Validation of an adenosine triphosphate (ATP) model for 10-50 μm plankton

A recent model demonstrated that the adenosine triphosphate (ATP) content of spherical aquatic organisms with a 10 to 50 μm diameter is between 0.16 and 19.9 pg cell^-1. Here, the model is validated by comparing microscopy-based counts with ATP concentrations from a commercial ATP kit. The measured ATP content of both freshwater and marine organisms 10 to 50 μm size range falls in the 0.16 to 19.9 pg cell^-1 model range. On average, freshwater organisms contain 0.33 pg ATP cell^-1, have a spherical equivalent diameter (SED) of 13 μm, while marine organisms have 0.89 pg ATP cell^-1 and a SED of 18 μm. In addition, their 13 to 18 μm size is within the 10 to 50 μm ballast water size range and in agreement with the 15 μm mean SED of a coastal plankton size-distribution model. This study concludes that the ATP-model is reliable, emphasizing the need for caution when converting three-dimensional biomass proxies into linear cell concentrations.