Auxospore formation of a giant diatom, Coscinodiscus wailesii (Bacillariophyceae), in culture

Coscinodiscus wailesii Gran, Bacillariophyceae, is known to be one of the most noxious diatom species which causes a great damage to Nori (Porphyra) culture by the consumption of nutrients during the period from winter to spring in the eastern Seto Inland Sea. To understand the life cycle of C. wailesii, it is essential to clarify the mechanisms of the bloom occurrences, but the information on the life cycle has been so far limited. We investigated the auxospore formation and the cell size variation caused by the vegetative cell division in culture. The mean valve diameters of two, sets of cultures, which had been about 250 micrometer and 100 micrometer initially, decreased with the rates of about 7.4 micrometer and 5.6 micrometer per month respectively, under the conditions of temperature of 10 degrees C with irradiance of 3000 lx, on an 8 h light and 16 h dark photo-cycle. When the cell size decreased to about 70 micrometer, they became unable to divide any more and died. The vegetative enlargement of cells, i.e. asexual auxospore formation, and sexualization of vegetative cells were induced by electing the incubating temperature from 10 to 20 degrees C, and by changing the light intensity from high through low to high again. The formation of spermatogenia and the probable oogonia was observed in the culture with cell diameters ranging from 90 to 200 micrometer. In the cultures in which the vegetative cell enlargement was observed, the valve diameters of mother cells ranged from 90 to 350 micrometer. The initial cell sizes after auxospore formation were 240-520 micrometer. The present study demonstrates that C. wailesii obtained from Harima-nada can restore their cell sizes by both sexual and asexual auxospore formations, suggesting an ecological advantage wherein sexual process keeps the genetic diversity in the population, and the asexual process enables the rapid restoration of cell sizes with lower energy