Reproduction in algae occurs by vegetative method (fission, fragmentation, hormogonia, amylum stars budding etc.), asexual method (akinete, zoospores, aplanospores, exospores etc.) and sexual method (autogamy, isogamy, anisogamy, oogamy etc.).
Reproduction in Algae
- Vegetative Reproduction in Algae
- Asexual Reproduction in Algae
- Sexual Reproduction in Algae
Vegetative Reproduction in Algae
In this type of reproduction of algae, any vegetative part of the thallus develops into a new individual without involving spore formation or alternation of generations. It is the most common method of reproduction in algae and occurs in the following forms:
- Cell Division or Fission
- This is the simplest method of reproduction, often called binary fission, commonly seen in unicellular algae like Chlamydomonas, Synechococcus, and Diatoms. In this process, the vegetative cell divides mitotically into two daughter cells, each of which becomes a new individual.
- Fragmentation
- In this method, the multicellular filamentous thallus breaks into many-celled fragments, each of which gives rise to a new individual. Fragmentation may occur accidentally or through the formation of separation discs or other mechanical forces or injuries. Examples include Spirogyra, Ulothrix, Oedogonium, Zygnema, and Cylindospermum.
- Hormogonia
- This method is found in blue-green algae. The trichomes present in blue-green algae breaks with sheath intact, into many-celled segments called hormogonia or hormogones. These segments are delimited by the formation of heterocysts, separation discs, necridia, or by the death and decay of intercalary cells. Hormogonia are commonly found in Nostoc, Oscillatoria, and Cylindosporium.
- Adventitious Branches
- In various large thalloid algae, adventitious branches form, which, when detached from the plant body, develop into new individuals (e.g., Fucus, Dictyota). Protonema-like adventitious branches form from the internodes of Chara and the stolons of Cladophora glomareta.
- Bulbils
- Tuber-like outgrowths develop due to the storage of food at the tips of rhizoids and on the lower nodes of Chara, called bulbils. As they separate from the plant body, bulbils grow into new plants.
- Amylum Stars
- Star-shaped aggregations of starch-containing cells develop on the lower nodes of Chara, called amylum stars. When separated from the plant body, they grow into new plants.
- Budding
- In Protosiphon, bud-like structures form due to the proliferation of vesicles delimited from the parental body by a septum. After detachment, these buds grow into new plants.
Asexual Reproduction in Algae
Asexual reproduction in algae involves the formation of various types of spores, either naked or walled, leading to the rejuvenation of the protoplast without any sexual fusion. Each spore germinates into a new plant, and there is no alternation of generations in this method.
- Zoospores
- These are motile, naked spores equipped with two, four, or many flagella, called bi-, quadri-, or multiflagellate zoospores, respectively. Biflagellate zoospores are found in Chlamydomonas and Ectocarpus, quadriflagellate zoospores in Ulothrix, and multiflagellate zoospores in Oedogonium.
- Aplanospores
- These non-motile spores form either singly or by the division of the protoplast into many aplanospores within a sporangium during unfavorable conditions, such as drought (e.g., Ulothrix, Microspora). Aplanospores with thickened walls and abundant food reserves are known as hypnospores (e.g., Pediastram, Sphaerella).
- Tetraspores
- Some diploid algae (e.g., Polysiphonia) produce special haploid aplanospores called tetraspores within a tetrasporangium.
- Akinetes
- Vegetative cells of certain filamentous algae develop into elongated, thick-walled spore-like structures with abundant food reserves, known as akinetes (e.g., Gloeotrichia). These can survive unfavorable conditions and germinate into new individuals when conditions improve.
- Exospores
- In some algae, spores are regularly cut off at the exposed distal end of the protoplast in a basipetal succession, forming exospores. These spores aggregate into groups and develop new colonies (e.g., Chamaesiphon).
- Endospores
- Small spores formed by the division of the mother protoplast, also known as conidia or gonidia, are called endospores. These are released after the mother wall dissolves and germinate directly into new plants without a resting period (e.g., Dermocarpa).
Sexual Reproduction in Algae
All algae, except Bluegreen algae, reproduce sexually. During sexual reproduction, gametes fuse to form a zygote. New genetic combinations can arise from the fusion of gametes from different parents. The different types of sexual reproduction are:
- Autogamy
- In this process, the fusing gametes come from the same mother cell and form a zygote after fusion. Plants developed through autogamy do not introduce new characteristics. Example: Diatoms.
- Hologamy
- In some unicellular members, the vegetative cells of different strains (+ and -) act as gametes and form a zygote after fusion. Though inefficient for multiplication, this process creates new genetic combinations. Example: Chlamydomonas.
- Isogamy
- This involves the union of two morphologically and physiologically similar gametes, called isogametes, which form a zygote after fusion. These gametes are usually flagellate. Examples: Chlamydomonas eugametos, Ulothrix Sp.
