Plastid retention, use, and replacement in a kleptoplastidic ciliate

Abstract

The marine oligotrich ciliate Strombid- ium rassoulzadegani retains and utilizes the chloro- plasts of its algal food. It does not appear to be able to induce its captured plastids to divide, and so the plas- tids must be replaced with new ones from recently ingested food. We measured the plastid replacement rate of S. rassoulzadegani, its growth and feeding rates, chlorophyll retention, and mortality when starved, and determined whether the ciliate showed differential grazing or plastid retention when pre- sented with different algal foods. S. rassoulzadegani had similar mortality rates when starved following growth on either Tetraselmis chui or Rhodomonas lens. When presented with a source for new plastids, the ciliate can incorporate its first new plastid within 30 min and completely replace all of its plastids within 48 to 72 h. S. rassoulzadegani did not show a preference for either Tetraselmis or Rhodomonas when grazing. However, it did preferentially acquire the Tetraselmis-derived plastids. Our results contrast with those for other mixotrophic ciliates; for example, Mesodinium rubrum can maintain its plastids for extended periods of time (weeks), while Strombid- ium capitatum can quickly lose (40 h) and replace (9 h) its prey-derived plastids. S. rassoulzadegani also seems to be able to grow more efficiently when grazing on Tetraselmis than on Rhodomonas. S. ras- soulzadegani may contribute to the primary produc- tion in the tide pools where it is found and by using autotrophic and heterotrophic nutritional strategies may be able to survive changes in food availability.