Reproduction in Algae

Algae (singular alga) are autotrophic organisms that can carry out photosynthesis. As of now, more than 30,000 species of algae are identified. Though algae possess chlorophyll similar to green plants, they lack true roots, rhizoids and leaves. Hence, algae are not categorized as plants, rather they are considered as a different organism. The structure of algae can be 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.

Reproduction 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 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 asexual and sexual reproduction in algae along with examples.

Asexual Reproduction
Asexual reproduction 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 of asexual division in Sargassum and other colonial algae, whereby the parent cell divides into two or more fragments that grow into new organisms. Another method of asexual reproduction in algae is by 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 vegetative cells or specialized cells. They are either motile or non motile.

Sexual Reproduction
As already mentioned, sexual reproduction takes place by the union of male and female gametes. The gametes may be identical in shape and size (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 serves as a receiver. After exchanging the genetic material, two strands separate from each other. The receiver then give 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, 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 meiosis cell division to give rise to haploid spores, which grow into gametophytes. This way, the gametophyte and sporophyte generations alter with each other.