Algae have a wide range of sizes and shapes. Many species occur as single cells that may be spherical, rod shaped, club shaped, or spindle-shaped. Others are multicellular and appear in every conceivable form, shape and degree of complexity, including membranous colonies, filaments grouped singly or in clusters with individual strands that may be branched or unbranched, and tubes. Some colonies are simply aggregations of single, 'identical' cells that cling together after division; others are composed of different kinds of cells specializing in particular functions. These colonies become quite complex and superficially resemble higher plants in structure.
Algal cells are eukaryotic. In most species the cell wall is thin and rigid. Cell walls of diatoms are impregnated with silica, making them thick and very rigid; they are often delicately sculptured with intricate designs characteristic of the species or variety. The motile algae such as Euglena have flexible cell membranes called periplasts. The cell walls of many algae are surrounded by a flexible, gelatinous outer matrix secreted through the cell wall, reminiscent of bacterial capsules. As the cells age, the outer matrix often becomes pigmented and stratified.
Algae contain a discrete nucleus. Other inclusions are starch grains, oil droplets, and vacuoles. Chlorophyll and other pigments are found in membrane bound organelles known as chloroplasts. These chloroplast may be massive structures situated near the wall or embedded in the midst of the cytoplasm. They may occur as one, two, or many per cell; they may be ribbon like, netlike, or in the form of discrete disks, as in green plants. Within the plastid matrix or stroma are found flattened membranous vesicles called thylakoids.
Algal pigments
The chloroplast of different divisions of algae containing similar pigments appear to have similar thylakoid arrangements. Chloroplast ultrastructure and pigment chemistry have been used as markers for algal phylogeny.
It should be pointed out that several divisions of algae include colorless members, e.g., certain species and genera in the Euglenophycophyta, the Pyrophycophyta and the Chlorophycophyta. These are sometimes considered Protozoa. Nevertheless, some colorless flagellates have been shown to posssess chloroplasts. There are three kinds of photosynthetic pigments in algae; Chlorophylls, carotenoids, and biloproteins.
Chlorophyll: There are five chlorophyll: a, b c, d, and e. Chlorophyll a is present in all algae, as it is in all photosynthetic organisms other than anoxygenic photosynthetic bacteria. Chlorophyll b is found in the Euglenophycophyta and Chlorophycophyta and in no other algal division. Chlorophyll c is more widespread and is present in members of Xanthophycophyta, Bacillariophycophyta. Chlorophyll d appears to be present only in the Rhodophycophyta. Chlorophyll e is rare and has been identified in only two genera of Xanthophycophyta, namely, Triboneara and the zoospores of Vaucheria.
Carotenoids: There are two kinds of carotenoids: carotenes and xanthophylls. Carotenes are linear, unsaturated hydrocarbons, and xanthophylls are oxygenated derivatives of there.
Biloproteins: These are water soluble pigments, whereas chlorophylls and carotenoids are lipid-soluble. Phycobilins are pigment-protein complexes and are present in only two algal divisions; the Rhodophycophyta and Cryptophycophyta. There are two kinds of phycobilins: phycocyanin and phycoerthrin. The proportion of one kind of pigment to another can vary considerably with changes in environmental conditions. Pigment quantitation is therefore not too reliable for use as a taxonomic features.
Motility: The motile algae, also called the swimming algae, have flagella occurring singly, in pairs or in clusters at the anterior or posterior ends of the cell. Since the advent of electron microscopy, considerable variation of taxonomic significance has been found in algal flagella. It will suffice for this discussion to mention three types: whiplash, tinsel and ribbon or straplike. Some algae have no means of locomotion and are carried about by tides, waves and currents. Others move about by means other than flagella. In some forms only the zoospores, the asexual reproductive cells, are motile. Some attach themselves to the substrate in the body of water where they are living and are occasionally broken loose by currents, which move them to new locations.
A small red or orange body, the eyespot, is often present near the anterior end of motile algae. Other structures occurring in certain algae include spines or knobs on their exteriors and gelatinous stalks by which they may be anchored to some objects.
Reproduction: Algae may reproduce either asexually or sexually. Some species are limited to one of these processes, but many have complicated life cycles employing both means of propagation.
Asexual reproductive processes in algae include the purely vegetative type of cell division by which bacteria reproduce. A new legal colony or filament may even start from a fragment of an old multicellular type from which it has broken. However, most asexual reproduction of unicellular spores, many of which, especially in the aquatic forms, have flagella and are motile; these are called zoospores. The nonmotile spores, or aplanospores, are more likely to be formed by the terrestrial types of algae. However, some aplanospores, are more likely to be formed by the terrestrial types of algae. However, some aplanospores can develop into zoospores.
All forms of sexual reproduction are found among the algae. In these processes there is a fusion of sex cells, called gametes, to form a union in which 'blending' of nuclear material occurs before new generations are formed. The union of gametes forms a zygote. If the gametes are 'identical' i, e if there is no visible sex differentiation, the fusion process is isogamous. If the two gametes are unlike, differing in size, the process is heterogamous. As we proceed to the higher, through not necessarily larger, forms of algae, the sexual cells become more characteristically male and female. The ovum is large and nonmotile, and the male gamete is small and actively motile. This type of sexual process is termed oogamy. Exclusively male or exclusively female thalli also exist. Although these thalli may look alike, they are of opposite sex types, since one produces male gametes and the other ova. Such plants are called unisexual or dioecious. Plants in which gametes from the same individual can unite are said to be bisexual or monoecious.