Life Cycle of Algae

Algae are a diverse group of photosynthetic organisms that play a crucial role in aquatic ecosystems. They have a complex life cycle that involves both sexual and asexual reproduction. Let's explore the life cycle of algae in detail:

1. Haploid Phase (Gametophyte stage):
- Algae begin their life cycle as haploid cells, which means they possess only one set of chromosomes.
- In this phase, algae undergo asexual reproduction through various methods such as cell division, fragmentation, or spore formation.
- During cell division, a parent cell divides into two daughter cells, each carrying the same genetic material. This process is known as mitosis.
- Fragmentation occurs when a part of the algal body breaks off and develops into a separate individual.
- Some algae produce spores, which are single cells capable of developing into new individuals under favorable conditions.

2. Sexual Reproduction (Gamete Formation):
- Under specific environmental conditions, algae enter the sexual phase of their life cycle.
- During sexual reproduction, specialized structures called gametangia develop on the haploid thallus (body) of algae.
- Gametangia produce gametes, which are reproductive cells. Algae can be either isogamous (producing similar gametes) or anisogamous (producing dissimilar gametes).
- Isogamous algae release motile gametes that fuse to form a zygote. Anisogamous algae produce two types of gametes: smaller male gametes (sperm) and larger female gametes (eggs).
- Fertilization occurs when a sperm fertilizes an egg, resulting in the formation of a diploid zygote.

3. Diploid Phase (Sporophyte stage):
- The zygote represents the beginning of the diploid phase of the algal life cycle.
- The zygote undergoes mitotic division to form a multicellular structure called a sporophyte.
- The sporophyte generation is diploid and produces spores through meiosis.
- Meiosis reduces the chromosome number from diploid to haploid, ensuring genetic diversity.
- The spores are released from the sporophyte and can disperse through various means such as wind, water, or animal-mediated transport.

4. Haploid Phase (Gametophyte stage, repeated):
- The released spores germinate under favorable conditions, giving rise to new haploid individuals.
- The haploid individuals grow into mature gametophytes, completing the life cycle of algae.

Conclusion:
The life cycle of algae involves alternating phases of haploid and diploid generations. The haploid phase includes asexual reproduction, while the diploid phase involves sexual reproduction and the production of spores. This complex life cycle allows algae to adapt to changing environments and ensures genetic diversity within their populations.

Pattern # 1. Haplontic Life Cycle: The plant body is gametophyte (haploid) and sporophyte (diploid) stage is represented only by zygote. The gametophytic plant develops hap­loid gametes in the gametangium. The fusion between gametes results the formation of zygote, the only diploid stage i.e., sporophytic phase of the life cycle. The zygote undergoes meiotic division and forms four meiospores. These meiospores develop into haploid plants. The alternation of generations can be interpreted by chromosome number (Fig. 3.19). This life cycle is also known as monogenic life cycle. This type of life cycle is found in majority of Chlorophyceae like Chlamydomonas, Ulothrix, Oedogonium, Spirogyra, Chara etc. and all members of Xanthophyceae. Pattern # 2. Diplontic Life Cycle: The plant body is sporophyte and develops sex organs. Sex organs produce gametes by meiosis. The gamete only represents the gametophytic stage. The gametes undergo fertilization immediately and form zygote. The zygote does not undergo meiosis and give rise to new sporo­phytic plant body (Fig. 3.20). This type of life cycle is found in majority of the members of Bacillariophyceae, some mem­bers of Chlorophyceae like Cladophora glomerata. Fucus and Sargassum of Phaeophyceae also show this type of life cycle. Pattern # 3. Diplohaplontic Life Cycle: ADVERTISEMENTS: In this type the haploid and diploid phases are equally prominent and are represented by two distinct vegetative individuals. They differ only in chromosome number and function. The haploid gametophytic plant reproduces by sexu­al method, while diploid sporophytic plant by asexual process. In this life cycle alternation of two vegetative individuals occurs by sporogenic meiosis and fusion of gametes. It is of two types: i. Isomorphic or Homologous Diplohap­lontic Type: In this type both sporophytic and gameto­phytic plants are morphologically similar and free living. The gametophytic plant (haploid) pro­duces gametes, undergo sexual reproduction and form zygote. The zygote germinates directly into a sporophytic (diploid) plant. The sporophytic plant forms haploid zoospores by meiosis. These zoospores can develop new gametophytic plant. This type of life cycle is found in Clapdohora, Ulva, Draparnaldiopsis of Chlorophyceae and Ectocarpus of Phaeophyceae. ii. Heteromorphic or Heterologous Diplo­haplontic Type. In this type both sporophytic (diploid) and gametophytic (haploid) plants are morphologi­cally dissimilar. Generally the sporophyte is complicated and much elaborate, but the gametophyte is simple and small as found in Laminaria of Phaeophyceae. In some cases like Cutlaria etc. gametophyte is dominant over sporophyte. In Laminaria the gametophytic plant body is made up of minute filaments which produce gametes. The gametes undergo fusion and form zygote, which germinates directly into a sporophytic plant. The sporophytic plant body is macroscopic and several meters in length. ADVERTISEMENTS: The sporophytic plant bears zoosporangia and produce zoospores after meiotic division. The haploid zoospores on germination produce haploid gametophytic plant. Pattern # 4. Triphasic Life Cycle: In this type, there is succession of three distinct generations. It is of two types: i. Haplobiontic Type: In this cycle the gametophytic (haploid) phase is elaborate, dominant and persists for long time than sporophytic (diploid) phase which is represented only by zygote i.e., haplobiontic type and two successive haploid generations are interrupted only by diploid zygote stage indicate its triphasic nature. This type of life cycle is found in the primi­tive members of Rhodophyceae like Batrachos­permum and Nemalion (Fig. 3.22A). In Batrachospermum the gametophytic plant body develops sex organs and produces male (spermatium) and female (egg) gametes. The gametes by fusion form zygote. The zygote immediately undergoes meiosis and produces another haploid gametophytic plant, the carposporophyte. The carposporophyte develops carposporangium which produces haploid carpospores. The carpospores germinate and develop new free-living gametophytic plant. So in this cycle, three phases are : i. Haploid carposporophyte, ii. Haploid gametophyte, and iii. Diploid zygote. ii. Diplobiontic Type: In this type there is one gametophytic phase and two sporophytic phases indicate its triphasic nature and the sporophytic phase is more elabo­rate and persists for long duration than the game­tophyte i.e., diplobiontic type (Fig. 3.22B). This type of life cycle is found in Polysiphonia, a member of Rhodophyceae. In Polysiphonia, the gametophytic phase is represented by two types of gametophytic plant i.e., male and female plant, those bear spermatangium and carpogonium respectively. Later, the spermatangium and carpogonium develop sperms and egg respectively. The male and female gamete i.e., sperm and egg undergo fusion and form zygote. The zygote (2n) develops into a diploid carpospophytic phase. The diploid carpospores are formed in the carposporophyte. The carpospores on germination develop the diploid tetrasporophytic plants. The tetrasporo- phytic plant develops diploid tetrasporangia each of which produce four tetraspores (n) by meiotic division. They are liberated by splitting of sporangial wall. Out of four tetraspores two produce male gametophyte and the other two into female gametophyte. So in this cycle, three phases are: i. Haploid gametophyte, ii. Diploid carposporophyte, and iii. Diploid tetrasporophyte. Affinity with Fungi: The affinity between algae and fungi are given: 1. Plants are thalloid. 2. Sex organs are very simple and not protec­ted by jacket wall. 3. No embryo formation after sexual union.