Role of sinking and ascent during sexual reproduction in the marine diatom Ditylum brighlwelli

Abstract

The induction of sexuality was documented during a fall bloom of the marine diatom Ditylum brightwellii. Large numbers of cells bearing male and female gametes and some resting spores occurred simultaneously. When laboratory cultures of D. brightwellii became sexual after nitrogen starvation, sinking rates of cells in different stages of sexual reproduction were measured for the first time. Cell sinking rates increased from a low of 0.02 m d-1 for asexual cells to a high of 1.4 m d-1 for early post-auxospore cells. These high sinking rates decreased rapidly, and within a day after post-auxospore formation, a measurable fraction (up to 90 %) of the post- auxospore cells had substantial ascent rates (up to 3 m d-1 upwards). Physiologically stressed cells might therefore sink from the photic zone during sexual reproduction, and then recolonize the surface as new post-auxospore cells. In a temporally and spatially patchy environment, the increase in sinking rate followed by the development of an ascending fraction of a population could increase the recolonization potential of a species. Ultimately, sinking and ascent patterns during sexual reproduction could influence the evolutionary balance of costs and benefits of sex in diatoms.