Predator-induced defense reduces growth rate and carrying capacity in a toxic diatom, Pseudo-nitzschia seriata.

Phytoplankton employ a wide variety of defenses to reduce mortality from zooplankton grazing. Many such defenses are inducible, that is, they are upregulated in the event of increased predation. Thus, theory predicts that they should come at a cost to the organism. When exposed to predatory cues from a copepod predator, the diatom Pseudo-nitzschia seriata upregulates the production of the neurotoxin domoic acid (DA). DA has recently been found to have grazer deterrent effects, but whether its production comes with trade-offs is still unclear. Here, we expose a strain of P. seriata to cues from copepods to induce increased DA production through the population growth phases. We find that not only do the induced cells grow slower, but because they take up more nitrogen per cell from the media, they produce less cells overall. <1 % of cellular nitrogen went into DA production, suggesting that the lower growth rate does not reflect a direct allocation cost, but rather that cells manipulate growth rate to reach higher toxin content per cell, or other costs associated with a higher toxin load. Such a mechanistic understanding of phytoplankton trade-offs is essential to accurately describe and model ocean plankton dynamics.