Phytoplankton   CERTIUM2 Eucampia Coccolithophore _my _drawing Prorocentrum _drawing Tha Nitzc Thalass

Are divided into two groups: the diatoms and the dinoflagellates.

Diatoms:  are made up of two halves of a silica case that fits together to form a pill box shaped cell containing chlorophyll. They are found singularly and in chains and can form very beautiful structures. The chained cells are a very good floatation mechanism helping the phytoplankton maintain its position in the water column and therefore preventing it from sinking out of the photic zone. Diatoms reproduce by simply dividing in two, however this reduces cell size and after several divisions an 'auxospore' is formed to recover the original size. Some diatoms also form 'resting spores' when conditions become adverse. This happens when the cell contents are concentrated inside a thick cell wall which can be very different to the original cell shape. The resting spore then sinks to the depths to wait for more suitable conditions, passing into a resting state during the winter and re starting an active life in the spring.

Dinoflagellates: come in many different forms and are characterised by having two whip like flagella, which they use to move through the water column. They use this flagella as a floatation mechanism, to help them stay up in the photic zone. Flagellates are a source of debate amongst scientists because some have the characteristics of plants and animals. Some posses chlorophyll and photosynthesise like true plants, whereas others lack chlorophyll and instead feed by absorbing organic substances through their surface. Some flagellates combine the methods of plant and animal feeding, however most planktonic dinoflagellates are in fact plants.

Dinoflagellates don't have a silca case like diatoms they have cellulose walls. They are usually solitary organisms and rarely form chains. They reproduce like diatoms through cell division, however there is no gradual diminution in size.
Some dinoflagellates are capable of producing toxins that are released into the water. When dinoflagellates bloom and become very numerous, the cumulative effect of the toxin can affect other marine organisms causing mass mortality in fish and invertebrates. Diruetic Shellfish poisioning (DSP) is caused by the dinoflagellate Dinophysis and Paralytic Shellfish poisioning (PSP) is caused by the dinoflagellate Gonyaulax. Both can produce the legendry 'Red Tides' seen in some oceans around the world. Many species of dinoflagellates can produce brilliant phosphorescence, illuminating the sea with sparks of light at night.