Single cell ICP-MS reveals diversity in cadmium response across individual cells of three marine phytoplankton species.

Cadmium (Cd) is a persistent toxic metal that threatens marine ecosystems, yet species-specific links between phytoplankton tolerance and cellular Cd accumulation remain poorly understood. Triplicate cultures of Prymnesium parvum, Oltmannsiellopsis viridis and Eutreptiella gymnastica were exposed to 0 -100 μmol L-1 nominal Cd for 21 days in seawater-based media; growth, projected cell area, Fv/Fm and per-cell Cd mass were measured on the same cultures. At 10 μmol L-1 Cd, cell densities of P. parvum and E. gymnastica were significantly lower than their corresponding same-day controls on some sampling days, whereas most corresponding same-day differences in Fv/Fm were not significant, indicating that cell density was a more sensitive response endpoint than photochemical efficiency. At 100 μmol L-1, only O. viridis sustained positive net specific growth and measurable late-stage Fv/Fm; the other two showed little or no net growth and late photophysiological impairment. Projected cell area increased markedly at 100 μmol L-1, most strongly on day 7, but in a species- and day-dependent manner. Cell-associated Cd mass rose with concentration but not with time, being 4.31-, 2.04- and 6.18-fold higher at 100 than at 1 μmol L-1 in P. parvum, O. viridis, and E. gymnastica, respectively, with cell-to-cell heterogeneity increased at higher concentrations. O. viridis showed the smallest relative increase yet carried a high per-cell Cd mass (2.83 fg cell-1 on day 21 at 100 μmol L-1), indicating that greater cell-associated Cd accumulation did not necessarily correspond to greater physiological impairment. Overall, O. viridis was the most Cd-tolerant of the three species under the tested conditions and priority candidate for removal-focused evaluation.