Figure 1.

Schematic drawing of the life cycle of a centric diatom: When a vegetative cell divides mitotically (1), each daughter cell inherits one of the two halves (thecae) of the rigid silica cell wall (orange and green lines) and builds a new smaller theca (purple lines). This leads to a progressive reduction of the cell size of the cell population (2). Above a species-specific sexualization size threshold (SST), (3) diatom cells are incapable of performing sexual reproduction. Below the SST, if a proper trigger is present (such as a salinity shock for Skeletonema marinoi), meiosis is induced and cells can either produce one egg or four sperms (4). Gametes conjugation (syngamy) leads to the formation of a zygote that becomes an auxospore, a soft stage which can expand (5). A new cell wall is built inside the auxospore, which eventually becomes an initial cell of the maximum cell size (6).