Retention of blue-green cryptophyte organelles by Mesodinium rubrum and their effects on photophysiology and growth

Abstract

As chloroplast-stealing or "kleptoplastidic" lineages become more reliant on stolen machinery, they also tend to become more specialized on the prey from which they acquire this machinery. For example, the ciliate Mesodinium rubrum obtains > 95% of its carbon from photosynthesis, and specializes on plastids from the Teleaulax clade of cryptophytes. However, M. rubrum is sometimes observed in nature containing plastids from other cryptophyte species. Here, we report on substantial ingestion of the blue-green cryptophyte Hemiselmis pacifica by M. rubrum, leading to organelle retention and transient increases in M. rubrum's growth rate. However, microscopy data suggest that H. pacifica organelles do not experience the same rearrangement and integration as Teleaulax amphioxeia's. We measured M. rubrum's functional response, quantified the magnitude and duration of growth benefits, and estimated kleptoplastid photosynthetic rates. Our results suggest that a lack of discrimination between H. pacifica and the preferred prey T. amphioxeia (perhaps due to similarities in cryptophyte size and swimming behavior) may result in H. pacifica ingestion Thus, while blue-green cryptophytes may represent a negligible prey source in natural environments, they may help M. rubrum survive when Teleaulax are unavailable. Furthermore, these results represent a useful tool for manipulating M. rubrum's cell biology and photophysiology.

Keywords: Hemiselmis pacifica; acquired photosynthesis; ciliate; cryptophyte; functional response; kleptoplasty.

FIGURE 1

Mesodinium rubrum ingestion functional response (A) and growth functional response (B) to increasing concentrations of Hemiselmis pacifica prey. Both ingestion and growth saturate as prey abundance increases.

FIGURE 2

No evidence of selective grazing by Mesodinium rubrum offered Hemiselmis pacifica and Teleaulax amphioxeia. (A) Population dynamics of H. pacifica and T. amphioxeia in the presence (filled symbols) and absence (open symbols) of M. rubrum. Lower (negative) growth rates in the presence of M. rubrum provide evidence of grazing on both cryptophytes by the ciliate. (B) During the experiment, the relative abundance of H. pacifica (which was initially inoculated slightly below the 50:50 intended ratio) did not differ in the two experimental treatments. Because the growth rates of the two cryptophytes were similar (though H. pacifica grew nonsignificantly slower, C), the initial difference in population size led to a difference in abundances that persisted throughout the experiment (D). As a result of these numerical differences in prey abundance, ingestion rates of T. amphioxeia were higher (E), but the attack rates were identical (F).

FIGURE 3

Confocal images of Mesodinium rubrum subject to different feeding treatments. (A–E) Mesodinium rubrum‐fed Teleaulax amphioxeia and then subject to 3 days of starvation still have numerous phycoerythrin‐bearing red plastids. (F–J) After 2 weeks of feeding on Hemiselmis pacifica, M. rubrum cells are filled with phycocyanin‐bearing blue‐green plastids; H. pacifica nuclei colocalize with these plastids. Typically, one or two T. amphioxeia plastids remain. (K–O) After only 2 days of feeding with T. amphioxeia, however, M. rubrum cells quickly eliminate most H. pacifica plastids and re‐equip themselves with T. amphioxeia plastids.

FIGURE 4

Changes in Mesodinium rubrum cell volume occupied by (A) Teleaulax amphioxiea plastids, (B) Hemiselmis pacifica plastids, and (C) all plastid types varied by treatment. While starved cells gradually lost plastid volume over time, H. pacifica‐fed cells rapidly replaced T. amphioxiea plastids after day 5, though the total cell volume occupied by plastids was lower in the H. pacifica‐fed cultures.

FIGURE 5

Photophysiology of Mesodinium rubrum starved (open points) or fed Hemiselmis pacifica (filled points). (A) Cellular chlorophyll content decreased in both treatments, but (B) photosynthetic efficiency remained higher for H. pacifica‐fed cultures. Overall, photosynthesis rates at growth irradiance (C), maximum photosynthetic rates (D), light sensitivity (E), and saturating light intensity (F) were lower for H. pacifica‐fed cultures. Where applicable, reference lines representing parameters from Teleaulax amphioxiea (dashed line) and H. pacifica (dotted line) photosynthesis–irradiance curves are shown. Note that H. pacifica‐fed M. rubrum cells still retain some T. amphioxeia plastids, which contribute to measurements, especially prior to day 14 (Figure 4).

FIGURE 6

Long‐term growth of Mesodinium rubrum is enhanced by the presence of Hemiselmis pacifica. (A) When fed H. pacifica (filled points), M. rubrum populations grew faster for a longer period of time, ultimately achieving larger population sizes over the 3‐week experiment. (Note that M. rubrum population data have been rescaled to account for dilution; actual population sizes were below 10,000 cells/mL throughout the experiment.) (B) We used a sliding 7‐day window to calculate growth rates, and found that feeding with H. pacifica increased M. rubrum growth rates for the first 12 days of the experiment, but in the final 10 days growth rates were similar between starved and fed treatments.