Ni (II) and Mg (II) ions as factors enhancing biohydrogen production by Rhodobacter sphaeroides from mineral springs

► Ni2+ and Mg2+ affected the phototrophic growth and H2 production by Rhodobacter sphaeroides MDC6521. ► The highest growth specific rate was obtained with 4-6 μM Ni2+ and 5 mM Mg2+. ► The changes in pH increase were observed with Mg2+ but not with Ni2+. ► Ni2+ and Mg2+ increased drop in external redox potential during the growth. ► 2-4 μM Ni2+ and 5-10 mM Mg2+ were effective for significant increase of H2 production.