Measurements of cell density by spectrophotometry
A 1 mL sample of culture was centrifuged at 3,000 × g for 10 seconds to separate sulfur from the solution. The remaining solution was then centrifuged at 17,000 × g for 1 minute to pellet cells. The supernatant was removed, and the cells were resuspended in 1 mL of 5 mM Tris-HCl, adjusted to pH 8.5. The suspended cells were incubated at 95 °C for 10 minutes. In a 96-well plate, 200 µL of the cell solution was added to 50 µL of a 5× SYBR Green I solution diluted with 5 mM Tris-HCl and incubated at room temperature for 10 minutes. The fluorescence was measured at an excitation wavelength of 497 nm and an emission wavelength 520 nm using a Molecular Devices SpectraMax M2 spectrophotometer after a background subtraction using 200 µL ddH2O with the SYBR stain. Any fluorescence values below the background were set at zero.
The fluorescence measurements were converted to OD600measurements using a power function regressing known OD600 concentrations of A. ferrooxidanssuspensions with the above protocol (Fig. S1).