Green algae and cyanobacteria are two types of photosynthetic organisms mainly found in aquatic habitats. Therefore, both green algae and cyanobacteria are autotrophs. Green algae are eukaryotes while cyanobacteria are prokaryotes. Hence, green algae consist of membrane-bound organelles including the nucleus, mitochondria, Golgi apparatus, etc. while cyanobacteria do not contain them. Green algae consist of chloroplasts while cyanobacteria lack chloroplasts.
Key Areas Covered
1. What are Green Algae
– Definition, Characteristics
2. What are Cyanobacteria
– Definition, Characteristics
3. How are Green Algae Different from Cyanobacteria
– Difference Between Green Algae and Cyanobacteria
Key terms: Chloroplasts, Cyanobacteria, Green Algae, Membrane-Bound Organelles, Origin, Reproduction
What are Green Algae
The green algae are the algae that consist of chlorophyll, the green color photosynthetic pigment. Green algae live in freshwater habitats. They are unicellular, multicellular or live in colonies. Some green algae form symbiotic relationships with fungi, producing lichens.
The two types of chlorophyll found in green algae are chlorophyll a and chlorophyll b. They also contain beta-carotene and xanthophyll. Chloroplasts are the organelles that contain photosynthetic pigments within the cells of green algae. A single green algal cell may contain one to several chloroplasts. Hence, green algae are photoautotrophs. The simple organic compounds produced by photosynthesis are stored in the form of starch and fat. Green algae are shown in figure 1.
Green algae asexually reproduce by budding, fragmentation or by formation of zoospores. The sexual reproduction of them occurs by the production of isogamous (both gametes are motile and same size) or anisogamous (female non-motile and male motile) gametes. Most green algae show alteration of generations with a haploid phase and diploid phase in their life cycle.
What are Cyanobacteria
Cyanobacteria are photosynthetic bacteria. They live in the soil, freshwater or marine water habitats. Cyanobacteria are prokaryotes. They may be either unicellular or multicellular organisms. The colonies of cyanobacteria can be spherical-shaped, filamentous or sheet-like. Sheet-like structures cover some colonies of cyanobacteria. Chlorophyll a, phycocyanin (blue color), and phycoerythrin (red color) are the photosynthetic pigments found in cyanobacteria. The food is stored in the form of starch in cyanobacteria. Cyanobacteria is shown in figure 2.
Since they are prokaryotes, Cyanobacteria do not contain any type of membrane-bound organelles inside the cell. However, they contain vacuoles inside the cell. They do not possess flagella. But, they show a gliding movement, which occurs due to the trichome. The movement helps to change the depth inside the water. Some cyanobacteria are capable of fixing gaseous nitrogen. The asexual reproduction of cyanobacteria occurs by fission. They do not undergo sexual reproduction.
How are Green Algae Different from Cyanobacteria
Both green algae and cyanobacteria are photosynthetic organisms that live mainly in aquatic habitats. Both of them store food in the form of starch. Both types of organisms can be unicellular or multicellular. However, green algae and cyanobacteria share some differences, which are described below.
Green algae are eukaryotes while cyanobacteria are prokaryotes. Therefore, green algae contain membrane-bound organelles such as nucleus, mitochondria, etc. But, cyanobacteria lack membrane-bound organelles.
Green algae belong to the clade Viridiplantae under the kingdom Protista. The two phyla of green algae are Chlorophyta and Charophyta. Most Chlorophyta are found in marine water, freshwater or subaerial. The Chlorophyta includes Trebouxiophyceae, Chlorophyceae, Bryopsidophyceae (seaweeds), Ulvophyceae (seaweeds), Dasycladophyceae, and Siphoncladophyceae. However, the Charophyta entirely live in freshwater habitats.
Cyanobacteria is a phylum under kingdom Eubacteria. The three classes of cyanobacteria are Hormogoneae, Chroobacteria, and Gloeobacteria.
The photosynthetic pigments found in green algae are chlorophyll a, chlorophyll b, carotenoids, and xanthophyll. But, chlorophyll a, phycoerythrin and phycocyanin are found in cyanobacteria.
The photosynthetic pigments of green algae are arranged into chloroplasts in green algae. One to several chloroplasts are found per green algal cell. However, cyanobacteria do not contain chloroplasts. Their photosynthetic pigments are found in the cytoplasm. Hence, a homogenous color can be identified throughout the cytoplasm of cyanobacteria when observed under the microscope.
The cell wall of green algae is made up of cellulose while that of cyanobacteria is made up of peptidoglycan. The bacterial cell wall is also made up of peptidoglycans, and it can be digested with lysozymes.
Green algae are not involved in the nitrogen fixation but, cyanobacteria have this ability. Hence, farmers use cyanobacteria as a biofertilizer.
Asexual reproduction of green algae occurs by budding, fragmentation or by the formation of zoospores. Asexual reproduction in cyanobacteria occurs by fission.
Sexual reproduction of green algae occurs by the production of isogamous or anisogamous gametes. Alteration of generations can be identified in green algae. No sexual reproduction occurs in cyanobacteria.
Green algae and cyanobacteria are two types of photosynthetic organisms. Green algae are eukaryotes while cyanobacteria are prokaryotes. Green algae contain chloroplasts while cyanobacteria do not.
1. GreenWater Laboratories. “Algae & Cyanobacteria.” GreenWater Laboratories – Algal Toxin Analysis, Algal Identification, Research, Available here.