(ORDO NEWS) — Green algae represent a large group of about 20,000 species. Their habitat is freshwater reservoirs and damp places on land, only a few species survive in salty sea water.
All of them are distinguished by a bright green color due to the high content of chlorophyll. The modern theory of evolution considers them to be the ancestors of land plants.
How are green algae arranged?
The main feature of green algae is their color and special structure without a clear division into a stem with shoots, a root system, leaves and flowers.
The body of the algae is called the thallus or thallus. The chlorophyll present in the cells masks other pigments, but sometimes it is covered by the hematochrome, a red pigment in species that cause red blooms in ponds and snow. The thallus of the most ancient unicellular algae evolved in two directions:
The unicellular uninuclear thallus has turned into a multinuclear non-cellular, supercell, like in caulerpa;
A mobile unicellular thallus equipped with flagella developed into a multicellular filamentous organism, the precursor of higher land plants and charophytes.
The structure of green algae cells:
- Pectin or cellulose mucous membrane, sometimes germ cells naked, devoid of cover;
- Protoplasm is located along the walls and is the basis for the life processes of the cell;
- The nucleus can be single or multiple, located in the parietal protoplasm or in the center, suspended on plasma filaments;
- The central vacuole occupies a large part of the cell, it contains cell sap with tannins and dyes, calcium oxalate and calcium sulphate;
- Plastids are membrane organelles involved in protein synthesis. They have cup-shaped cells and, in addition to the green pigment, include yellow and orange-red;
- Chromatophores have a protein structure and are very diverse. It is these organelles that contain dyes, including chlorophyll A and B, carotene and xanthophyll, as well as colorless dense pyrenoids. The photosynthesis product of green algae is starch or oil droplets.
Some unicellular green algae have ocelli for recognizing light and flagella for locomotion. Thus, the cell is able to choose the most favorable place for nutrition and development, because photosynthesis occurs in the light.
By structure, green algae are divided into unicellular, multicellular planktonic and filamentous organisms. There are five main classes:
- Ulvophycia include filamentous and lamellar algae, lichens;
- Briposodes have a non-cellular thallus and live in fresh water;
- Chlorophyceous – some of them are able to combine into giant colonies, others are asexual and reproduce by cell division;
- Trebuxia are unicellular organisms that live in the soil and at the bottom of fresh water bodies;
- Prasins are distinguished by a unicellular structure – coccoid, palmelloid or flagellated.
For the development of green algae, heat and sufficient lighting are necessary. That is why the flowering of water begins mainly in spring and summer.
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