Date of Completion
protist, mixotrophy, ciliate, physiology, ecology
George B. McManus
Field of Study
Doctor of Philosophy
Mixotrophy is a common nutritional strategy that uses both heterotrophy and photosynthesis. Kleptoplastidic mixotrophs do not make their own plastids but acquire them from their algal prey. Before we can add mixotrophs to standard ecological models we need to understand how much each nutritional mode contributes to mixotrophic growth, and how this balance may be influenced by plastid acquisition, retention, and turnover.
In order to examine the role of captured chloroplasts in the metabolism of the oligotrich ciliate Strombidium rassoulzadegani. I evaluated the uptake and retention of chloroplasts, the ability of two different algae to supply functional chloroplasts, and the photosynthetic uptake of inorganic carbon by the chloroplasts once inside the ciliate. In addition, the ability of the ciliate to take up inorganic forms of nitrogen and its role as net mineralizer or utilizer of inorganic nitrogen was examined, using stable isotope tracers of N in nitrate and ammonium. I compared mixotrophic ingestion and inorganic uptake to that of the heterotrophic ciliate Strombidinopsis.
The kleptoplastidic ciliate had higher growth efficiency (GGE) on a chlorophyte diet than a cryptophyte diet. When compared to the heterotroph, the mixotrophic ciliate had improbably high GGEs at low algal food concentrations. However, mixotrophic ingestion did not saturate as the food concentration increased as it did with the heterotroph, suggesting that the mixotroph continues to consume algae at a high rate in order to maintain a fresh supply of chloroplasts.
Mixotrophic inorganic carbon uptake did not change with algal food concentration, but its importance to the ciliate’s carbon budget did increase. Although there was measurable inorganic carbon uptake due to still-active algae in food vacuoles, it did not contribute significantly to the growth of the heterotrophic ciliate. In terms of nitrogen, mixotrophic and heterotrophic ciliates had similar GGEs with the mixotroph’s being slightly higher. Furthermore, ammonium uptake was slightly higher in the mixotroph. Inorganic nitrogen uptake did not contribute significantly to the nitrogen budget of either the heterotrophic or mixotrophic ciliate, and nitrogen GGEs were always less than one.
Kleptoplastidic ciliates like S. rassoulzadegani may survive algal scarcity by ensuring that they have the freshest and most suitable plastids they can at any given time. Like the heterotrophs, kleptoplastidic ciliates are net remineralizers of nitrogen, and thus not competing with algae for this resource. However, they may outcompete heterotrophs due to higher growth efficiency and high ingestion rates.
Schoener, Donald M., "Inorganic Carbon and Nitrogen Utilization in Mixotrophic Ciliates" (2013). Doctoral Dissertations. 90.