Yeast extract as an effective nitrogen source stimulating cell growth and enhancing hydrogen photoproduction by Rhodobacter sphaeroides strains from mineral springs

The suitability and limitation of yeast extract as nitrogen source to support cell growth and to enhance hydrogen photoproduction by Rhodobacter sphaeroides strains MDC6521 and MDC6522 isolated from mineral springs in Armenia was investigated during the anaerobic growth. Yeast extract (2 g L−1) was indicated to be an effective nitrogen source for bacterial cell growth stimulation and enhanced H2 production (compared to glutamate). Both strains followed similar growth patterns in medium with yeast extract as nitrogen source and succinate or malate as carbon source. The highest growth rate was obtained for bacterial cells with yeast extract: the latter added gave a stimulated (2–3.5 fold) growth rate than using glutamate. R. sphaeroides suspension oxidation–reduction potential (ORP), which was measured with a platinum electrode, decreased down to low negative values with nitrogen source for both strains. ORP decreased down to more negative values (−610 ± 25 mV) in the presence of yeast extract than when adding glutamate (−405 ± 15 mV) compared to the control (without nitrogen source addition): the significant decrease of ORP indicated enhanced (∼6 fold) H2 yield. The noticeable ORP decrease measured with the titanium-silicate electrode and simultaneously the increase of extracellular pH ([pH]out) were observed; ORP was more negative at alkaline [pH]out. Thus, the optimal culture conditions with nitrogen and carbon sources for bacterial growth stimulation and enhanced H2 production were established. The ORP decrease together with the increase of [pH]out point out a significant role of reduction processes in cell growth and ability of bacteria to live.