2.6. Cell counting

Cell counting was performed before inoculation and at the end of experiments. A Marienfeld Thoma counting chamber was used with an inverted microscope in phase contrast imaging mode. First, a glass coverslip was placed over the counting chamber such that there is 100 µm between the counting grid and the glass slide. Then, 10 µl of control‐diluted samples was injected under the coverslip with a precision pipette. The Thoma chamber has 16 large squares in the counting grid in which the cells are counted per square and summed up. Each sample was counted three times and the average is used to calculate the original concentration.

On the last day of the culture experiments, samples are extracted from the wells. They were first actuated at 2000 rpm for 1 min to resuspend microalgae into the medium. All content was then extracted with a sterile syringe and stored in a 0.5 ml vial. To remove the remaining attached algae as thoroughly as possible, 0.5 ml nutrient medium was used to wash the well with repeated in and out motion using a pipette and is also stored in a 0.5 ml vial. Finally, the same counting procedure was applied to determine the final microalgae concentration in each well.