• cell cycle;
  • dinoflagellates;
  • flow cytometry;
  • heterotrophy;
  • meiosis;
  • mixotrophy;
  • nuclear cyclosis;
  • phagotrophy;
  • planozygote;
  • reproduction

Cryptoperidiniopsoids are an unclassified group of delicately thecate heterotrophic dinoflagellates known to be common in eastern U.S. estuarine waters. Over the past 10 years cryptoperidiniopsoids were isolated from different geographical regions and cultured with cryptophyte algal prey. In the seven clonal isolates examined, reproduction was strongly linked to the availability of prey cells. The dinoflagellates phagocytized the contents of prey cells through a tube-like peduncle, similarly as close relatives of Pfiesteria spp. and several other heterotrophic species. Cell division occurred while encysted, most commonly yielding two biflagellated offspring. Abundant fusing gametes, phagotrophic planozygotes, and cysts with a pronounced nuclear cyclosis characterized persistent sexuality. Cysts with nuclear cyclosis produced two flagellated offspring cells. The resistance of reproductive cysts to antimicrobial treatments was examined, and a simple high-yield technique was developed for population synchronization while ridding the dinoflagellates of most contaminating vacuolar prey DNA and external contaminants. The DNA content and population DNA profiles of synchronously excysted cryptoperidiniopsoids from different isolates were measured using flow cytometry and were related to the life history of these and other dinoflagellates. Cryptophyte-fed cultures with versus without extracellular bacteria were compared, and bacteria apparently promoted cryptoperidiniopsoid feeding and growth. Externally bacteria-free dinoflagellates were cultured in media enriched with dissolved organic nutrients, and nutritional benefit may have occurred in some treatments. The potential for mixotrophic nutrition from maintenance of cryptophyte chloroplasts was examined using flow cytometrically sorted cells, but evidence of kleptoplastidy was not found in these isolates under the conditions imposed.