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

Various metal ions play a key role in biohydrogen (H2) production by phototrophic bacteria through incorporation into or stimulating the responsible enzymes and/or related pathways. The Ni (II) and Mg (II) ions effects on growth and H2 production by Rhodobacter sphaeroides strain MDC6521 isolated from mineral springs in Armenia were established. The highest growth specific rate was obtained with 4–6 μM Ni2+ and 5 mM Mg2+. pH of the growth medium changed from 7.0 to 9.2–9.4 during the bacterial growth up to 72 h in spite of Ni2+ added but pH increased in different manner with Mg2+. In the presence of 2–4 μM Ni2+ external oxidation-reduction potential (ORP) decreased to more negative values (−800 ± 15 mV). This decrease of ORP indicated ∼2.7-fold enhanced H2 yield (9.80 mmol L−1) with Ni2+ compared with the control (without Ni2+). The H2 yield determined in the medium with Mg2+ was ∼2.2 fold higher than that with 1 mM Mg2+. These results reveal new regulatory ways to improve H2 production by R. sphaeroides those were depending on Ni2+ and Mg2+ of different concentrations. ► 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.