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Q33 - Symbiodinium Nutrient Limitation

Theoretical 2 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Coevolution & Species Interactions

Corals form mutualistic associations with a dinoflagellate, Symbiodinium of the family Symbiodiniaceae. Symbiodinium has two major life stages: a flagellated stage, called mastigote, and an aflagellated stage, called coccoid.

When Symbiodinium is grown in culture, each coccoid cell undergoes cell division to produce two mastigote cells. The mastigotes swim for a few days before they shed their flagella and metamorphose into the coccoid stage.

However, within the host coral cells, there is a dramatic change in Symbiodinium life history, with a marked reduction in cell proliferation and a severe reduction in the number of mastigotes. It is assumed that the coral host regulates the metamorphosis of Symbiodinium, and evidence suggests that certain nutrients may serve as limiting factors.

In an experiment, Symbiodinium microadriaticum cells were cultured outside their host cells, under nutrient-limited free-living conditions, to determine the importance of different nutrients for regulating cell proliferation and differentiation.

Figure A: Cell proliferation curves of S. microadriaticum from Days 6 to 16, with selective depletion of nitrogen, iron, and phosphate versus control

Figure B: Relative proportions of mastigotes and coccoid cells under nutrient depletion conditions

True or False?

Q33.1. All three nutrients tested affected the mean values of both proliferation and differentiation of S. microadriaticum cells.
Q33.2. Nitrogen was the only limiting nutrient for the differentiation of S. microadriaticum cells within host coral cells.
Q33.3. Given the above data, it is possible that the changing coccoid / mastigote ratios in the cultures are a side effect of the marked reduction in cell proliferation.
Q33.4. Phosphate had relatively the greatest restrictive effect on cell proliferation over the experimental period.
Q33.5. These results suggest that it may be possible for coral host cells to increase the levels of certain nutrients to regulate the differentiation of S. microadriaticum cells.
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Question reproduced from IBO 2023, Theoretical Paper 2, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF