Evaluating a most probable number method for assessing the viability of Laurentian Great Lakes protists

To support testing of ballast water management systems, we evaluated freshwater viability assessments for protists from western Lake Superior using the most probable number (MPN) method. Evaluations of protists in ballast water assessments have mainly used microscopy facilitated by cellular stains, whereas MPN uses a dilution series and grow-out period in incubated tubes containing growth media. Tests under varying temperatures and growth media compared MPN to microscopic methods for determining live organism densities. Protist growth was favored under higher temperatures and a medium comprising a 50% solution of Bold Modified Basal Media. Based on microscopy, a ballast treatment system reduced protist densities from 554–3000 to 12–52 cells/mL. Corresponding assessments using MPN estimated respective densities of 1 651–6 060 and 0–2.8 cells/mL, suggesting MPN overestimated viable cells while underestimating cell densities in low-density samples. MPN-estimated protist densities matched the microscopically measured protist size class including only cells 10–50 μm in minimum dimension; protist densities including smaller cells were much higher than MPN estimates. There was no correlation between MPN- and microscopy-determined densities. Certain protist taxa were poorly favored during the MPN growth period, while others thrived. These findings contribute freshwater data to the field of efficacy testing of ballast water treatment systems.