Phylum Bacillariophyta (Diatoms)

Diatoms, are the most important phytoplankton in the ocean. Diatoms are usually single celled but often occur in chains of cells. The cell shape of diatoms varies. There are two basic forms. Centric diatoms are radially symmetrical while Pennate types are bilaterally symmetrical. Both types of diatoms have an external cell wall, or theca, composed of silicon dioxide. The theca is usually in two parts, with a slightly larger epltheca fitting snugly over the hypotheca. The cell contents are contained completely within the test. Most diatoms exhibit fine lines on the test surface. These lines are actually rows of very small pores. Exchange across the cell wall occurs through these pores.

Phylum Pyrrhophyta (Dinoflagellates)

The dinoflagellates are almost entirely marine.They are second only to the diatoms in importance as marine phytoplankton. They have a cell wall composed of cellulose which has a groove around its equator in most species. The equatorial groove contains a flagellum wrapped around it. Another groove perpendicular to the equatorial groove contains another flagellum. The long flagella permits the cells a limited degree of mobility.

Many species of dinoflagellates are bioluminescent and are quite noticeable when disturbed. They glow in the moist sand when disturbed by footsteps. Their glow may also be seen in the wake of a boat and in the crashing waves.

Some species of this division produce strong neurotoxins (poisons that affect the nervous system). When certain environmental conditions are right, a bloom of these algae will occur forming what is commonly called a red tlde. The water is discolored by the dinoflagellates' storage products of oil that are often bright red. Filter-feeding organisms, such as mussels, concentrate these dinofiagellates in their bodies in such great numbers during periods of red tide that they become poisonous. Individuals eating shellfish during the periods of red tide may suffer paralytic shellfish poisoning, which can cause death. The poisons secreted into the waters by these dinoflagellates can also cause death among fish and some fish-eating birds and mammals.