Algae can be unicellular or multicellular organisms. Algae habitats in both fresh and marine waters, in the form of free-floating or attached to the substratum.
The multicellular algae lack true stems, leaves, or roots but able to develop specialized tissues.
Algae play an important role in the aquatic ecosystem. They are primary producers.
Most of the food chains depend upon algae. Around 60% of the total oxygen in the atmosphere is produced by algae.
Types of Algae
Seven major types depend upon distinct sizes, functions, and color.
1. Euglenophyta (euglenoids)
The Euglenophyta or euglenoids are unicellular species, protozoan-like algae, and dominant in the freshwater environment. Most of the euglenoids are autotrophic and photosynthetic. They have chloroplasts.
The primary pigments of euglenophytes are chlorophyll ‘A’ and ‘B’, while their secondary pigments are carotenoids and xanthophylls.
The euglenoids do not have a true cell wall and are protected by a protein sheath covering, known as a pellicle.
The main characteristic feature of photosynthetic euglenoids is an eyespot, flagella, and cell organelles (nucleus, chloroplasts, and vacuole).
Some species of euglenoids are heterotrophic and feed on organic material in the water.
Some euglenoids can survive in darkness for some time with suitable organic material, and Euglenoids store energy as a carbohydrate called paramylon.
2. Chrysophyta (golden-brown algae)
The Chrysophyta are the golden-brown algae and diatoms. These algae occur in both marine water and freshwaters with dominancy in marine water. The cell walls are made of cellulose and pectic materials, a type of hemicellulose.
In the diatoms, especially, the cell wall is saturated with silica, and that makes diatom resistant to decay. Diatoms stores energy as a carbohydrate called leucosin.
The golden-brown algae have flagella for locomotion. The photosynthetic pigments of golden-brown algae are chlorophylls ‘A’ and ‘C’, and the secondary pigments are carotenoids and xanthophylls, with a special pigment known as fucoxanthin.
Bacillariophyceae, the diatoms are a diverse form of algae found in rivers, lakes, ponds, sea, Diatoms have double shells called frustules, mainly composed of silica. Diatom species are differentiated based on the shape of their frustules
Some species of Chrysophyceae, the golden-brown algae lack cell walls, while others have pectin-rich walls.
Golden-brown algae are dominant in marine water, marked the productivity of the ocean. They are generally called as nanoplankton, having cells diameter only 50 micrometers.
3. Pyrrophyta (fire algae)
Fire algae are unicellular, dominant in oceans, and few are in some freshwater that uses flagella for motion.
They are classified into two classes:
- dinoflagellates and
The dinoflagellates have cell walls made up of cellulose and have two flagella. Pyrrophyta stores energy in the form of starch. The primary photosynthetic pigments are chlorophylls a and c, and the secondary pigments are carotenoids and xanthophyll, including fucoxanthin.
The Dominance of Dinoflagellates can cause a phenomenon known as a red tide, in which the ocean appears red due to their large abundance.
Some dinoflagellates are bioluminescence due to this; their abundance causes the ocean to seem to be ablaze. Dinoflagellates are poisonous that they produce a neurotoxin that harms other organisms.
4. Chlorophyta (Green Algae)
Chlorophyta or green algae are dominant in freshwater, but few are marine also. Mainly, they are microscopic, but a few species multicellular and macroscopic like the genus Cladophora.
Their cell walls are mostly made up of cellulose, but in few species, the cell wall also has hemicellulose and calcium carbonate. Multicellular species usually form colonies ranging in size from four cells to several thousand cells.
The green algae stores energy in the form of starch and have flagella for locomotion. The primary photosynthetic pigments are chlorophylls a and b, and their secondary pigments are carotenoids and xanthophylls.
Charophyceae, the stoneworts, is a very peculiar group that sometimes classify under a separate division (the Charophyta). These are found in fresh or brackish waters, and their cell walls mainly made up of calcium carbonate.
They have complex growth forms as their tissue nodes have associated with whorls of “branches.” Charophytes are the exception in algae as they develop multicellular sex organs but not like in the higher plants.
5. Rhodophyta (Red Algae)
Red algae are mainly found in tropical marine locations having a size range from microscopic to macroscopic.
The higher species of red algae grow attached to a hard solid substrate, or they occur as epiphytes on other algae.
These algae lack flagella and centrioles and store energy in the form of polysaccharide called Floridian starch.
The cell wall of red algae contains cellulose and polysaccharides. The primary photosynthetic pigments of red algae are chlorophylls ‘A’ and ‘D’, and their secondary pigments are carotenoids, xanthophyll, and phycobilins. Red algae reproduce asexually by monospores that are carried by water currents until germination.
6. Phaeophyta (brown algae)
Brown algae mainly consist of varieties of seaweed and kelp found in marine environments.
These species of algae are macroscopic in size. Brown algae have cell walls made up of cellulose and polysaccharides called alginic acids.
Brown algae have an anchoring organ, photosynthetic organs, air pockets for buoyancy, and reproductive tissues that produce spores and gametes, also have a supporting stalk for nutrient exchange.
Brown algae have a food reserve called laminarin, a carbohydrate polymer. Their primary photosynthetic pigments are chlorophylls ‘A’ and ‘C’, while the secondary pigments are carotenoids and xanthophylls, including fucoxanthin, this pigment is responsible for the brown color to brown algae
Some examples of brown algae include sargassum species, which are dominant species of the Sargasso Sea and Giant kelp, having 100 meters in length.
7. Xanthophyta (Yellow-Green Algae)
The Xanthophyta or yellow-green algae have the least number of species and are dominant in freshwater.
Only a few of them are found in the marine environment. Cell walls are constructed of cellulose and pectic compounds, and some cases have silica.
The yellow-green algae store energy in the form of carbohydrates called leucosin. These algae have flagella for locomotion.
These algae prefer small colonies of few cells only. The primary photosynthetic pigment of yellow-green algae is chlorophyll a, and secondary pigments are carotenoids and xanthophylls, the pigment responsible for yellow-green color.
Based on growth size
There are two types of algae are classified into two different categories
- Microalgae: These algae are small microscopic, maybe in the form of a single cell or group of cells, photosynthetic that requires a microscope for recognition. They are either free-floating (phytoplankton) or attached to the substrate (periphyton). Dominant in freshwater rivers, ponds, and lakes.
- Macroalgae: These algae are large enough to be seen without a microscope. They are mainly dominant in the marine environment and most often called seaweeds.