Algae is a common term for a broad and distinct group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping, including species from many different clades. Included organisms range from single-cell microalgae, such as Chlorella and the diatoms, to multicellular forms. One of them is the giant kelp, a large brown alga that may grow up to 50 m in height. Most are autotrophic and aquatic and lack much distinct tissue and cell types, such as xylem, stomata, and phloem, found in land plants. The most massive and most complex marine algae are called seaweeds. In contrast, the most complex freshwater forms are the Charophyta, a sub-species of green algae, including stoneworts and Spirogyra.
How do Algae Reproduce?
Algae regenerate and revive by sexual reproduction, involving female and male sex cells, by asexual reproduction, or both.
Asexual reproduction is the creation of progeny without the fusion of cells or nuclear matter. Many small algae reproduce asexually by normal cell division or fragmentation, whereas more massive algae reproduce by spores. Some red algae produce monospores (walled, non-flagellate, spherical cells) carried by water currents and upon germination produce a new body. Some green algae build nonmotile spores called aplanospores, while others grow zoospores, which lack actual cell walls and bear one or more flagella. These flagella allow zoospores to race to a favorable environment, whereas aplanospores and monospores have to rely on enduring water currents.
Sexual reproduction is defined by the method of meiosis, in which progeny cells accommodate half of their genetic information from each parent cell. Environmental events usually control sexual reproduction. When salinity, temperature, inorganic nutrients (e.g., phosphorus, nitrogen, and magnesium), or day length become favorable, sexual reproduction is produced in many species. A sexually reproducing organism has two phases in its life cycle. In the first step, each cell has a unique set of chromosomes and is called haploid, whereas, in the second stage, each cell has two sets of chromosomes and is called diploid. When one haploid gamete blends with another haploid gamete during fertilization, the resulting combination, with two sets of chromosomes, is called a zygote. A diploid cell indirectly or directly undergoes a unique reductive cell-division process (meiosis) either immediately or after a while. Diploid cells in this step are called sporophytes because they create spores. During meiosis, the chromosome quantity of a diploid sporophyte is halved, and the consequent daughter cells are haploid.
Lifecycle of Algae
Chlorophyta, Rhodophyta, and Heterokontophyta, the three main algal divisions, have lifecycles that show significant complexity and variation. An asexual condition exists where the seaweed’s cells are diploid, a sexual step where the cells are haploid, accompanied by fusion of the male and female gametes. Asexual reproduction allows an efficient population to increase, but less modification is possible. Commonly, in sexual reproduction of colonial and unicellular algae, two specialized, sexually compatible, haploid gametes make natural contact and fuse to form a zygote. To ensure a flourishing mating, gametes’ growth and release are highly regulated and synchronized; pheromones may play a vital role in these processes. Sexual reproduction allows for more variety and presents the benefit of effective recombinational repair of DNA damages during meiosis, a critical stage of the sexual cycle.