Types of Algae Reproduction and Life Cycle of Algae

November 27, 2023 Agric4Profits

In this article, we will discuss the types of algae reproduction and life cycle of algae taking suitable representative examples from various groups. Algae show all three types of reproduction vegetative, asexual and sexual.

The vegetative method solely depends on the capacity of bits of algae accidentally broken to produce a new one by simple cell division. Asexual methods on the other hand involve the production of new types of cells, zoospores.

In sexual reproduction, gametes are formed. They fuse in pairs to form zygotes. A zygote may divide and produce a new thallus or it may secrete a thick wall to form a zygospore. You will see that sexual reproduction in algae has many interesting features.

Types of Algae Reproduction

Reproductive processes found in various groups of algae can be broadly divided into three types: vegetative, asexual, and sexual methods.

1. Vegetative Reproduction

The most common type of reproduction in algae is by binary fission. In unicellular prokaryotic algae like Anacystis, it is the only method of reproduction found in nature.

In filamentous and multicellular forms, the algae may get broken accidentally into small pieces, -each developing into a new one.

The above methods of propagation are known as vegetative reproduction.

2. Asexual Reproduction

When vegetative reproduction takes place through specialized cells (other than sex cells), it is described as asexual reproduction.

i. Anabaena and Nostoc

The cells accumulate food materials; develop thick walls to become spores or akinetes. Akinetes can withstand dryness (lack of water) and high temperatures for a long time, but when conditions are suitable they germinate to form new filaments.

ii. Ulothrix

Filamentous algae (like Ulothrix) may reproduce by producing motile cells called zoospores. The protoplast of a single cell divides many times by mitosis to produce several zoospores.

Types of Algae Reproduction and Life Cycle of Algae — Fig.: Formation of zoospores in Ulothrix

Each zoospore has 2-4 flagella with which it swims for some time and then settles by its anterior end. It subsequently divides into a lower cell which becomes the holdfast and the upper cell which by further divisions becomes the vegetative filament. Zoospores are produced in other algae also.

Asexual reproduction in other algae is described below.

iii. Chlamydomonas

Although this is a unicellular motile algae but it produces zoospores. The parent cell divides inside the cell – envelope and each daughter cell develops two flagella each.

These zoospores look exactly like the parent cell except they are smaller in size. When the zoospores are fully developed the parent cell wall dissolves, releasing them free into the surrounding water.

Sometimes when there is less water outside, zoospores may lose flagella and round up. These non-motile spores are called aplanospores which develop into thick-walled hypnospores.

On moist soil when zoospores cannot be released due to lack of free water, they get embedded within a gelatinous material formed from the parent cell wall.

Such cells do not have flagella but whenever they become flooded with water they develop flagella and swim away in the water. These gelatinous masses containing thousands of non-motile cells are known as palmella stage of Chlamydomonas.

Oedogonium Zoospores are produced singly in a cell. Each has one nucleus and a crown of flagella at the apex.

Many zoospores are produced from a single cell, as in Ulothrix. They have a single nucleus and 2-4 flagella.

iv. Ectocarpus

Zoospores are produced in sporangia which are of the following two types:

Plurilocular Sporangia: The sporangium is made up of many cells and several biflagellate zoospores are produced.

Unilocular Sporangia: The sporangium is made up of one cell which produces single biflagellate zoospore.

3. Sexual Reproduction

Sexual reproduction in algae like in other organisms involves the fusion of two cells from opposite sex called gametes, resulting in the formation of a zygote. Some basic features of this method of reproduction are as follows:

Gametes are always haploid and may or may not be different in morphology. If both the sex cells look alike, they could be male called plus (+) or female called minus (-) mating types of strains. Gametes can fuse only when one is plus and the other is minus.

Both of them + and – may be produced by a single parent. This is called monoecious or homothallic condition. When they come from different plus or minus thallus types it is called dioecious or heterothallic condition.

There are three types of gametic fusion.

Isogamy: When both the gametes are of the same size and morphology.

Anisogamy: The two gametes are distinctly different in size or shape, the larger of the two is minus (female) type.

Oogamy: The female gamete, egg or ovum is big in size and has no flagella hence it is non-motile. Male gametes are flagellated and highly motile. They are also known as antherozoids, spermatozoids, or sperms.

