Is toxin production in phytoplankton costly? How do environmental conditions affect the cost of toxin production?
Many species of phytoplankton produce toxins that may provide protection from grazing. In that case one would expect toxin production to be costly; else all species would evolve toxicity. However, experiments have consistently failed to show any costs. Then when, if ever, are there costs? How do environmental conditions affect the cost?
We develop a fitness optimization model to estimate rate, costs, and benefits of toxin production, using PST (paralytic shellfish toxin) producing dinoflagellates as an example. Costs include energy and material (nitrogen) costs estimated from well-established biochemistry of PSTs, and benefits are estimated from relationship between toxin content and grazing mortality. The model reproduces all known features of PST production: inducibility in the presence of grazer cues, low toxicity of nitrogen-starved cells, but high toxicity of P- and light-limited cells. The costs of toxin production are environment dependent but can be high. The model predicts negligible reduction in cell division rate in nitrogen replete cells, consistent with observations, but >20% reduction when nitrogen is limiting and abundance of grazers high. Such situation is characteristic of coastal and oceanic waters during summer when blooms of toxic algae typically develop. The investment in defense is warranted, since the net growth rate is always higher in defended than in undefended cells.
The paper can be read here.
Subhendu Chakraborty, Marina Pančić, Ken H Andersen, Thomas Kiørboe (2018). The cost of toxin production in phytoplankton: the case of PST producing dinoflagellates: The ISME Journal (DOI: 10.1038/s41396-018-0250-6)