Editing Content :: Admin Scraper

Chapter

Content type

Position

Value

26 BIOLOGY Algae are useful to man in a variety of ways. At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis. Being photosynthetic, they increase the level of dissolved oxygen in their immediate environment. They are of paramount importance as primary producers of energy-rich compounds which form thebasisofthefoodcyclesofallaquaticanimals.ManyspeciesofPorphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food. Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), e.g., algin (brown algae) and carrageen (red algae) which are used commercially. Agar, one of the commercial products obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies. Chlorella, a unicellular alga rich in proteins, is used as food supplement even by space travellers. The algae are divided into three main classes: Chlorophyceae, Phaeophyceae and Rhodophyceae. 3.1.1 Chlorophyceae The members of chlorophyceae are commonly called green algae. The plant body may be unicellular, colonial or filamentous. They are usually grass green due to the dominance of pigments chlorophyll a and b. The pigments are localised in definite chloroplasts. The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species. Most of the members have one or more storage bodies called pyrenoids located in the chloroplasts. Pyrenoids contain protein besides starch. Some algae may store food in the form of oil droplets. Greenalgaeusuallyhavearigidcellwallmadeofaninnerlayerofcellulose and an outer layer of pectose. Vegetative reproduction usually takes place by fragmentation. Asexual reproduction is by flagellated zoospores produced in zoosporangia. The sexual reproduction shows considerable variation in the type and formation of sex cells and it may be isogamous, anisogamous or oogamous. Some commonly found green algae are: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara (Figure 3.1a). 3.1.2 Phaeophyceae The members of phaeophyceae or brown algae are found primarily in marine habitats. They show great variation in size and form. They range from simple branched, filamentous forms (Ectocarpus) to profusely branched forms as represented by kelps, which may reach a height of 100 metres. They possess chlorophylla, c, carotenoids and xanthophylls. Theyvaryincolourfromolivegreentovariousshadesofbrowndepending upon the amount of the xanthophyll pigment, fucoxanthin present in Reprint 2026-27