Green Algae are eukaryotic organisms characterized by the presence of chlorophylls a and b in them, and these chlorophylls act as major photosynthetic pigments. The photosynthetic pigments of green algae are usually colorless, and heterotrophic taxa are also present in them, such as Hyalogonium, Polytomella, and Polytoma. Green Algae are associated with Cyanobacteria (which is also known as Blue-green Algae), and they are also found in freshwater habitats. Green Algae is also known as Freshwater Algae, and they play a very important in ecological successions (both primary and secondary). We will study Green Algae in this article, and we will learn their habitats, origin & evolution, reproduction pattern, and many more things about them.
Green Algae are the eukaryotic organisms that constitute the most heterogeneous group of photoautotrophic protists that are inhabiting our biosphere. They are characterized by the presence of chlorophylls a and b, which are also their major photosynthetic pigments. In green algae, flagella are of the whiplash (smooth) type, and starch is located within their chloroplasts which act as the major storage product. Green algae show an enormously wide range in their habitats, shapes, and sizes, making their study even more interesting. Green Algae work as the primary producers, and that's why their importance on our planet is comparable to that of rainforests. From the taxonomic point of view, we will find that green algae constitute a paraphyletic group because organisms that belong to this group are likely to have a common ancestor with plants. Green algae have the same type of pigments present as that of plants, and they produce the same type of carbohydrates during their photosynthesis process.
Green Algae: An Overview
We can distinguish green algae from other eukaryotic algae members because of their stellate structure in the flagellar transition region and the presence of two chloroplasts membranes in them. The ability of green algae as single-cell species to exploit resources can be very different concerning resistance to flushing, light-harvesting, and nutrient uptake. This unique ability makes it even more difficult to summarize their ecology in a few words. The different combinations of all the parameters of green algae, which we have discussed above, may fluctuate very strongly in the inland water ecosystem in short time scales. Thus it makes it very hard to study them. The fluctuations in these parameters of green algae in a very short time raise a variety of assemblages, showing an amazingly high biological diversity of green algae. Green algae are also represented as an organism archetypical of the incredible vast possibilities of adaptation usually shown by life on the planet.
Green Algae: Cellular Structure
The bright green color of green algae is because of chloroplasts that contain both chlorophylls a and b. In addition, green algae also have accessory pigments, i.e., xanthophylls (yellow) and beta carotene (red-orange), in their stacked thylakoids. Green algae store carbohydrates in the form of starch, and their cell walls usually contain cellulose. Green Algae are anchored by fibrous strands and a cross-shaped system of microtubules, and only motile male gametes of charophytes have flagella in them. The flagella are also present in the motile male gametes of Ginkgo, cycads, pteridophytes, and bryophytes but these flagella are absent from the gametes of flowering plants and pinophyta.
All species of green algae have mitochondria with flat cristae, and whenever paired flagella are present in them, they are used to move the cell. The members of green algae, which belong to the class Chlorophyceae, undergo closed mitosis, which is the most common form of cell division among all the species of green algae. The closed mitosis process in green algae occurs via a phycoplast. Unlike this, green algae species belong to land plants (class embryophytes) and, from class charophytes, undergo open mitosis without centrioles. The phragmoplast, a 'raft' of microtubules, is formed from the mitotic spindle instead of centrioles. Also, cell division in these species of green algae involves the use of this phragmoplast to produce the cell plate.
Green Algae: Habitats and distribution
Most of the species of green algae are found in the freshwater environment where plenty of freshwater is present. In this habitat, they are usually attached to wood or submerged rocks or sometimes as a scum on the stagnant water (still water). But there are also marine and terrestrial habitat species of green algae, making their habitats and distribution very vast. There are almost 22,000 species of green algae in which most of these species live as single-cells. Still, some of them form long filaments, coenobia (colonies of green algae), or highly differentiated macroscopic seaweeds.
Many species of green algae are also found symbiotically associated with the Hydra viridissima and in the flatworms and ciliate Paramecium. We all may have heard of lichens; these are also the associated species of green algae. Some species of green algae, which belong to class Trentepohlia (also known as class Ulvophyceae) and class Trebouxiophyceae (genera Trebouxia), are also found to have a symbiotic association with fungi, and thus they form lichens. Also, in lichens, the fungal species, which partner with green algae species to form lichens, cannot live independently (as solo surviving species).
A filamentous green alga species, Trentepohlia, can live independently and survive on tree bark, rocks, and humid soil or form a photosymbiont relationship with fungal species of family Graphidaceae in the form of lichens. Besides this, Prasiola crispa is a species of green algae that live in the supralittoral zone (A zone on the terrestrial and can be on the Antarctic where they form large humid soil carpets), especially near the bird colonies. The macroalga Prasiola calophylla species of green algae, which belongs to class Trebouxiophyceae, are also terrestrial distributed species.
Green Algae: Reproduction
The reproduction pattern in green algae is very diverse, and it is also a very interesting field to study. Many different species of algae follow different reproduction pattern and some of them shares a common pattern too. First, Ulva, the diplobiontic species, species of green algae, follows a unique reproduction cycle which is known as Alternation of generation. In this reproduction pattern, the two multicellular forms of species, i.e., diploid and haploid alternate and these alternate forms may or may not be having the same morphology (isomorphic to each other). The fertilized egg cell of the diploid zygote undergoes the meiosis process of cell division, which gives rise to haploid cells, which give rise to the haploid generation of species, and later these haploid cells of these species will become new gametophytes. Now, the haplobiontic generation from the species is the only haploid generation, and they will be multicellular, and later they form the diploid generation of them. Thus, a diploid form of the species evolved from the haploid ancestors' generation has both a multicellular haploid generation and multicellular diploid generation. All diploid generation of these species is considered to have evolved independently from Ulvophyceae more than once. Studies have also found that all land plants have a common diploid ancestor. The same process is also found in many species of brown & red algae.
Reproduction is also varying in many diplobiontic generations of green algae species. It can vary from fertilization of a large not-motile cell by a smaller motile one through oogamy to the fusion of multiple identical species through isogamy. Green algae species from diplobiontic generation also include many heteromorphic and isomorphic forms. In the heteromorphic form, these green algae species have different sizes and morphology in the sporophytes and gametophytes. In contrast, in the isomorphic form, species of green algae have the same morphology in the identical diploid and haploid generations. Many traits of green algae species, especially related to reproduction, show some variations, which is also most notably among the prasinophytes (It is a basal green algae species). Haploid cells of the green algae species can fuse with other haploid cells to form diploid zygotes, which results in a diploid generation. When this same thing happens in the filamentous species of green algae, they form bridges between the haploid cells and leave their empty cell wall behind, which we can easily see using a light microscope. The process of forming bridges between cells and leaving the empty cell wall behind is called conjugation and occurs in many green algae species such as Spirogyra, etc.
Green algae: Sex pheromone
Production of sex pheromones is a common feature of green algae, but the detailed study of this feature is only studied in a few species of green algae. The sexual development in green algae is initiated by some of these sex pheromones, such as glycoprotein pheromones discovered by Hallmann in 1998. These glycoprotein pheromones are one of the most potent known biological effector molecules, which is very effective in initiating sexual development in many species of green algae.
Green Algae: Physiology
Green algae are often used as model study organisms to study many factors and processes happening in the living cell bodies. All green algae species (including the characean alga species) have served as a model experimental organism to understand the following natural processes happening inside a living cell:
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