Reproduction and Life Histories of Algae

Algae are a group of predominantly aquatic, photosynthetic, and nucleus-bearing organisms that lack the true roots, stems, leaves, and specialized multicellular reproductive structures of plants.

Algae represent  ancient plants comprising different evolutions such as photoautotrophic organisms. Algae are thallophytic and have  vegetative bodies which are not  organized in roots and leafy stems. Many algae are living in solitary cells, colonies, filaments, or primitive vegetation bodies and do not have a vascular system. The  algae are cryptogams that propagate with the hidden  reproductive strategies. Following a conception of subdivision of living organisms into five kingdoms that is Monera, Protista, Fungi, Animalia, and Plantae. The prokaryotic algae are placed in the Monera and the eukaryotic algae in the Protista. Hence, the algae do not belong to the kingdom of Plantae. However , it is widely accepted that photosynthesis is a mutual characteristic. The perception is that algae are ‘lower plants’ in distinction to the vascular ‘higher plants.’

How do algae reproduce? 

A large and diverse community of photosynthetic eukaryotic species is known as algae. It's a polyphyletic assemblage of species from different clades. The species included in the study range from unicellular microalgae like Prototheca, Chlorella, and diatoms to multicellular types like the giant kelp, a massive brown alga that can develop up to 50 metres (160 feet) in length.

Many are marine and autotrophic, lacking most of the various cell and tissue forms present in land plants, like xylem, stomata, and phloem. Seaweeds are the biggest and most complex marine algae, whereas the Charophyta, a division of green algae that comprises Spirogyra and stoneworts, seem to be the most complex aquatic algae.

There is no widely recognized description of algae. Algae "have chlorophyll as their part of primary photosynthetic pigment which does not have a sterile coating of cells surrounding their reproductive cells," according to one description. Similarly, the colourless Prototheca in Chlorophyta is all chlorophyll-free. 

While cyanobacteria are widely called "blue-green algae," several organisations classify algae to exclude all of the prokaryotes. Algae are a polyphyletic group as they lack a common ancestor and, while their plastids tend to have had a single source, cyanobacteria, they were probably acquired in a number of ways. Algae with primary chloroplasts originating from endosymbiotic cyanobacteria are known as green algae. Algae containing secondary chloroplasts originating via an endosymbiotic red alga include diatoms and brown algae. Algae use a number of reproductive techniques, ranging from basic asexual cell division to complicated sexual reproduction.

Reproduction in Algae

The algae reproduce by three different methods, namely, vegetative reproduction, asexual reproduction and sexual reproduction.

Vegetative Reproduction in Algae: 

Any vegetative part of the thallus grows into a fresh new organism in this form. This does not entail the development of spores or the alternation of generations. This is the most typical method for algae to reproduce.

The modes of vegetative reproduction in algae are as follows:

1. Cell Division or Fission:

It is the most basic type of reproduction. Synechococcus,  Chlamydomonas, diatoms, and other unicellular algae generally reproduce through this simple mechanism, known as binary fission. The vegetative cell undergoes mitotic division and results in two daughter cells, which then function as new individuals in this process.

2. Bulbils:

Food is processed at the peak of rhizoids as well as on the basal part of Chara, resulting in tuber-like outgrowths known as bulbils. Bulbils expand into new plants until detaching from the plant body.

3. Fragmentation:

The multicellular filamentous thallus is broken into many-celled fragments in this process, all of which produces a new organism. Fragmentation in algae may occur by chance, as a result of the formation of separation discs, or as a result of another mechanical force or injury. Spirogyra, Zygnema, Oedogonium, Ulothrix, Cylindrospermum, and other plants contain it.

4. Hormogonia:

Blue-green algae use this form of vegetative reproduction. The trichomes of blue-green algae part ways and get divided into many-celled segments termed as hormogonia or hormones inside the sheath. The development of separation discs, or necridia, heterocysts, as well as the death and decay of trichome intercalary cells, keep them delim­ited. Hormogonia can be found in Nostoc, Oscillatoria, and other places.

5. Budding:

Bud-like structures are defined in Protosiphon as a result of the proliferation of vesicles that are separated from the parental body by a septum and develop into a fresh new plant following detachment.

6. Formation of Adventitious Branches:

Various big thalloid algae create adventitious branches, that when detached from the body of the plant grow into new individuals (for instance, Fucus, Dictyota). Stolons of Cladophora glomerata, Internodes of Chara, and other stolons form protonema-like adventitious branches.

7. Amylum Stars:

On the basal part of Chara, a star-shaped accumulation of starch-containing cells forms. Amylum stars are indeed the name given to these structures. They evolve into new plants once they are separated from the plant body.

Asexual Reproduction in Algae:

The creation of some types of spores — whether naked or freshly walled spores — is needed for asexual reproduction. It is a mechanism of protoplast rejuvenation that does not involve sexual fusion. Every single spore develops into a plant. There occurs no alternation of generations in this process.