- Anisogamy
- In this process, the uniting gametes are morphologically and physiologically different. The smaller, more active gamete is the microgamete (male), and the larger, less active gamete is the macrogamete (female). Example: Chlamydomonas braunii.
- Oogamy
- This advanced process involves fertilization between a small motile (or non-motile in Rhodophyceae) male gamete (sperm or antherozoid) and a large non-motile female gamete (egg or ovum). Male gametes develop within the antheridium, while the female gamete develops within the oogonium. Examples: Oedogonium, Vaucheria, Chara, Laminaria, Sargassum, Polysiphonia, Batrachospermum etc.
Life Cycle in Algae
The sequence of events through which one generation transitions to the next is called a life cycle. Sexual reproduction in algae involves an alternation between haploid and diploid generations, known as alternation of generations. In algae, there are five main types of life cycles or alternation of generations. These are:
- Haplontic Life Cycle
- Diplontic Life Cycle
- Diplohaplontic Life Cycle
- Haplobiontic Life Cycle
- Diplobiontic Life Cycle
Haplontic Life Cycle
- In this life cycle, the main plant body is a gametophytic (haploid) structure that produces mitospores during the growing season. These mitospores develop into gametophytic plants. As the growing season ends, the gametophyte generates haploid gametes.
- Following gametic fusion, a diploid zygote or zygospore forms, representing the sole diploid phase in the life cycle.
- Shortly after formation, the zygote or zygospore undergoes meiosis to produce meiospores, which then germinate into a gametophytic thallus.
- This life cycle, known as haplontic, is the most primitive type, featuring zygotic meiosis and lacking a diploid sporophytic thallus.
- It is commonly found in most green algae, Charophytes, and Bangia of red algae.
Diplontic Life Cycle
- In this life cycle, the dominant plant thallus is diploid. The thallus reproduces sexually through gametes formed by meiosis in the sex organs, representing the haploid phase of the life cycle.
- These gametes fuse to form a diploid zygote or zygospore, which develops into the diploid plant body.
- Unlike the haplontic life cycle, there is no true alternation of generations.
- This life cycle is termed diplontic and is observed in diatoms (Bacillariophyceae), certain members of Siphonales, Siphonocladiales, and Dasycladiales of green algae, as well as Fucales of brown algae.
Diplohaplontic Life Cycle
- This life cycle is seen in Ulvales and Cladophorales of Chlorophyceae and some brown algae like Ectocarpus and Dictyota.
- In this life cycle, two distinct generations alternate, exhibiting true alternation of generations. It features two different vegetative individuals alternating with each other, hence called diplohaplontic.
- Two types of diplohaplontic life cycles are found to occur: isomorphic and heteromorphic.
- Isomorphic – In this type of diplohaplontic life cycle, the sporophyte and gametophyte are morphologically similar. The zygote produces a sporophytic thallus, which generates meiospores in the sporangium through reduction division. These meiospores germinate into a gametophytic thallus that forms gametes in sex organs. Syngamy between gametes results in a zygote that develops into a diploid thallus. Examples include Ulvales, Cladophorales, Ectocarpales, Dictyotales, and some red algae.
- Heteromorphic – In this type, the alternating generations are morphologically different. The sporophyte is more elaborately developed compared to the gametophyte. Examples include Laminariales and Desmarestiales.
Haplobiontic Life Cycle
- This life cycle can be either diphasic or triphasic. In the red alga Nemalion, there are two haploid phases and a diploid zygote, making it a diphasic haplobiontic life cycle, also known as haplo-haplontic.
- In Nemalion, the dominant phase is the gametophyte, which produces gametes. After gametic fusion, a zygote forms and develops into a carposporophyte after meiosis. The carposporophyte produces carpospores, which germinate into the main gametophytic plant body.
- In Batrachospermum (another red alga), the haplobiontic life cycle is triphasic, involving three prominent haploid phases: the main gametophyte, carposporophyte, and chatransia phase. Therefore, this life cycle is referred to as haplo-haplo-haplontic.
- The zygote is the only diploid phase. The gametophyte, which is the main plant body, produces gametes. These gametes fuse to form a zygote that undergoes meiosis and develops into a carposporophyte.
- The carpospores from the carposporophyte germinate into the chatransia stage, which then develops into the normal gametophyte.
Diplobiontic Life Cycle
- This triphasic life cycle, also known as the diplodiplohaplontic life cycle, includes three phases: two diploid and one haploid.
- The mature plant body is a gametophytic and produces gametes. After syngamy, a zygote forms and differentiates into a diploid carposporophyte.
- Within the carposporophyte, diploid carposporangia develop and produce diploid carpospores.
- Once released, these carpospores develop into a diploid tetrasporophyte. Meiosis occurs inside the tetrasporangia to produce tetraspores, which then develop into the main gametophytic plant thallus.
- This type of life cycle is exhibited by some members of red algae, such as Polysiphonia.
