Comparative effects of Ni(II) and Cu(II) ions and their combinations on redox potential and hydrogen photoproduction by Rhodobacter sphaeroides

- Discrimination between Cu2 + and Ni2 + and their combination effects was revealed.
- Low concentrations of Cu2 + inhibited the growth and H2 yield in R. sphaeroides.
- Low concentrations of Ni2 + enhanced the decrease of redox potential and H2 yield.
- Cu2 + and Ni2 + combinations have different action mechanisms on R. sphaeroides.
- Interaction between heavy metal ions and the proton FOF1-ATPase is suggested.

The aim of the present work was the study of comparative effects of Cu(II) and Ni(II) and their mixture on growth, redox potential, hydrogen (H2) yield and ATPase activity in phototrophic purple bacteria R. sphaeroides MDC6522 from Jermuk mineral spring in Armenia. It was ascertained, that Cu2 + and Ni2 + have different effects on bacterial specific growth rate: in the presence of 5 μM Cu2 + growth rate was ~ 3.2-fold lower in comparison with control (no addition), and increased ~ 1.5-fold in medium with 5 μM Ni2 +. These changes may be resulted by action of the ions on redox potential (Eh). Low concentrations of Ni2 + had an enhancing effect on the Eh drop and H2 production. The increase of concentration from 1 to 5 μM enhanced the stimulatory effect of Ni2 +. H2 yield in R. sphaeroides (72 h of growth) was enhanced ~ 3-fold with 5 μM Ni2 +, whereas in the presence of 5 μM Cu2 + H2 yield was ~ 1.2 fold lower in comparison with control. Cu2 + + Ni2 + combinations effects were differed from the effect when ions used separately. When Cu2 + and Ni2 + were added together, the Ni2 + stimulatory effect disappeared, which indicated that heavy metal ions mixture may have different action mechanisms. Moreover, N,N′-dicyclohexylcarbodiimide-sensitive ATPase activity of R. sphaeroides membrane vesicles has been increased in the presence of both ions, but in the presence of Сu2 + the influence was feebly marked in comparison with Ni2 +. The results suggest an interaction between these ions and the FOF1-ATPase. Thus, the results obtained point out discrimination between Cu2 + and Ni2 + and their combinations effects and reveal new regulatory pathways to enhance H2 yield in R. sphaeroides.