Sexual Reproduction and Cyst Formation in the Freshwater Dinoflagellate Peridinium cunningtonii

Abstract

In order to make clear the mechanism of the bloom of Peridinium cunningtonii, which periodically occur in summer in Nagase Reservoir, Kochi Prefecture, the conditions regulating encystment of this species were examined by use of clonal population and a synthetic medium (MW1), The maximum cell concentration was I x 105vegetative cells -ml-1in the chemically defined medium (MW1) for about 2 weeks at 20°C. Sexual reproduction was induced in the dinoflagellate Peridinium cunningtonii when late logarithmic phase cells in MW1 medium were inoculated into nitrogen and phosphorus deficient medium. Sexual reproduction was isogamous and homothallic. Small, naked vegetative cells produced by division of thecate cells acted as gametes. The planozygote retained two red bodies and remained motile for 8-10 days. About 10 days following plasmogamy the zygote became nonmotile, the protoplast expanded, a large red body appeared and the wall thickened. The theca of planozygote split at the girdle region and the hypnozygote (cyst) was produced. The cysts were produced under 10,000 lx illumination at 15°C in N- and P-free medium at the maximum frequency of 70 %. The frequencies of encystment of transferring the cell into N-free medium, P-free medium, and fresh medium containing N and P were about 63, 41, and 0%, respectively. Optimum temperature for encystment was 15 to 20°C and optimum light intensity was 5,000 to 10,000 lx. The dormant cysts were resistant to low temperature. It is strongly suggested that the cysts of P. cunningtonii develope into the seed population for blooms of this dinoflagellate for the next year. © 1984, The Japanese Society of Fisheries Science. All rights reserved.