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Algae - Lesson Summary

Algae are thalloid autotrophic organisms that usually thrive in aquatic habitat. Algae reproduce through vegetative, asexual and sexual methods. Vegetative reproduction occurs through fragmentation wherein each fragment develops into a thallus, while asexual reproduction takes place through zoospores, which germinate into new plants. During sexual reproduction, two gametes fuse to form a new organism. Based on the size of the gametes, sexual reproduction can be further divided into isogamous, anisogamous and oogamous reproduction. When the fusion is between two gametes of similar size, the reproduction is called isogamous. When the fusion is between two gametes of dissimilar size, the reproduction is called anisogamous, while fusion between a large, non-motile female gamete and a small, motile male gamete is called oogamous reproduction.
Based on the type of pigment and the type of stored food, algae are classified into these classes—Chlorophyceae, Phaeophyceae and Rhodophyceae. Chlorophyceae includes algae such as chlamydomonas, volvox and spirogyra, commonly called green algae. Chlorophyll a and b pigments in chloroplast give the algae its green colour. The cell has several chloroplasts that contain pyrenoids. They can reproduce vegetatively through fragmentation and asexually by flagellated zoospores, as well as sexually.

Phaeophyceae includes algae such as ectocarpus, dictyota and laminaria, commonly called brown algae.  The varying concentration of xanthophylls, fucoxanthins, chlorophyll a, c, and carotenoids pigments impart the algae its brown colour. The protoplast inside the cell contains plastids and a centrally located vacuole and nucleus. They reproduce through fragmentation and asexually through biflagellate spores. On the other hand, in sexual reproduction, the gametes unite either in water or within the oogonium.
Rhodophyceae includes algae such as polysiphonia, gracilaria and gelidium, commonly called red algae. The high concentration of r-phycoerythrin pigment imparts the algae its red colour. The thallus of red algae are multicellular and food is stored as floridean starch. Red algae reproduce vegetatively, asexually through non-motile spores and sexually through motile spores.


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