Clear benefits without direct costs in dinoflagellate defenses

Some defense costs only become noticeable under resource limitation where the phytoplankton have the ‘choice’ of investing in defense or growth, but this effect has not been thoroughly explored for toxin producing dinoflagellates. In our study, we grew the dinoflagellate Alexandrium minutum in chemostats at a range of different nitrogen concentrations, and induced increased toxin production by exposing the cells to chemical cues extracted from copepods.

The benefits of toxicity were clearly demonstrated by direct video observations: Induced cells have increased toxin content that leads to cells being rejected by copepods. However, we did not find any direct costs of toxin production in terms of reduced growth rate, but observed several additional effects of the copepod cues, some of which were dependent on both nitrogen availability and the grazer-cue concentration. The most important discovery is that induced cells are up to 25% smaller than non-induced cells, leading to a higher specific nutrient affinity. Consequently, induced cells grow faster, not slower, than cells that were not exposed to grazer cues. That is, exactly the opposite of what we expected. Since there must be trade-offs –else all phytoplankton would be equally defended – we argue that the costs to toxin production are mainly indirect that only materialize in the field. 

Read the paper here: https://www.nature.com/articles/s41396-021-00908-y

Ryderheim, F., Selander, E. & Kiørboe, T. Predator-induced defence in a dinoflagellate generates benefits without direct costs. ISME J (2021). https://doi.org/10.1038/s41396-021-00908-y

There is also a blog post on our work at the Nature Microbiology community ‘Behind the paper’: https://naturemicrobiologycommunity.nature.com/posts/new-insights-into-predator-induced-dinoflagellate-defenses?channel_id=346-behind-the-paper