Algae (singular: alga) are autotrophic organisms that can carry out the process of photosynthesis. As of now, more than 30,000 species of algae are identified. Though algae possess chlorophyll similar to the green plants, they lack true roots, rhizoids, and leaves. Hence, they are not categorized as plants; rather they are considered as a different organism altogether.
The structure of algae can vary from simple unicellular (for example, Micromonas) to complex multicellular (for example, Kelps) forms. Usually, algae are found in any type of habitat: freshwater, marine water, swampy areas, moist soil and rocks. Based on the characteristic features, there are four major types of algae, namely cyanobacteria, green algae, red algae, and brown algae.
Procreation in Algae
The reproduction of algae can be discussed under two types, namely asexual reproduction and sexual reproduction. The former type refers to reproduction in which a new organism is generated from a single parent. In case of the sexual type, two haploid sex cells are fused to form a diploid zygote that develops into an organism. Let's discuss in brief about the vegetative, asexual, and sexual reproduction in algae along with examples.
Vegetative breeding in algae is quite diverse. Some unicellular forms of algae like Euglena reproduce by binary fission, in which the parent cell divides (longitudinal or transverse) into two similar parts. These two cells develop as organisms and are similar to the parent cell. Fragmentation is a process that is classified under vegetative reproduction in algae. It occurs in Sargassum and other colonial algae, whereby the parent cell divides into two or more fragments that grow into new organisms.
Asexual reproduction occurs by the formation of spores; the algal species Chlamydomonas and Chlorella reproduce by this method. Depending upon the algal species, the spores can be produced in normal or specialized cells. They are either motile or non-motile. Different types of spores are zoospores, synzoospores, aplanospores, hypnospores, autospores, and tetraspores.
As mentioned earlier, sexual reproduction takes place by the union of male and female gametes. The gametes may be identical in shape, size, and structure (isogamy) or different (heterogamy). Some of the simplest forms of algae like Spirogyra reproduce by the conjugation method of sexual reproduction. In the process of conjugation, two filamentous strands (or two organisms) of the same algae species exchange genetic material through the conjugation tube. Among two strands, one acts as a donor and another behaves as a receiver. After exchanging the genetic material, two strands separate from each other. The receiver then gives rise to a diploid organism.
In the higher forms of algae, for example Ulva and Laminaria, an alternation of generation is usually observed. Both asexual and sexual reproduction occur in such organisms. Thus, the mature forms of haploid organisms called gametophyte and diploid organisms called sporophyte are present in the life cycle. If gametophyte and sporophyte organisms are similar in appearance, then they are referred to as isomorphic, whereas algae with different gametophyte and sporophyte forms are called heteromorphic.
The gametophyte produces haploid gametes by mitosis cell division, which unite to form diploid zygote that develops into a sporophyte. The sporophyte then undergoes meiotic cell division to give rise to haploid spores, which grow into gametophytes. This way, the gametophyte and sporophyte generations alter with each other.