric diatoms: a) Melosira, b) Coscinodiscus, c) Actinoptychus and Thalassiosira.
Some ornamented diatoms

10. TAXONOMY Diatoms divided into two main orders: Order Centrales
(centric diatoms)
Radially symmetric
dominate marine planktonic communities
Example, Genus: Melosira
Order Pennales
(pennate diatoms)
bilaterally symmetric
benthic marine and fresh water communities
Example, Genus: Pinnularia

11. Members of both classes may be, in general, found in either fresh or salt water, though centrate forms tend to predominate in marine habitats, while pennate diatoms are more typical of freshwater environments.
The Pennate diatoms are a monophyletic group derived from centric diatoms. Pennate diatoms have long been viewed as a derived group, and molecular studies agree with this interpretation. Centric diatoms appear in the fossil record about 120 ma, while pennate diatoms do not appear until about 70 ma.