1.5 ml reaction tubes

The cell vitality was determined using an MTT assay (Mosmann 1983 (link)) as described in Sachs et al. (2017 ). This approach measures the activity of mitochondrial and cytosolic dehydrogenases, which reduces the yellow, water soluble 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) to a blue, water-insoluble formazan product (Lindl and Gstraunthaler 2008 ).
After cell exposure to Eu(III) or U(VI), 50 mg of fresh cells were weighed into 1.5-mL reaction tubes (Greiner) followed by the addition of 1-mL phosphate-buffered saline solution without Ca²⁺ and Mg²⁺ (PBS; Biochrom, Berlin, Germany) and 200 μL MTT solution (5 mg/mL; Duchefa, Harlem, The Netherlands). Subsequently, the assay was performed as described in Sachs et al. (2017 ). The vitality of the Eu(III) and U(VI) exposed cells was determined as a percentage of the control samples according to Eq. (1). $$\text{Cell vitality}\left(\%\text{of control}\right)=\frac{\text{Absorbance of exposed cells}}{\text{Absorbance of control cells}}\times 100$$
The results represent data from five independent experiments, each with two to three samples for control and each metal concentration.

Cells were retrieved from the capsules for determination of cell numbers. For this, capsules were collected in 15 mL reaction tubes (Greiner Bio-One, Kremsmünster, Austria) and weighed. About 300 mg per sample were collected, the alginate shell was dissolved with 100 mM sodium citrate pH 7.4 for 2 min at room temperature. Before centrifugation at 500× g for 5 min sodium citrate was diluted with 5-fold volume culture medium. Supernatant was removed and the cell pellet was resuspended in 200 µL Accumax™ and incubated for 30 min at 37 °C and 100 RPM. Afterwards, Accumax™ was inactivated by 3-fold the volume of cell culture medium. After centrifugation at 500× g for 7 min cell pellet was resuspended in fresh medium and counted manually using Neubauer counting chambers and Trypan Blue exclusion assay. Results are presented as both the number of cells/capsule and number of cells/g capsules.