· A term alga is derived from latin word (alga- sea weed) used for chlorophyll bearing non-vascular thalloid organisms.
· Carolus Linnaeus (1754) first coined term algae which he placed along with liverworts and lichen under class cryptogamia of conventional plant kingdom.
· But in Whittaker classification, all members of algae are redistributed in three kingdom i.e, monera (Blue Green Algae), protista (Diatoms, Dinoflagellates, Euglenoids) and plantae (Green algae, brown algae and red algae).
· Study of algae is known as phycology or algology.
· F.E. Fritsch is known as father of algology.
· M O P Iyenger from Madras University is known as Father of algology in India.
Occurrence and Distribution:
Most of the algae are aquatic either living in fresh water or marine, few are terrestrial. A few species are even growing in extreme condition like thermal springs, glaciers and snow. Some are found free floating or free swimming which are known as phytoplankton. Species that are found attached to the bottom of shallow water along the edges of seas and lakes are called Benthic. Some of the algae are found in symbiotic association with fungi, they are called as lichens. A few species of algae are epiphytes I.e. they live on another plant or on other algae) and some of them are lithophytes i.e. they grow attached to rocks.
Salient features of algae:
Ø Algae are eukaryotic organisms that have no roots, stems or leaves. Such a plant body is known as thallus. They don’t have vascular tissues and do not have chlorophyll and other pigments for carrying out photosynthesis.
Ø Algae is relatively simple unicellular or multicellular. Unicellular thallus may be non-motile, rhizopus or coccoid. Phytoplankton is the population of free floating micro- organisms composed primarily of unicellular algae. Multicellular thallus may be colonial, palmelloid, dendroid, filamentous.
Ø Cells contain plastid and three classes of pigments namely chlorophyll (a, b, c, d and e) and carotenoids (α, β, γ, theta) and phycobilins.
Ø Most algae are photoautotrophic and carry out photosynthesis. However, some forms are chemoheterotrophic and obtain energy from chemical reaction and nutrients from preformed organic matter.
Ø Most species are saprobes and some are parasitic.
Ø Reproduction occurs by vegetative, asexual and sexual methods.
Ø Vegetative reproduction is by fragmentation, akinesis etc.
Ø Asexual reproduction is by motile zoospores or by non- motile aplospores, autospores, hypnospores, exospores, endospores etc. Spores are produced in sporangia.
Ø Sexual reproduction may be isogamous, anisogamous or oogamous and possess anthridia and oogonia.
Ø Sporophytic and gametophytic generations which are present in the life cycle are independent. So, there are no algae with sporophyte parasite on gametophyte.
Classification of Algae:
Algae are classified into six classes:
I. Eugenophyta or Euglenoids:
It lacks a true cell wall. They are bounded by proteinaceous cell covering known as pellicle. Euglenophytes have 1-3 flagella for locomotion. They store carbohydrate as their reserve food material which is known as paramylon. The primary photosynthetic pigments of euglenophytes are chlorophyll a and b, while their accessory pigments are carotenoids and xanthophylls.
II. Chrysophyta (Golden Brown Algae):
This includes golgen brown algae and diatoms. These algae occur in both marine and fresh water. Cell wall made up of cellulose and pectic material, a type of hemicelluloses. Photosynthetic pigments are chlorophyll a and c and the accessory pigments are carotenoids and xanthophylls. Xanthophyll includes a specialized pigment called as fucoxanthin.
III. Pyrrophyta or Fire algae:
Pyrrophyta includes dinoflagellates. Mostly occur in marine ecosystem but some are found in fresh water. Dinoflagellates have cell wall which is composed of cellulose and have two flagella. These algae store energy as starch. The photosynthetic pigments of the pyrrophyta are chlorophyll a and c and the accessory pigments are carotenoids and xanthhophyll includes fucoxanthin. Its examples are Dinoflagellates
IV. Chlorophyta ( Green algae)
Mostly Chlorophyta occur in fresh water, others are marine. Most Green algae are microscopic, but few species such as those in genus cladophora are multicellular and macroscopic. Cell wall of green algae is mostly constructed of cellulose, hemicelluloses and calcium carbonate. In some species of algae reserve food material of algae is starch. Their cells can have two or more organelles known as flagella, which are used in whip like fashion for locomotion. Photosynthetic pigments of green algae are chlorophyll a and b and their accessory pigments are carotenoids and xanthophylls. Examples are Euglena, chlamydomonas, Spirogyra.
V. Rhodophyta (Red algae):
They are most diverse in tropical water, larger species are typically grown by attaching to the hard substrate, or they occur as epiphytes on other algae. Cell wall of red algae is constructed of cellulose and polysaccharides, such as agar and carageenin. These algae lack flagella and they store energy as a specialized polysaccharide called as floridean starch. Examples are Gelidium, Chondrus, Plumaria. The photosynthetic pigment of red algae is chlorophyll a and d and their accessory pigments are carotenoids, xanthophylls and phycobilins.
VI. Phaeophyta (Brown algae):
Phaeophyta occur in marine environment. These seaweeds are especially abundant in cold water. Brown algae have cell wall which is constructed of cellulose and polysaccharides which is known as alginic acid. Examples are Dictyota, Fucus, Sargassum, Padina.
Gymnosperms are small group of plants which act as connecting link between pteridophytes and angiosperm. Gymnosperms are classified into four classes;Cycadophyta, ginkophyta, gnetophyta, coniferophyta.
Angiosperms are the largest group of plant on this earth. They grow as trees, shrubs , bushes, herbs and small flower plants. Angiosperms are classified in two classes; Monocot and Dicot.
Pteridophyta are first true land plant that evolved after bryophytes. Hence they are known as amphibians of plant kingdom. Pteridophytes are classified into following four classes; Pterophyta, lycophyta, sphenophyta and psilophyta.