1. Bacillariophyta: Diatoms

2. Diatom morphology

3. More diatom morphology

4. Centrales: centric diatoms • Radial symmetry • Striae composed of linear punctae • May be single-celled like Cyclotella (above )or colonial: in filaments or like Tabellaria (above)

5. Pennales: pennate diatoms

6. Pennales • Bilateral symmetry • Many groups possess a raphe-these are motile; some have a pseudoraphe • Many are epilithic, benthic, etc. (provides substrate for gliding • Generalization: pennate diatoms are more common in eutrophic waters, centrics in oligotrophic waters; • hence, A/C (araphidinieae : centric) index is one indicator of water quality (in lakes of low alkalinity)

7. Preparation of diatoms for microscopic observation

8. Characteristics • Frustules made of silica (often a limiting nutrient) • Unicellular or colonial • Form diatomaceous earth • Opal is the mineralized form of sedimented diatoms • Reproduction: produces auxospores