A Sea Comprising Green Algae
A Colony Of Volvox - A Green Alga
Carolus Linnaeus (1707-1778)
Introduction To The World Of Algae
What do we imagine when we hear “Plants”? We imagine a thick brown bark rising up from the ground to give out branches in all directions having showery green leaves, right? Alas, those “Plants” with thick brown bark and lots of leaves (Angiosperms and Gymnosperms) represent a very small portion of the entire Plant Kingdom. Lower groups of plants such as the Algae are the Most Primitive Plants of the world.
The Angiosperms (Flowering plants with closed seeds) and Gymnosperms (Flowering plants with naked seeds) had probably evolved millions of years ago from simpler Algal plants through hundreds of intermediate stages, that is, a variety of plant forms (Bryophytes and Pteridophytes). The most probable Algae to give rise to the Bryophytes are Chara, which has affinity to both Algae and Bryophytes.
The term “Algae” meaning “Sea-weeds” in Latin was given by the famous Swedish Scientist Carolus Linneaus in the year 1753. Algae are of various sizes and habitat. They range from minute forms to larger forms.
Terrestrial Algae On Rocks
Definition Of Algae
- British Professor F.E.Fritsch gave the definition of Algae as “The holophytic organisms (as well as their numerous colorless derivatives) that fail to reach the higher level of differentiation characteristic of the archegoniate plants” in his book 'The Structure and Reproduction of the Algae' in 1935.
- Gerald Webber Prescott in 1969 had defined Algae as “Those chlorophyll bearing organisms (as well as their colorless derivatives) which are thalloid, that is, having no true roots, stems and leaves or leaf like organs”
- The modern definition of Algae could be written by summing up the most important characteristic features – “Algae are a diverse group of primitive autotrophic plants containing chlorophyll, thus, they can prepare their own food. Algae are mostly aquatic plants with a few terrestrial species found on the lands. The plant body of Algae isn’t differentiated into a “root system” and “shoot system” as we see in the higher group of plants. Thus, we say that Algae has a thalloid plant body.
Cell of Chlorella
Distribution Of Algae
Algae are present everywhere. Based on their habitat, Algae are classified into three types-
1. Aquatic Algae- Algae are mostly aquatic. Aquatic Algae are of two types:
- Freshwater Algae- Freshwater Algae grow in Water where salinity is low, less than 10 %.
(Example-Chara, Chlamydomonas, Oedogonium)
- Marine Algae- Marine Algae grow where the salinity is (33-40) %. Some Algae may grow in Brackish water also (having salinity in between fresh and marine water).
(Example-Ectocarpus, Laminaria, Polysiphonia)
2. Terrestrial Algae- They may grow on rocks, on or under the surface of soil, woods, on the bottom of ponds etc. Certain Blue-Green Algae grow under the surface of the soil and are known as cryptophytes. Terrestrial Algae in our surrounding can easily be identified as the “green cushion” like layer on rocks or soil.
(Example-Chlorella, Euglena, Vaucheria)
3. Algae living on bodies of other Plants and Animals-
- Epiphytic Algae- Algae growing on other Plant species.
a) Some Algae grow on another Algal species.
(Example- Diatoms growing on Spirogyra)
b) Some Blue-Green Algae grow on Bryophytes.
c) Some Algae grow on Angiosperms.
- Epizoic Algae- Some Algae may grow on bodies of animals.
(Example-Stigeoclonium lives in the gills of fishes)
- Symbiotic Algae- Algae forming association with another member.
a) Algae forming symbiotic association with Fungi, which is called Lichen.
(Example- Nostoc and any fungal species of Ascomycotina)
b) Some Algae form symbiotic association with bryophytes.
(Example- Nostoc and Anthoceros)
c) Some Algae form symbiotic association with Gymnosperms.
(Example-Anabaena on Cycas)
- Parasitic Algae- Some Algal species grow on other Plants and Animals as parasites.
(Example- Cephaleuros grows parasitically on Tea leaves)
4. Algae living in unique habitat- There are many Algae which grow in extreme conditions. They are-
- Thermophillic Algae- Some Blue-Green Algae can grow in hot springs, where normal life forms are not possible, that is, at a temperature of 60-80 degree Celsius.
(Example- Some species of Oscillatoria)
- Halophytic Algae- Some species of Chlamydomonas grow in highly saline water bodies, where any other form of life is impossible.
(Example- Some species of Chlamydomonas)
- Algae growing on Snow- Some Algal species grow on Snow. They give the snow various colors.
(Example- Algae Haemotococcus nivalis gives red color to snowy mountains)
Classification of Algae Based On Cellular Differences
Algae were the first organisms on Earth to contain chlorophyll. It was from these primitive life forms that gradual evolution took place and more complex forms of plants developed. A proof of their evolutionary significance is that, they have different cellular organizations. Based on the cytological features, Algae are classified into the following groups -
1. Prokaryotic Algae - Prokaryotic cell structure is found in the primitive groups of Algae. In recent years, members of Cyanophyceae have been renamed as Cyanobacteria because of their closer proximity to Bacterial cell structure, that is, prokaryotic characteristics. The characteristics of Prokaryotic Algae are-
- The presence of a naked nucleus, that is, the nucleus lacks a nucleus membrane.
- There are no membrane bound organelles (mitochondria, endoplasmic reticulum, golgi apparatus,etc).
- Plastids are absent. Chlorophyll pigments in the cell are found in thylakoids.
- The cell wall is made up of Mucopolysaccharides (the same substance which the bacterial cell wall are made up of)
- Basic proteins are absent in the DNA.
- Sexual reproduction is completely absent.
2. Eukaryotic Algae - Eukaryotic Cell Structure is found in most Algae, which have evolved from the simpler ones by millions of years of evolution.
The proof of evolution of Algae is - When an Algal cell engulfed another Algal cell, it digested all of it except the plastid. Thus, the plastid developed an extra membrane around it.
If we carefully think about the justification for the presence of the "membrane bound organelles" of the Eukaryotic Algal cells, we will understand that they are the result of evolution. In fact, Algae have been classified by Robert Edward Lee in 1999 on the presence of the surrounding membranes of the chloroplast. The characteristics of Eukaryotic Algae are-
- It contains a nucleus with a proper nuclear membrane.
- Membrane bound organelles such as endoplasmic reticulum, golgi apparatus and mitochondria.
- Plastids present.
- Cell wall contains Cellulose.
- Basic proteins present in DNA
- It reproduces by all forms of Reproduction, that is, Vegetative, Asexual and Sexual forms.
3. Mesokaryotic Algae- Some Algae are rather peculiar like these. The characteristics of a Mesokaryotic Algal cell are-
- The nucleus has a proper nuclear membrane and sexual reproduction does take place (Eukaryotic nature).
- The cell lacks basic proteins in its DNA (Prokaryotic nature).
Mesokaryotic Algae contain both the characteristic features of Prokaryotic and Eukaryotic Algae. Further research is required about its cellular details.