Algae present a reproduction cycle (or life cycle) with an alternation of two phases: a sexual phase and a non sexual one. Each phase constitutes a generation characterized by a specific organism.
For example, in many brown algae, during their life cycle, there is an alternation between the sporophyte (organism dedicated to the sexual phase) and the gametophyte (organism dedicated to the non sexual phase). The sporophyte is diploid which means it has 2n chromosomes in each of its cells and the gametophyte is haploid which means it has n chromosomes: only one sample of each one, in each of its cells.
Ectocarpus siliculosus life cycle
Document taken from the web site of the Algal Genetics group in the Station Biologique de Roscoff : http://www.sb-roscoff.fr/UMR7139/en/gene-rech.html
In brown algae the sporophyte produces spores. Spores will develop into gametophytes which are different independent individuals. The gametophyte will produce the gametes which will fuse when they meet and form a zygote which will then become a new sporophyte, etc. The alternation between the creations of these organisms is called the alternation of generations.
The case of another algae, the unicellular dinoflagellate belonging to the genus Alexandrium:
The life cycle in Alexandrium: the diploid cyst (resting cell), dormant at the seabed, excysts and forms two haploid vegetative cells, which divide vegetatively until they form gametes, which fuse to eventually produce the diploid cyst; diploid phases of the life cycle are in black. See web site: http://www.irishscientist.ie/2003/contents.asp?contentxml=03p95.xml&contentxsl=is03pages.xsl
Besides the fact that the diploid individual has a resting period, these organisms' life cycle is of the same type of Ectocarpus' cycle as there is an alternation between two phases: a diploid phase and an haploid one.
Contributed by Stephanie Ries