The male gametes are attracted by the female cells because of special hormones called gamones (a violatile hydrocarbon) produced by them. Fusion of the gametes leads to the formation of a zygote.

If the conditions are unsuitable for growth, the zygote may develop a thick wall and become a resting zygospore. Gametes being haploid, are produced by mitosis in a haploid thallus. If the thallus is diploid as in Fucus the reproductive cells undergo meiosis or reduction division to form haploid gametes.

In haploid thallus, after the fusion of gametes, the diploids zygote undergoes meiosis during germination. However, in diploid algae a zygote may divide mitotically and give rise to a diploid thallus (Fucus). Both haploid and diploid thallus are found in Ulva. They look very similar in size and shape.

Reproduction and Life Cycle of Algae

We have given above the basic modes of reproduction in algae. Now we take up some specific algal types to illustrate their life cycle in nature. It is to be noted that the life cycle of an alga is very much controlled by environmental factors like temperature, light, seasons, and availability of nutrient, and also salinity, wave action and periodicity of tides in the case of marine forms.

Observations made by people during different times from various geographical locations and sometimes experimentally studied under controlled conditions, give us fairly comprehensive if not a complete picture of the life cycle of an alga.

1. Chlamydomonas

Sexual reproduction in this alga shows all the three different types depending on the species. Isogamy is found in C. reinhardii, C. gynogama and C. media.

Isogamy is of two types:

In clonal population (cells obtained by the repeated divisions of a single parent cell) fusion may take place between gametes which are homothallic or in self-compatible strains. For example, fusion occurs between any two cells of C. gynogama and C. media.

In C. moewusii and C. reinhardii fusion of gametes can take place only when they come from two different unrelated (heterothallic, self-incompatible) strains.

In many isogamous species the parent cell may divide to produce 16 to 64 biflagellate gametes while in some the adult cells themselves may directly behave as gametes and fuse.

Anisogamous form of gametic fusion is found in C. braunii. A female cell divides and produces four large cells. Each of these cells have two flagella but are less active. The male cells are about 8 in number but smaller in size.

Oogamy is the advanced type of sexual reproduction found in C. coccifera. A parent cell discards its flagella and directly becomes a non- motile egg or ovum. While male parent cell by repeated divisions produces sixteen male gametes. These are biflagellate and highly motile.

The process of gametic attraction, fusion and related phenomena have been studied in some detail in the laboratory. Under proper light condition and carbon dioxide concentration, production of gametes can be initiated by nitrogen starvation.

The formation of male or female gametes (even in the case of isogamy) is attributed to the varying concentration of gamones produced by them.

The attraction between gametes was found due to the presence of glycosidic mannose at the tips of the flagella of one strain which in a complementary way binds with the substance present in the flagella of the gamete of the opposite stain.

Once this sticking of the flagella of plus and minus gametes takes place, flagella twist about each other bringing the anterior ends of the gametes close. This is followed by cellular and nuclear fusion.

The zygote secrets a thick wall and accumulates large amount of food materials like starch. Lipids and orange – red pigments. It is now known as zygospore which remains dormant till the environmental conditions are favourable for its germination

It has been shown that during germination of zygospore meiosis takes place followed by mitosis resulting in haploid Chlamydomonas cells.

Life Cycle of Algae

Chlamydomonas is unicellular, haploid and reproduces asexually many times by forming zoospores. Under unfavourable environmental conditions it produces gametes which fuse to form diploid zygospore.

During germination reduction division takes place and haploid cells are formed.

Chlamydomonas is of great interest to biologists. Its study has brought to light several interesting features of biological importance, some of which are listed below.

i. Presence of DNA in the chloroplasts of the alga;

ii. Presence of cytoplasmic genes.

iii. Production of genetic mutations – affecting nutrition, photosynthesis and production of mutants without flagella or cell wall.

iv. Dicovery of gamones and their role in sexual reproduction.

v. Presence of isogamy, anisogamy and oogamy in a single genus.

vi. Control of reproduction by environmental conditions.

Alternation of Generations

The type of life cycle of an organism in which reproduction alternates with each generation between sexual reproduction and asexual reproduction is called alternation of generations.

