Bicarbonate enhances synchronous division of the giant nuclei of sporophytes in Bryopsis plumosa

Abstract

A mature sporophyte of Bryopsis plumosa (Hudson) C. Agardh forms a huge number of zoospores in its cell continuum. Zoospore formation starts with the division of a single giant nucleus and subsequent repeated mitosis. We found that an elevation of photosynthetic activity triggered the division of a mature giant nucleus. Transfer to short-day conditions was not necessary. Giant nuclei did not divide in darkness or in the presence of 1 µM DCMU. Giant nuclei of as many as 90% of sporophtyes started to divide following the addition of 5 mM NaHCO₃ to the growth medium under continuous white light (6–12 W m⁻²). Frequency of nuclear division increased with increased light intensity. By combining those parameters that promoted the division of giant nuclei, we developed the "two-step culture method" which is composed of preliminary and main cultures. This new method guarantees that giant nuclei of more than 90% of all sporophytes synchronously divide between 72 and 96 h after the transfer to the main culture (continuous white light of 12 W m⁻² in PES medium supplemented with 5 mM NaHCO₃).