Photosynthetic Protists

They are also known as protistan algae or plantlike protists. They are represented by mainly the unicellular algae. These organisms are mostly planktonic and represent the phytoplanktons which account for nearly 80% of the total photosynthetic activity in the biosphere. Following are major groups of photosynthetic protists.

Dinoflagellates

These protists belong to a division called Pyrrophyta of algae. They are a well defined group of unicellular, photosynthetic forms. Most of them are flagellated and motile but some forms are non-flagellated.

Dinoflagellates are mostly marine forms. There are some fresh water forms also. Most of the dinoflagellates occur on the surface of water bodies as plankton, imparting characteristic colours to the water. The so-called 'red tide' is due to the characteristic red colour imparted by dinoflagellates belonging to the genus Gonyaulax.

Some dinoflagellates like Noctiluca and Pyrocystis contain phosphorescent granules that make the surface of the sea glow in darkness. This phenomenon is called bioluminescence.

A cell wall may be present or absent. When present it is composed of numerous plates made up of cellulose and is called theca or lorica. Cytoplasm is generally vacuolated and forms numerous radiating strands. The cells usually possess two flagella, differing from each other, arising through the pores in the lorica.

The nucleus is relatively large and prominent and is called as mesokaryon. There are numerous disc- shaped chloroplasts. They contain chlorophyll-a, chlorophyll-c, -carotene and Xanthophylls which provide a shade of yellowish brown colour.

Dinoflagellates reproduce asexually by cell division. Some members form zoospores or cysts. Sexual reproduction can be either isogamous (fusion of similar gametes) or anisogamous (fusion of dissimilar gametes). Life cycle may involve either zygotic meiosis as in Ceratium or gametic meiosis as in Noctiluca.

fig. 8.10 - Noctiluca and Ceratium

Golden Brown Algae or Diatoms

These protistans fall under thephylum - Chryosophyta and are commonly called as diatoms. There are more than 5,000 species of diatoms. They are microscopic, unicellular, protistans which represent a major part of phytoplanktons both in fresh water and marine water.

Diatoms occur in various shapes and colours. They may be circular, rectangular, triangular, elongated, spindle shaped or filamentous.

fig. 8.11 -Types of Diatoms

The cell body of a diatom is called as a frustule. It is bound by a cell wall composed of cellulose impregnated with silica forming characteristic ornamentations. The cell wall has two overlapping halves called valves or theca that fit together like the two parts of a soap box. The upper valve is called as epitheca and the lower valve is called as hypotheca.

A flagella is absent except in the reproductive stages. The cells may exhibit gliding type of movement with the help of mucilage secretion.

The cell has a large central vacuole in which the diploid nucleus lies suspended with the help of cytoplasmic strands. Disc-like chloroplasts are present containing the pigments

They have chlorophyll a and c, fucoxanthin and xanthophyll.

Diatoms reproduce commonly by binary fission which involves simple cell division. During this process, each daughter individual retains one half of the parent cell and the other half is newly synthesized. What is retained, always becomes the epitheca of the daughter cell and what is newly synthesized will be the hypotheca. As a result, during each fission, one of the two daughter individuals becomes progressively smaller in size.

fig. 8.12 - Reproduction in Protists

Sexual reproduction involves the fusion of gametes produced by meiotic division of the parent cell. The gametes fuse in pairs to form a diploid zygote which gets transformed into rejuvenating cells called auxospores. The auxospores develop into frustules of normal size.

The siliceous cell walls of diatoms are indestructible and do not decay easily. These get deposited at the bottom of the sea to form diatomaceous earth. It is mined out from the sea and is used in several industrial processes.