The two generations are termed as gametophytic and sporophytic generations. The gametophytic generation is haploid (n) and the sporophytic generation is diploid(2n)

The fusion of two gametes (n) results in zygote (2n) which on germination forms the plant / thallus called sporophyte. The sporophyte in turn produces haploid spores by meiosis. When a spore germinates it develops into gametophyte which bears male or female gametes or both on the same plant / thallus.

In some bryophytes the gametophytic generation is more conspicuous. While in ferns the sporophytic generation is more prominent.

In angiosperms main plant body is sporophyte and the gametophytic generation is reduced to a few cells. You will see that all type of situations prevail in algae. In some algae gametophyte is prominent while in others sporophyte is prominent.

1. Ulothrix

Sexual reproduction takes place by means of isogamous, biflagellate.

Fusion takes place only between plus and minus mating types. The gametes are from different filaments (heterothallic).

The zygote develops a thick wall and remains dormant till the conditions are favourable for germination.

When conditions become favourable meiosis takes place and 4 – 16 haploid zoospores are produced which settle down and give rise to vegetative filaments.

It has been found that Ulothrix produce gametes when grown under long day conditions while short day conditions initiate the formation of zoospores.

Life cycle of Ulothrix

The thallus of Ulothrix is haploid and the diploid stage is represented by the zygote only.

We would like to draw your attention to the fact that in some species (U. speciosa, u. flcca and and in U. implexa) the zygote develops into an independent, unicellular, thallus which is diploid in nature. It produces zoospores asexually by meiosis. The zoospores develop into haploid filaments.

Thus in Ulothrix two types of life cycles can be distinguished:

Haplobiontic: The thallus is haploid and only the zygote is diploid e.g. U. zonata?

Diplobiontic: In diplobiontic cycle, the alga consists of a haploid thallus that produces gametes and a diploid unicellular stalked thallus which produces zoospores after meiotic division.

The two generations – haploid and diploid, alternate with each other (Alternation of generations). Because the two thalli are very different in size and morphology it is known as heteromorphic, diplobiontic life cycle.

2. Fucus

Uocus has advanced type of reproductive structure, termed as receptacles, which are swollen at the tips of branches. Distributed over the surface of each receptacle are small pores, known as ostioles which lead into the cavities, called conceptacles.

Each conceptacle may produce only eggs, only sperms or as in some cases both. A thallus may be unisexual – either having male receptacle or only female ones.

At the base, inside the conceptacle is a fertile layer of cells which develops into oogonia. Each oogonium has a basal stalk cell and an upper cell which undergoes reduction division and produces eight haploid eggs.

These are liberated in the conceptacle. Some of the cells inside the conceptacle produce unbranched multicellular hairs called paraphyses which emerge out of the ostiole as tufts.

Antheridia are produced on branched paraphyses inside the concptacle. Each antheridium is like a unicellular sporangium which divides meiotically and then by further divisions produced 64 haploid sperms.

The biflagellate sperm has a longer flagellum pointing backwards and a shorter one projecting towards the front. It has a single chloroplast and a prominent orange eye spot

The release of the gametes is connected with the sea tides. At low tide, Fucus fronds shrink due to loss of water, and when such fronds are exposed to an on coming tide, the eggs and sperms are released into the surrounding sea water.

The egg of Fucus are known to attract sperms by secreting a gamone. Immediately after fertilization all is secreted around the zygote. It has been shown that unfertilized eggs can develop into gremlins parthenogenetically if treated with dilute acetic acid.

The diploid zygote germinates by producing a rhizoidal outgrowth on one side. It is later cut by wall formation to form a lower rhizoidal cell and apical cell which by further divisions gives rise to the Fucus fronds.

In summary, reproduction in algae could be by vegetative method (binary fission), asexual through specialized cells or sexual by fusion of two cells from opposite sex called gametes.

The life cycles of Chlamydomonas, Ulothrix and Ficus were discussed. There are other genera in this group. It should be noted that algae also exhibit alternation of generations in their life cycles. Reproduction in algae is by asexual and sexual methods. Asexual method involves fission of cells are regeneration of new ones

Sexual method involves fusion of male gamete and female gamete resulting in the formation of a zygote. The life cycle in algae demonstrates clearly a marked alternation of generations, especially in the higher forms like Ulva, Laminaria, and Ficus.

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