Asexual reproduction in algae can come in a variety of forms:

1. Zoospores:

Zoospores are motile exposed spores containing two, four, or several flagella, and are respectively referred to as bi-, quadri-, or multi flagellated zoospores. Ulothrix, Chlamydomonas, Ectocarpus, and other bacteria produce biflagellate zoospores; Ulothrix produces quadriflagellate zoospores, and Oedogonium produces multiflagellate zoospores.

2. Aplanospores:

Aplanospores are spores that are not mobile. Under unfavourable conditions, such as drought, such spores can develop singly or their protoplast can split to form several aplanospores within the sporangium (e.g., Ulothrix, Microspora). Some algae in semi-aquatic habitats may also produce aplanospores.

3. Autospores are cells that tend to be similar to their parent cell (for example, Scenedesmus, Chlorella etc.). Hypnospores are aplanospores possessing a thickened surface and a large food reserve (examples may include., Sphaerella, Pediastrum, etc.).

4. Tetraspores:

Tetraspores are haploid aplanospores produced by diploid algae (– for example, Polysiphonia). Tetraspores are produced inside tetrasporangia. A tetrasporangium's diploid nucleus undergoes meiotic division and leads to the creation of four haploid nuclei, each of which grows into four tetraspores with just a minor fraction of protoplasm. Tetraspores germinate to produce male and female gametophytes upon libe­ration.

5.  Akinetes:

Some filamentous algae's vegetative cells grow into akinetes, which are lengthened thick-walled spore-like formations with ample food reserves (for example., Gloeotrichia). They have the ability to weather the storm. They germinate into new individuals as ideal conditions arise.

6. Exospores:

Exospores are spores that are chopped off at the uncovered distal end of the protoplast throughout the basipetal succession of certain algae. Such spores clump together and form new colonies, including Chamaesiphon.

7. Endospores:

These are tiny spores produced by the mother protoplast's divisions. Conidia and gonidia are other names for them. After the breakdown of mother wail, they were set free. The spores germinate immediately and grow into a new plant, such as Dermocarpa, without having to rest.

Sexual Reproduction in Algae:

Except for individuals of the Cyanophyceae class, almost all algae undergo sexual reproduction. Gametes unite to produce zygotes while sexual reproduction. The combination of gametes from different parents will result in a new genetic establishment.

Sexual reproduction in algae are divided into five groups based on the structure, physiological activity, and complexity of sex organs:

1. Autogamy: 

Fusing gametes are formed from the very same mother cell throughout this process, and then after fusion, these produce a zygote. For the reasons mentioned above, autogamous plants do not display the emergence of any new characteristics, such as Diatoms (Amphora normani).

2. Hologamy:

Vegetative cells of various strains (+ and -) act as gametes in certain unicellular members, and then after fusion, they result in the formation of a zygote. This seems to be an inefficient method in terms of multiplication, however, it does result in the creation of new genetic varieties, such as Chlamydomonas.

3. Isogamy:

It is the merger of two gametes that are physiologically and morphologically identical, resulting in the formation of a zygote. Isogametes are a form of gamete. These are typically flagellates, such as Chlamydomonas Eugametos, Ulothrix, and others.

4. Anisogamy:

The uniting gametes are physiologically and morphologically distinct during this phase. The microgamete (male) is small and more aggressive, while the macrogamete (female) is bigger and less active, such as Chlamydomonas braunii. Physiological anisogamy differs from traditional anisogamy in that the uniting gametes share morphological similarities but vary physiologically. Zygnema, Spiro­gyra, can be some examples.

5. Oogamy:

It is a complex process in which a small motile (non-motile in Rhodophyceae) male gamete (sperm or antherozoids) is fertilised by a large non-motile female gamete (egg or ovum). Male gametes grow in antheridium, while female gametes grow in oogonium, such as Polysiphonia, Oedogonium, Chara, Batrachospermum, Vaucheria, Sargassum, Laminaria, and so on.

Some fun facts about algae

Did you know that if the conditions are favourable enough for algae, this one-celled plant can even grow in your swimming pool? 

Algae are known to produce oxygen which is then utilised by various other forms of aquatic life. Not only that but when you breathe, half the oxygen you inhale has actually been made by algae because they oxygenated the Earth’s atmosphere first. 

There are more than 400,000 varieties of algae that have been found. 

It is said that if you drink even one drop of ocean water, you’d be swallowing about a thousand microscopic algae along with it!

Algae are incredibly important as they serve as the primary base for the marine and aquatic food chain because, in their absence, there would be absolutely no trace of any fish or other such sea animals. 

Inside the corals, the symbiotic algae that reside there tend to produce a certain type of sugar through the process of photosynthesis. That sugar tends to fulfil a massive percentage of the energy needs required by the corals.