Extending the cell digestion assay to quantify dead phytoplankton cells in cold and polar waters

The cell digestion assay (CDA) is an effective method to quantify living/dead cells in complex natural phytoplankton communities. The CDA involves the incubation of fresh samples at 35°C for 45 min for the enzymatic digestion of dead cells. The relatively high incubation temperature has raised concerns as to the applicability of the CDA to phytoplankton samples from polar and cold environments. Here, we examined the applicability of the CDA to phytoplankton growing at low temperatures and developed modifications and recommendations to reliably use the CDA in polar and cold-water communities. We carried out different tests with polar and temperate phytoplankton species, and with natural communities from the Arctic Ocean. Increased cell death when applying the standard CDA method was evident in some of the species tested, when compared with results obtained using alternative staining membrane permeability tests. We confirmed that the use of an incubation temperature of 25°C with Arctic communities and cultured species growing at 5°C results in the complete digestion of dead cells and no significant effect on the percentage of living cells. We recommend the use of 25°C as the incubation temperature when applying the CDA to phytoplankton communities inhabiting cold environments.