The below mentioned article provides a note on algae.
Algae are eukaryotic photosynthetic thallophytes lacking archegonia. Although most of the taxonomic groups of algae include multicellular macroscopic organisms, there are also unicellular forms in majority of such groups. That is what justifies their inclusion among microorganisms.
As a major group of living organisms, algae are highly heterogenous in size, morphological complexity and reproductive behaviour starting from microscopic unicellular motile or non-motile types, some algae may attain an enormous size. The unicellular forms can be motile with the help of flagella or without any locomotive organelle, or may be non-motile.
Again, the unicellular algae can be free-living organisms or may form characteristic colonies, known as coenobia. Multicellular forms may be filamentous which may be branched or un-branched. The branched filamentous algae may have a heterotrichous habit with a prostrate and an erect system.
More complex thallus organization having a tissue differentiation is also observed among some algal groups, specially growing in the marine environments. There are also many coenocytic algae, both unicellular or filamentous.
Most of the algae are aquatic, growing in fresh-water or marine habitats. Some are terrestrial growing on moist soil or rocks, or on bark and leaves of trees. The fresh-water and marine unicellular forms growing as planktons produce huge amount of organic matter and oxygen by photosynthesis.
About 80% of the earth’s oxygen has been estimated to be produced by such planktonic algae. As primary producers they also provide the base of the aquatic food chain serving as food for aquatic animal organisms inhabiting both marine and fresh-water environments.
All types of algae as also the cyanobacteria which are considered as blue-green algae perform oxygenic photosynthesis using water as exogenous hydrogen or electron donor. They possess different types of chlorophylls and carotenoids as photosynthetic pigments. Some, like the red algae and cyanobacteria also contain phycobilirubins as accessory pigments.
All groups of algae including cyanobacterra have chlorophyll a as a common photosynthetic pigment. Besides, chlorophyll a, other chlorophylls, like chlorophyll b, c, d and e are present in different taxonomic groups. Thus, green algae have chlorophylls a and b, brown algae have chlorophylls a and c, red algae have chlorophylls a and d, and so on.
Apart from being a very important component of the biosphere, serving as primary producers of organic matter and oxygen, many algae have important economic value. Various useful products of commerce are obtained from different species of algae. Among such products are alginic acid, carrageenan, agar-agar, keiselguhr or diatomaceous earth, etc.
Alginic acid and its salts, alginates obtained from some brown algae, like Laminaria, Macrocystis, Fucus etc. are used in the manufacture of various food products, as well as in paper, textile and paint manufacture. Carrageenan’s and agar-agar are extracted from several species of red algae, like Chondrus, Gigartina, Gracilaria etc.
These products are chemically sulfated galactans and are used as stabilizer in food industries as well as thickening agent. Agar-agar is universally used for solidification of microbiological growth media. Diatomaceous earth contains the silicified remains of marine diatoms. This product finds use as a fine abrasive in preparation of various commercial products including toothpaste.
Most of the earth’s petroleum reserves are believed to have originated from the diatoms and other marine planktonic organisms that lived millions of years ago. They were buried deep in the marine sediments. Under the anaerobic conditions prevailing in these sediments, the organic matters of the deposited organisms were slowly converted into hydrocarbons of the crude petroleum. This is why petroleum is also known as “fossil fuel’.
Although algae are generally non-pathogenic, some species are known to cause mild plant diseases. One such alga is Cephaleuros virescens causing red-rust disease of tea plants. Other species of this genus infect Rhododendron and Magnolia. Some diatoms and dinoflagellates have been reported to produce neurotoxins. Domoic acid is one such neurotoxin elaborated by some diatoms.
Mussels feeding on diatoms retain the toxin. Consumption of these mussels by humans leads to development of neurotoxic symptom including diarrhoea and loss of memory. Some dinoflagellates, like Gymnodinium and Alexandrium are also known to produce neurotoxins.
Such toxins have been the cause of large-scale mortality of fish populations and other aquatic animals. The toxin, called saxitoxin produced by Alexandrium is concentrated in shell-fish, like mollusks and mussels. Consumption of these sea-animals by humans is known to cause paralytic symptoms. Another dinoflagellate, Pfiesteria has been identified as a major fish-killer.