ALGAE

  • Algae are chlorophyll containing thallophytes and are characterised by the absence of embryo stage and presence of unicelled non-jacketed gametangia. The study of algae is called algology or phycology.
  • The main characteristics of algae are:

1.    Algae are usually aquatic, either marine or fresh water, or may occur in a variety of habitats like moist stones, soil and wood. 
2.    Reserve food is mainly starch. 
3.    Vascular tissues are absent. Being aquatic, water conduction is not required even in giant forms. 
4.    Nutrition is autotrophic. 
5.    Vegetative reproduction by means of fragmentation. 
6.    Asexual modes of reproduction by means of Mitospores like -Zoospores, Aplanospores, Hypnospores, Akinetes, Palmella stage, etc. 
7.    Sex organs are non-jacketed. They are unicellular. In case the sex organs are multicellular (e.g., Chara), every cell is fertile. 
8.    Sexual reproduction involves gametic fusion, that may be isogamy, anisogamy and oogamy in different groups. 
9.    Life cycle is of various types such as haplontic, diplontic or diplohaplontic. 

Concept Builder

1.    Algae shows a wide range of forms like -unicellular (e.g., Chlamydomonas), colonial (e.g., Volvox), palmelloid (e.g., Tetraspora), dendroid (e.g., Prasinocladus), filamentous unbranched (e.g., Spirogyra) or branched (e.g, Cladophora), heterotrichous (e.g., Fritschiella, Coleochaete, Stigeoclonium), siphonaceous (e.g., Vaucheria), parenchymatous (e.g., Ulva). 

2.    F.E. Fritsch (1935) in his book Structure and Reproduction In Algae proposed a practical and sound classification of algae. He divided algae in 11 classes on the basis of pigmentation, reserve food, flagellation, thallus structure and mode of reproduction. 

3.    Kingdom Plantae of Whittaker (1969) includes mainly three types of algae -red algae, brown algae and green algae. 

Comparison of Some Characteristics of Algae

Algae

Algae are a class of creatures that are mostly aquatic, photosynthetic, and nucleus-bearing but lack the real roots, stems, leaves, and specialized multicellular reproductive systems that plants have. They can be found in a range of various environments, including damp stones, soils, and wood. Some of them are also found in the presence of fungi (lichen) and animals (e.g., on sloth bears).

Algae come in a wide range of shapes and sizes, from colony forms like Volvox to filamentous forms like Ulothrixand Spirogyra. Kelps, for example, are a type of marine organism that develops gigantic plant bodies. Algae reproduce vegetatively, asexually, and sexually. Fragmentation is the mode of vegetative reproduction. A thallus forms from each fragment.Asexual reproduction occurs by the formation of spores, the most common of which are zoospores. They are flagellated (motile) and produce new plants upon germination. The fusing of two gametes occurs during sexual reproduction. These gametes can be flagellated and comparable in size (as in Ulothrix) or non-flagellated and similar in size (as in Spirogyra). Isogamous reproduction refers to this type of reproduction. Anisogamous refers to the fusion of two gametes of different sizes, such as those found in Eudorina species. Volvox and Fucus are examples of oogamous fusion that occurs between a big, nonmotile (static) female gamete and a smaller, motile male gamete.

Algae are beneficial to humans in a number of ways. Algae photosynthesis accounts for at least half of the total carbon dioxide fixing on the planet. They enhance the amount of dissolved oxygen in their immediate environment because they are photosynthetic. They are crucial as primary producers of energy-rich chemicals, which are the foundation of all aquatic species' food cycles. Among the 70 species of sea algae used as food are many Porphyra, Laminaria, and Sargassum species. Certain coastal brown and red algae, such as algin (brown algae) and carrageen (red algae), produce huge amounts of hydrocolloids (water-holding compounds) that are commercially exploited.Agar, a commercial product derived from Gelidium and Gracilaria, is used to culture bacteria as well as in ice cream and jelly preparations. Chlorella, a protein-rich unicellular alga, is utilized as a nutritional supplement by astronauts. Chlorophyceae, Phaeophyceae, and Rhodophyceae are the three major groups of algae that are described below.

A. Chlorophyceae: Green algae are members of the Chlorophyceae family. Unicellular, colonial, or filamentous plant bodies are all possible. Because chlorophyll a and b pigments predominate, they are frequently grass green. Pigments are concentrated in specific chloroplasts. In different species, the chloroplasts might be discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped. In the chloroplasts, most members have one or more storage bodies called pyrenoids. Pyrenoids contain protein besides carbohydrates. Food may be stored in the form of oil droplets by some algae. Green algae have a stiff cell wall made up of an inner cellulose layer and an outside pectose layer.

(a) Volvox
(b) Ulothrix

Figure 1

Vegetative reproduction is usually accomplished through fragmentation or the creation of several spore types. Flagellated zoospores produced in zoosporangia are used for asexual reproduction. Sexual reproduction can be isogamous, anisogamous, or oogamous, with significant differences in the kind and development of sex cells. Chlamydomonas, Volvox, Ulothrix, Spirogyra, and Chara are some of the most common green algae.

B. Phaeophyceae: Brown algae, also known as Phaeophyceae, are mostly found in marine environments. They display tremendous variety in size and form. They range in complexity from simple branched filamentous forms (Ectocarpus) to profusely branched forms such as kelps, which can reach 100 meters in height. Chlorophyll a, c, carotenoids, and xanthophylls are all present. Depending on how much of the xanthophyll pigment fucoxanthin is present in them, they range in color from olive green to various colors of brown. Food is stored as laminarin or mannitol, both of which are complex carbs. The vegetative cells have a cellulose wall that is normally covered with a gelatinous layer of algin on the outside. In addition to plastids, the protoplast has a centrally positioned vacuole and nucleus.The plant body has a stalk, the stipe, and a leaf-like photosynthetic organ, the frond, which is normally linked to the substratum by a holdfast. Fragmentation is used in vegetative reproduction. Biflagellate zoospores, which are pear-shaped and have two unequally attached flagella, are used for asexual reproduction in most brown algae. Isogamous, anisogamous, and oogamous sexual reproduction are all possible. Gamete union can occur in water or within the oogonium (oogamous species). The gametes are pyriform (pear-shaped) and have two flagella connected to their sides. Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus are the most prevalent types.

(a) Fucus
(b) Laminaria

Figure 2

C.Rhodophyceae: Rhodophycea members are frequently referred to as red algae due to the preponderance of the red pigment r-phycoerythrin in their bodies.  The majority of red algae are found in the sea, with higher quantities in warmer climates. They can be found in well-lit areas near the surface of the sea as well as at considerable depths in oceans where light is scarce. The majority of red algae have multicellular red thalli. Some of them have a complicated body structure. The food is stored as floridean starch, which has a structure that is extremely similar to amylopectin and glycogen. In most cases, red algae reproduce vegetatively by fragmentation. Non-motile spores and non-motile gametes are used to reproduce asexually and sexually. Sexual reproduction is oogamous, with complicated post-fertilization processes.Polysiphonia, Porphyra, Gracilaria, and Gelidium are the most prevalent members.

(a) Polysiphonia
(b) Porphyra

Figure 3