(i) The Euglenoids
Kingdom Protista – Euglenoids have at various times been classified in the plant kingdom (with algae) and in animal kingdom (in protozans). Based on molecular data. Euglenoids are throught to be closely related to zooflagellates. They are plant like in their pigments. However. Some photosynthetic euglenoids lose their chlorophyll when grown in dark and obtain their nutrients heterotrophically by ingesting organic matter. Other species of euglenoids are always colourless and heterotrophic (fig. 7.8).
Fig 7.8:Euglenoids have special evolntionary significance as they resemble with plants and green algae in having similar pigments and, on the other hand, are also related to zooflagellates.
(ii) Dinoflagellates
One of the most unusual protest phyla is that of dinoflagellates. Most dinoflagellates are unicellular. Most dinoflagellates are unicellular. Their cells are often covered with shells of interlocking cellulose plates impregnated with silicates.
Ecologically, dinoflagellates are one of the most important groups of producers (second only to diatoms) in marine ecosystem. Dinoflagellates are known to have occasional population explosions or blooms. These blooms frequently colour the water orange, red or brown and are known as red tides (Fig. 7.9).
 (a) |  (b) |
Fig. 7.9(a) A dinoflagellate showing cellulose plates in the shell and flagella located in the grooves. (b) A red tide.
(iii) Diatoms
The cell wall of each diatom consists of two shells that overlap where they fit together, much like a petri dish. Silica is deposited in the shell, and this glasslike material is laid down in intricate patterns.
Fig. 7.10Diatoms have silica shells with extremely beautiful symmetrical patterns
Diatoms are the major producers in the aquatic (marine and freshwater) ecosystems because of their extremely large numbers. Diatoms are very important in aquatic food chains (Fig. 7.10).
Brown algae include the giants of the protest kingdom. All brown algae are multicellular and range from a few centimeters to approximately 75 meters in length. The largest brown algae, called the kelps are tough and leathery in appearance. They possess leaflike blades, stemlike stipes, and rootile anchoring holdfast. Brown algae are common in cooler marine waters, especially along rocky coastlines in the intertidal zone (Fig. 7.11).
Fig. 7.11laminaria, a brown alga showing blades, stipes and boldfast
(iv) Red algae
The multicellular body form of red algae is commonly composed of complex interwoven filaments that are delicate and feathery. A few red algae are flattened sheets of cells. Most multicellular red algae attach to rocks or other substances by a basal holdfast. Some red algae incorporate calcium carbonate in their cell walls from the ocean and take part in building coral reefs alongwith coral animals (fig. 7.12).
Fig. 7.12polysiphonia is a representative red alga with world wide distribution
(vi) Green Algae
Green algae have pigments, energy reserve products, and cells walls that are identical to those of plants. Green algae are photosynthetic, with chlorophylla, chlorophyll b, and carotenoids present in the chloroplasts. Their main energy reserves are stored as starch. Most green algae possess cell walls with cellulose. Because of these and other similarities it is generally accepted that plants arose from ancestral green algae. Evidence from RNA sequencing also indicates that green algae and the plants form a monophyletic lineage (Fig. 7.13).
 (a) |  (b) | ||
 (c) |  (d) |  (e) | |
Fig. 7.13Green algae exhibit diverse forms. (a) unicellular chlamydomonas (b) desmids have cells with two halves. (c) colonial Volvox (d) Filamentous spirogyra (e) Ulva, having sheet like body.
 | Chlorella is a unicellular non-motile green alga. Its habitat is fresh water ponds and ditches. It is easily cultured and has been used as an experimental organism in research on photosynthesis as well as being investigated as an alternate source of food. |
Importance of algae
Algae have great economic and environmental importance for us. Some algae such as kelps are edible and may be used to overcome shortage of food in the world. Marine algae are also source of many useful substances like algin, agar, carrageenan, and antiseptics. Algae are major producers of the aquatic ecosystem, thus they play a basic role in food chains, providing food and oxygen to other organism.