Trade-offs are the cornerstone of trait-based ecology. They describe the advantages and costs of a certain trait, and thus determine when and where organisms with that trait will occur. But, empirical correlations between traits that have no mechanistic interpretation may lead to false trade-offs. Observed correlations between toxin contents in dinoflagellates and their nutrient uptake half-saturation constant is one such example.
A recent paper in Ecology Letters by Brandenburg et al. (2018, 21: 1561–1571) demonstrates a positive correlation between the toxin contents in dinoflagellates and their half-saturation constant - the nutrient concentration at which the cells achieve half their maximum nutrient uptake rate. They interpret the half-saturation constant as a measure of the cost of toxin production, and conclude that there is a trade-off between the beneficial toxin production and the higher cost of nutrient harvesting.
However, this is a false trade-off. The half-saturation constant is not an organism trait with a mechanistic interpretation. A more appropriate trait is the nutrient affinity, which is the imaginary volume of water that the cells clear of nutrients per unit time at low nutrient concentrations. The affinity depends on the size of the cell and the number of uptake sites on the cell surface, both mechanistic organism traits. The half-saturation constant, in contrast, is the ratio of the maximum uptake rate and the nutrient affinity, i.e., not an organism trait. When we express the ability of the cells to harvest nutrients in terms of the affinity, rather than the half-saturation constant, the correlation reported by Brandenburg et al. vanishes. There is no measurable cost to toxin production in terms of reduced ability to harvest nutrients.
There is also no a priori reason to suspect such a cost. In another recent paper from the Centre (Chakraborty et al., 2018, ISME J.), we suggest that the real cost of toxin production relates to nitrogen allocation between growth and toxin production, not to nitrogen harvesting. We are currently testing this hypothesis.
Read the paper here.
Kiørboe T and Andersen KH (2019) Nutrient affinity, half-saturation constants and the cost of toxin production in dinoflagellates. Ecol Lett, doi: 10.1111/ele.13208