Table 1. Coded and actual values of experimental variables used in the central composite experimental design.

Independentvariables	Symbols			Levels
		−1.72*	−1	0	1	1.72*
Fe (µM)	x1	0.03	2.39	5.35	8.31	10.44
Mn (µM)	x2	0.02	2.67	6.36	10.05	12.70
Mo (µM)	x3	0	0.13	0.31	0.50	0.62
Ni (µM)	x4	0	0.71	1.69	2.68	3.39

^*Alpha values used for the axial points in this study.