Fig. 1

a Growth of Zymomonas mobilis mutant strains in biogas slurry medium at pH 6.0 which were obtained in 1st-round breeding. The Z. mobilis strains were as follows: D91 (Olive star), D92 (Brown up-triangle), D95 (Red diamond), D161 (Pink left-triangle), D172 (Light blue circle), ZM4 (control) (Gray square). b Growth of Z. mobilis mutant strains in biogas slurry medium at pH 3.8 which were obtained in 2nd-round breeding. The Z. mobilis strains were as follows: S1 (Purple right-triangle), S9 (Yellow circle), S10 (Orange cross). S11 (Green down-triangle), S912 (Blue up-triangle), D95 (control) (Red diamond), ZM4 (control) (Gray square). Growth was determined by measuring OD₆₀₀ value. The cultivations were carried out by automated turbidimeter with 10% inoculum at 30 °C. The biogas slurry with 50 g/L glucose was sterilized using a 0.22 μm filter membrane. All cultivations were carried out in triplicate. Error bars show standard deviation (n = 3)