Cost and benefits of toxin production in a dinoflagellate

Abstract

Many phytoplankton respond to chemical cues from grazers by upregulating defensive capabilities. Inducible defences like these are often assumed to come at a cost to the organism, but these trade-offs have not been experimentally established. A reason for this may be that costs only become evident under resource limiting conditions. Here, we exposed the toxin-producing dinoflagellate Alexandrium minutum to chemical cues from copepods under different levels of nitrogen limitation. Induced cells had higher cellular toxin content and a larger fraction of the cells were rejected by a copepod, demonstrating the clear benefits of toxin production. Induced cells also had a higher carbon and nitrogen content, despite an up to 25% reduction in cell size. Unexpectedly, induced cells seemed to grow faster than controls, likely owing to a higher nutrient affinity due to reduced size. We thus found no clear trade-offs, rather the opposite. However, we argue that indirect ecological costs that do not manifest under laboratory conditions are important and that the induction of toxins specific to particular defences prevents the cells from constantly synthesizing the large array of secondary metabolites that they are capable of producing.