Fragmentation in Algae

  Vegetative and Asexual Reproduction
    - Binary Fission or Cellular Bisection
    - Zoospore, Aplanospore, and Autospore
    - Autocolony Formation
    - Fragmentation
    - Resting Stages
  Sexual Reproduction
    - Haplontic or Zygotic Life Cycle
    - Diplontic or Gametic Life Cycle
    - Diplohaplontic or Sporic Life Cycles
This is a more or less random process whereby non-coenobic colonies or filaments break into two to several fragments having the capacity of developing into new individuals.

Resting Stages
Under unfavorable conditions, particularly of desiccation, many algal groups produce thick-walled resting cells, such as hypnospores, hypnozygotes, statospores, and akinetes.

Hypnospores and hypnozygotes, which have thickened walls, are produced ex novo by protoplasts that previously separated from the walls of the parental cells. Hypnospores are present in Ulotrix spp. (Chlorophyceae) and Chlorococcum spp. (Chlorophyceae), whereas hypnozygotes are present in Spyrogyra spp. (Chlorophyceae) and Dinophyta. Hypnospores and hypnozygotes enable these green algae to survive temporary drying out of small water bodies and also allow aerial transport from one water body to another for instance via birds. It is likely that dinophyceae cysts have a similar function.

Statospores are endogenous cysts formed within the vegetative cell by members of Chrysophyceae such as Ochromonas spp. The cyst walls consist predominantly of silica and so are often preserved as fossils. These statospores are spherical or ellipsoidal, often ornamented with spines or other projections. The wall is pierced by a pore, sealed by an unsilicified bung, and within the cyst lie a nucleus, chloroplasts, and abundant reserve material. After a period of dormancy the cyst germinates and liberates its contents in the form of one to several flagellated cells.

Akinetes are of widespread occurrence in the blue-green and green algae. They are essentially enlarged vegetative cells that develop a thickened wall in response to limiting environmental nutrients or limiting light. Figure 1.18 shows the akinetes of Anabaena cylindrica (Cyanophyta). They are extremely resistant to drying and freezing and function as a long-term anaerobic storage of the genetic material of the species. Akinetes can remain in sediments for many years, enduring very harsh conditions, and remain viable to assure the continuance of the species. When suitable conditions for vegetative growth are restored, the akinete germinates into new vegetative cells.

Akinetes (arrows) of Anabaena cylindrica. (Bar: 10 µm.) (Courtesy of Dr. Claudio Sili.)
FIGURE 1.18 Akinetes (arrows) of Anabaena cylindrica. (Bar: 10 µm.) (Courtesy of Dr. Claudio Sili.)