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Keywords:

  • Capsulan;
  • carboxylated polysaccharide;
  • decapore;
  • Golgi-decapore complex;
  • Prasinococcus capsulatus (Miyashita et Chihara);
  • Pycnococcaceae;
  • sulfated polysaccharide;
  • uriolide

A nonmotile green nanoalga was isolated from the waters over the Cayman Trench in March 1979 and has been maintained in culture as clone URI 266G (CCMP 1202). It was observed to form a copious polysaccharide capsule that presumably originated in the Golgi body and was secreted through a crown of 10 pores in the cell wall, the “decapore.” This multilaminate apical area, lying adjacent to the Golgi, underwent structural changes during morphogenesis. The polysaccharide precursors that coalesced to form the capsule apparently became stainable and visible as they exited the decapore when they cross-linked with divalent ions in seawater. Cell wall precursors, or a cell wall lamina, surrounded the daughter cells both during synchronous binary fission and after cell separation, with the maternal capsule perhaps acting as a template. Similar prasinophyte isolates have been obtained from widespread areas of the North Atlantic and were divided into two subgroups on the basis of their pigment complement (Hooks et al. 1988). One subgroup, typified by clone Ω 48-23 (CCMP 1203), was described by Guillard et al. (1991) as Pycnococcus provasolii Guillard within a new family, the Pycnococcaceae. The other subgroup, typified by clone URI 266G (CCMP 1202), contained two unique carotenoids, one of which was uriolide (Foss et al. 1986). Subsequently, Miyashita et al. (1993) described an alga from the western Pacific Ocean that is indistinguishable from URI 266G in both pigment composition and ultrastructure that they named Prasinococcus capsulatus Miyashita et Chihara and placed tentatively in the Pycnococcaceae. They described a curious asexual budding fission. Here we suggest an alternative form of cell division analogous to that observed in the other described Pycnococcaceae. We used theultrastructure of cells in exponential and stationary phases of growth to illustrate synchronous asexual binary fission, the “Golgi-decapore complex,” and its apparent role in capsule formation. A unique sulfated and carboxylated polyanionic polysaccharide named capsulan is released from this complex.