(ii) Chlorella

Chlorella is one of the microbiologically most important green algal protist. It has tremendous practical application and is rightly called the 'guinea pig' of many scientific investigations. It is probably used more extensively in studies of photosynthesis than any other organism.

Calvin and his coworkers (1946) proposed the pathway of CO2 reduction, the calvin cycle, while working on Chlorella pyrenoidosa. This algal protist was considered to be used in photosynthesis studies because of its following important features,

(a) it grows luxuriantly in different environmental conditions and avoids contamination,

(b) the entire vegetative body behaves like a single chloroplast,

(c) it divides very rapidly,

(d) its culture is easy to maintain,

(e) it contains chlorophyll pigments similar to higher plants and, therefore, the end products of photosynthesis are also similar, and if) the young cultures are rich in protein and while approaching to maturity the fat and carbohydrate contents increase.

(iii) Diatoms

Diatoms, the members of division Chrysophyta, are the most plentiful form of phytoplankton in the Arctic. The thousands of species of diatoms provide an ever present and abundant food supply for aquatic organisms.

They are morphologically diverse, non-flagellate, unicellular algal protists. The diatom cell has a prominent, single nucleus and massive ribbon like, or smaller lens-like plastids.

They produce shells (cell walls) impregnated with silica. The shells of Diatoms are called 'frustules'. Deposits of these shells resulting from centuries of growth are called 'Diatomite' or

Diatomaceous earth' which has enormous industrial significance as it is used in insulating materials as a filter for clarifying water, fruit juices, cane sugar and beverages; for cosmetic bases; and for polishing material.