In this article we will discuss about the morphological and cellular characteristics of algae.
Morphological Characteristics of Algae:
Algae exhibit a very wide range of morphological diversity. The simplest forms are unicellular, microscopic, motile or non-motile eukaryotic cells. They may be spherical (Protococcus, Chlorella), or pyriform (Chlamydomonas). When motile (Volvox, Chlamydomonas) the cells are generally provided with a pair of eukaryotic flagella. Diatoms show a characteristic type of non-flagellar locomotion.
Motile or non-motile algae may form a colony, known as a coenobium. There are also many multicellular algae. These may form uniseriate or multiseriate filaments which may be branched or un-branched. The branched filaments may have prostrate and erect branches (heterotrichous habit).
The multiseriate filaments may form a cylindrical thallus or sometimes a flat thalloid structure. The siphonaceous algae have coenocytic body (multinucleate, without septa) which may be simple or complex and elaborate. The brown algae which are exclusively marine and always multicellular, often have large complex thalli. Diatoms are unicellular algae, but they have a cell which is unique. It consists of two overlapping halves or valves, like those of a petridish.
Some lower forms of algae have a doubtful systematic position. Many of them, like the chrysomonads are amoeboid. Euglenoids, have a flexible cell-covering. They are without a rigid cell wall and resemble protozoa in many ways. The dinoflagellates are also peculiar in having a typically flattened cell with an equatorial constriction, known as a girdle. However, all such atypical organisms are photosynthetic which justifies their inclusion in algae.
Morphological features of some representative types of algae are shown in Fig. 5.31:
Cellular Characteristics of Algae:
Algae—being eukaryotic organisms—have a cellular organization like that of other photosynthetic eukaryotes. Algal cells have a double-membrane bound nucleus, mitochondria, vacuoles, chloroplastids, Golgi bodies, endoplasmic reticulum and 80S ribosomes. The prokaryotic blue-green algae are now considered as bacteria (cyanobacteria), though like other algae they carry out oxygenic photosynthesis. They have been treated elsewhere along with bacteria.
Motile unicellular or coenobial algae, as well as motile asexual and sexual spores, all have eukaryotic flagella with two central and nine pairs of fibrils surrounded by a membrane. The flagella are anchored to the basal bodies situated in the protoplast.
In some forms, the flagella are of two types — whiplash type having a stiff basal part and a flexible upper part, and tinsel type with fine hairy outgrowths. The flagella, often in a pair may be attached at the anterior end of the cell, or laterally. In some, like yellow-green algae (e.g. Botrydium, Vancheria), the pair of flagella are of unequal length in zoospores.
The algal chloroplasts vary greatly in size, shape and number. They may contain one or more pyrenoids, or none. The pyrenoids are colourless proteinaceous bodies which are involved in synthesis of starch during photosynthesis. Some photosynthetic flagellates, like Euglena, as well as zoospores of some algae e.g. Stigeoclonium have a red-coloured carotenoid-containing eye-spot which serves as a photo-receptive organ guiding locomotion. Some photo autotrophic flagellates have also contractile vacuoles.
Algal cell, in general, is bound by a cell-wall. The flagellates and the amoeboid forms lack a rigid cell wall. The composition of the cell wall is variable in different taxonomic groups. Generally, it is made of complex polymeric carbohydrates. In the green algae, the cell wall is mainly composed of cellulose. In many marine green algae, mannans are also present along with cellulose. Galactans are present in the cell wall of red algae.
Pectic substances are often associated with the polymeric carbohydrates. The walls of many algae are often reinforced with a variety of other materials, such as silica, calcium salts, alginic acid etc. The flagellates lack a cell wall and their cells are covered by a flexible modified membrane, which is generally known as a pellicle.
All types of algae contain photosynthetic pigments. These include different chlorophylls, carotenoids and phycobiliproteins. The pigments are located in chloroplasts. The proportion of different pigments imparts the characteristic colour of different groups of algae. Chlorophyll a is present in all groups of algae.
Chlorophyll b is present in mainly in the green algae and in traces in the englenoid flagellates. Chlorophyll is present in small amounts in the brown algae, yellow-green algae, golden-brown algae and the diatoms. Chlorophyll d is present specifically in the red algae. Chlorophyll e is found in some yellow-green algae.
Among the carotenoids, β-carotene is present universally in all algal groups, a-carotene is present in some green algae, brown algae, red algae and in diatoms, y-carotenes have been detected in some green algae and englenoids. Xanthophyll’s which are oxygenated carotenes also occur extensively in algae. Lutein is found in green algae and in small amount in red algae.
Fucoxanthin is the main xanthophyll’s of brown algae and gives the characteristic colour of these algae. It is also present in the golden-brown algae and diatoms. Zeaxanthin is present in red algae. Several other xanthophyll’s, like alloxanthin, dinoxanthin, heteroxanthin etc. occur in specific groups.
The phycobiiiproteins, characteristically present in cyanobacteria, also occur in red algae and brown crypto-monads. Phycobiiiproteins are of two types — phycocyanin which is a blue pigment, and phycoerythrin, a red pigment. Both types are found in algae, as also in cyanobacteria.
A non-plastidial pigment found in some algae is haematochrome. This pigment is present in some green algae, like Trentepohlia giving a red colour to these terrestrial algae. Sphaerella, a unicellular alga related to Chlamydomonas growing in the Arctic and Alpine regions, is rich in this pigment.
The alga grows so densely that the snow appears red and it causes the phenomenon called ‘red snow’. The chloroplasts of algae, specially those of green algae, are of various forms depending on the genus. Generally, in green algae there is 3 single chloroplast. In other groups, there are numerous small chloroplasts.
Some characteristic forms of chloroplasts in green algae are shown in Fig. 5.32:
Another cellular feature that varies in different algal groups is the nature of the storage carbohydrates. In green algae, as well as in crypto-monads and dinoflagellates, the storage product is starch. A starch-like polysaccharide, called floridean starch is the reserve substance in red algae.
In the brown algae, a dextrin-like carbohydrate, known as laminarin is the reserve material and a related polymer, chrysolaminarin occurs in the yellow-green and golden-brown algae, as also in diatom. The englenoids store paramylum. Besides the polysaccharides, most algae have oil-drops in the cells. Brown algae have also soluble mannitol.
Some useful characteristics of taxonomic importance of different algal divisions are summarized in Table 5.2: