Limits to the cellular control of sequestered cryptophyte prey in the marine ciliate Mesodinium rubrum

The marine ciliate Mesodinium rubrum is famous for its ability to acquire and exploit chloroplasts and other cell organelles from some cryptophyte algal species. We sequenced genomes and transcriptomes of free-swimming Teleaulax amphioxeia, as well as well-fed and starved M. rubrum in order to understand cellular processes upon sequestration under different prey and light conditions. From its prey, the ciliate acquires the ability to photosynthesize as well as the potential to metabolize several essential compounds including lysine, glycan, and vitamins that elucidate its specific prey dependency. M. rubrum does not express photosynthesis-related genes itself, but elicits considerable transcriptional control of the acquired cryptophyte organelles. This control is limited as light-dependent transcriptional changes found in free-swimming T. amphioxeia got lost after sequestration. We found strong transcriptional rewiring of the cryptophyte nucleus upon sequestration, where 35% of the T. amphioxeia genes were significantly differentially expressed within well-fed M. rubrum. Qualitatively, 68% of all genes expressed within well-fed M. rubrum originated from T. amphioxeia. Quantitatively, these genes contributed up to 48% to the global transcriptome in well-fed M. rubrum and down to 11% in starved M. rubrum. This tertiary endosymbiosis system functions for several weeks, when deprived of prey. After this point in time, the ciliate dies if not supplied with fresh prey cells. M. rubrum represents one evolutionary way of acquiring photosystems from its algal prey, and might represent a step on the evolutionary way towards a permanent tertiary endosymbiosis

Dynamics of Sequestered Cryptophyte Nuclei in Mesodinium rubrum during Starvation and Refeeding

The marine mixotrophic ciliate Mesodinium rubrum is known to acquire chloroplasts, mitochondria, nucleomorphs, and nucleus from its cryptophyte prey, particularly from species in the genera, Geminigera and Teleaulax. The sequestered prey nucleus and chloroplasts are considered to support photosynthesis of M. rubrum. In addition, recent studies have shown enlargement of the retained prey nucleus in starved M. rubrum and have inferred that enlargement results from the fusion of ingested prey nuclei. Thus far, however, little is known about the mechanism underlying the enlargement of the prey nucleus in M. rubrum. Here, we conducted starvation and refeeding studies to monitor the fate of prey nuclei acquired by M. rubrum when feeding on Teleaulax amphioxeia and to explore the influence of the retained prey nucleus on photosynthesis of M. rubrum. Results indicate that enlargement of the prey nucleus does not result from fusion of nuclei. Furthermore, the enlarged prey nucleus does not appear to divide during cell division of M. rubrum. The presence of a prey nucleus significantly affected photosynthetic performance of M. rubrum, while the number of retained chloroplasts had little influence on rate of carbon fixation. We interpret results within the context of a model that considers the dynamics of ingested prey nuclei during division of M. rubrum

Effects of irradiance and prey deprivation on growth, cell carbon and photosynthetic activity of the freshwater kleptoplastidic dinoflagellate <i>Nusuttodinium</i> (= <i>Gymnodinium</i>) <i>aeruginosum</i> (Dinophyceae)

<div><p>The freshwater dinoflagellate <i>Nusuttodinium aeruginosum</i> lacks permanent chloroplasts. Rather it sequesters chloroplasts as well as other cell organelles, like mitochondria and nuclei, from ingested cryptophyte prey. In the present study, growth rates, cell production and photosynthesis were measured at seven irradiances, ranging from 10 to 140 μmol photons m^-2s^-1, when fed the cryptophyte <i>Chroomonas</i> sp. Growth rates were positively influenced by irradiance and increased from 0.025 d^-1 at 10 μmol photons m^-2s^-1 to maximum growth rates of ~0.3 d^-1 at irradiances ≥ 40 μmol photons m^-2s^-1. Similarly, photosynthesis ranged from 1.84 to 36.9 pg C cell^-1 h^-1 at 10 and 140 μmol photons m^-2s^-1, respectively. The highest rates of photosynthesis in <i>N</i>. <i>aeruginosum</i> only corresponded to ~25% of its own cell carbon content and estimated biomass production. The measured rates of photosynthesis could not explain the observed growth rates at high irradiances. Cultures of <i>N</i>. <i>aeruginosum</i> subjected to prey starvation were able to survive for at least 27 days in the light. The sequestered chloroplasts maintained their photosynthetic activity during the entire period of starvation, during which the population underwent 4 cell divisions. This indicates that <i>N</i>. <i>aeruginosum</i> has some control of the chloroplasts, which may be able to replicate. In conclusion, <i>N</i>. <i>aeruginosum</i> seems to be in an early stage of chloroplast acquisition with some control of its ingested chloroplasts.</p></div

Light microscopy of <i>Nusuttodinium aeruginosum</i>.

<p>(A) Large chloroplast-containing cell. (B) Small cell showing chloroplast degradation. (C) Epifluorescence microscopy revealing numerous chloroplasts.</p

Effect of irradiance on cell volume and biomass production of <i>Nusuttodinium aeruginosum</i>.

<p>(A) Cell volume and cell carbon of <i>N</i>. <i>aeruginosum</i> as a function of irradiance. Data points represent means ± SE (n = 30–40). The curves were numerically fitted to Michaelis-Menten kinetics: cell volume = 22048*I/(21.04+I), R² = 0.25; and cell carbon = 2668*I/(15.45+I), R² = 0.28. Note different ordinate scales. (B) Biomass production (BP) and photosynthetic activity (PA) as a function of irradiances. The BP curve was numerically fitted to Michaelis-Menten kinetics: BP = 1144*(I-13)/(38.62+(I-13)), R² = 0.93. The data for PA were fitted to a linear line, R² = 0.97. Data for biomass production are the same as in Figs <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0181751#pone.0181751.g002" target="_blank">2A</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0181751#pone.0181751.g003" target="_blank">3A</a>.</p

Effect of irradiance on growth and photosynthetic activity of <i>Nusuttodinium aeruginosum</i>.

<p>(A) Growth rates as a function of irradiance. The curve was numerically fitted to Michaelis-Menten kinetics. μ = 0.45*(I-10)/(26.99+(I-10)), R² = 0.91. Data points represent means ± SE (n = 12). (B) Photosynthetic activity as a function of irradiance. The data was fitted to a linear line, R² = 0.96. Data points represent means ± SE (n = 4, except irradiance = 10 where n = 3). (C) Growth rates of <i>N</i>. <i>aeruginosum</i> as a function of photosynthetic activity. The curve was numerically fitted to Michaelis-Menten kinetics. μ = 0.41*(I-1.5)/(5.54+(I-1.5)), R² = 0.89.</p

Biomass production (BP) and photosynthetic activity (PA) of <i>Nusuttodinium aeruginosum</i> as a function of time under prey deprivation.

<p>Data points represent means ± SE (BP; n = 30, PA; n = 4).</p