Rhodopseudomonas palustris WP3-5細胞生長、產氫與PHB累積之關係及共培養試驗

Abstract

Hydrogen is a clean energy that generates only water upon burning and it releases high amount of energy (122 kJ/g) by combustion. Photo-biological hydrogen production by purple non-sulfur photosynthetic bacteria is one of promising methods. Purple non-sulfur photosynthetic bacteria can produce hydrogen by nitrogenase under photo-heterotrophic condition. In recent years, many researchers make efforts to improve its efficiency of biological hydrogen production. Poly-β-hydroxybutyrate (PHB) is accumulated as carbon and energy storage material under unbalanced growth. The synthesis of PHB may compete with hydrogen production for energy and reducing power. In addition, maximum hydrogen production rate usually takes place during early stationary phase. Therefore, energy distribution between cell growth, hydrogen production and PHB accumulation is an important issue. Besides, dark fermentation effluent containing various volatile organic acids is suitable for purple non-sulfur photosynthetic bacteria to produce hydrogen. To treat wastewater and recycle clean energy efficiently by biological method are also essential. In this research, Rhodopseudomonas palustris WP3-5 and its PHB synthase-deficient mutant Rps. palustris M23 were used in batch experiments to explore energy distribution between cell growth, hydrogen production and PHB accumulation under different culture conditions. This study also tried to isolate purple non-sulfur photosynthetic bacteria from environment, and tested their ability to produce hydrogen using different volatile organic acids. Then, hydrogen production and treatment efficiency in co-culture system with Rps. palustris WP3-5 were evaluated and continuous photosynthetic hydrogen production was performed. From the results, Rps. palustris WP3-5 accumulated PHB in growth phase, and PHB could be utilized as another carbon and energy source when its content reached maximum value. Microorganism ceased growing when nitrogen source was exhausted, and used excess energy to produce more hydrogen in stationary phase. Rps. palustris WP3-5 could not synthesize PHB under low concentration of carbon source because substrate was degraded rapidly, and there were no difference of hydrogen production between wild-type strain and mutant Rps. palustris M23. When concentration of carbon source was three times higher, cumulated hydrogen volume and substrate conversion efficiency of Rps. palustris WP3-5 were better than Rps. palustris M23. This result coincided with other experiments in different culture conditions. But when using malate as carbon source, Rps. palustris WP3-5 had lower substrate degrading rate and cumulated hydrogen volume, and PHB was not accumulated. Different species of purple non-sulfur photosynthetic bacteria had different substrate assimilation pathway when using same carbon source and accumulated PHB content might be different. Rps. palustris WP3-5 used most energy to produce hydrogen in stationary phase and PHB content was below 10% cell dry weight, accounting for less than 5% of the substrate electrons utilized. This portion of energy might partially redistribute to synthesize soluble microbial product (SMP). Therefore, the competition relationship between hydrogen production and PHB accumulation was insignificant. In second part, several purple non-sulfur photosynthetic bacteria were isolated, and there were no obvious difference in utilizing volatile organic acids to produce hydrogen between these strains. Hydrogen production was not improved and competition for energy might exist in co-culture system. Additionally, new designed photo-bioreactor was used for Rps. palustris WP3-5 to produce hydrogen continuously when using tungsten filament lamp as light source. Operating hydraulic retention time from 2 day to 3 day, gas production rate was highest in initial period, up to 317.6 mL gas/L culture-day. MLSS was always between 220 and 360 mg/L, and MLSS wasn’t enhanced by changing operational strategies. Thinking of light transmission efficiency and problem of light shielding effect caused by high biomass concentration, experimental results was not as expected, and this photo-bioreactor might not favorable for continuous hydrogen production by purple non-sulfur photosynthetic bacteria.