Term Coefficient Value Standard error of coefficient t- value p-value Constant b0 44.5 6.292 7.07296 0.000034* Glucose concentration b1 60.0 10.210 5.87255 0.000157* (NH4)2SO4 b2 -35.5 5.402 -6.56617 0.000063* Metal ion concentration (Mn2+) b3 29.2 3.759 7.77009 0.000015* Glucose concentration × Glucose concentration b11 -7.5 0.879 -8.51224 0.000007* (NH4)2SO4 × ((NH4)2SO4 b22 617.2 173.295 3.56154 0.005168* Metal ion concentration (Mn2+) × Metal ion concentration (Mn2+) b33 -20382.1 2202.723 -9.25315 0.000003* Glucose concentration × (NH4)2SO4 b12 -5.8 2.950 -1.98299 0.075496 (NH4)2SO4 × Metal ion concentration (Mn2+) b23 373.7 147.504 2.53382 0.029676 Metal ion concentration (Mn2+) × Glucose concentration b31 3.7375 1.47504 2.53382 0.029676* * Significant (p≤0.05)

Table 3: Coefficients, t-statistics and significance probability of the model