7 Algae
The different ways algae reproduce
Meur a vod gouennañ zo d'ar bezhin
Reproduction in algae is a complex and varied field. Three main forms of sexual reproduction can be distinguished.
The monogenetic cycle
Example: the wracks (Fucales).
Wracks release male or female gametes according to the sex of the plant. Dispersed in the seawater, these gametes are drawn to each other by chemotaxis. From the egg formed by their fusion, a male or female seedling germinates.
The digenetic cycle
Example: the kelps.
Kelps release asexual spores. As these germinate they form tiny clusters of cells called gametophytes. The male gametophytes produce gametes, or sperm cells, which “swim” towards the egg cell carried by the female gametophytes. The fertilised egg cell generates an embryo, which in turn gives rise to a seedling and finally to an asexual adult plant.
The trigenetic cycle
Example: the red algae.
This last form of reproduction resembles the digenetic cycle up to the formation of the egg. The egg, however, does not germinate straight away: it undergoes a series of cell divisions, cloning itself. Only once this multiplication is complete is it released. Each copy of the egg — the carpospore — will produce a seedling.
And without sexual reproduction
Some algae can reproduce asexually, by fragmentation. This is the case of the sea lettuces and the wireweeds, which multiply rapidly and, when growing conditions are right, form green or brown tides.