# Photo-Fermentative Bacteria Used for Hydrogen Production
## 3 Discussion
A maximum hydrogen production rate of 0.79 mM/h with acetate, 0.71 mM/h with propionate, and 0.65 mM/h with a mixture of acetate, propionate, and butyrate was observed at pH 6.8 and a temperature of about 32 \({}^{\circ}\)C [53]. By increasing the temperature to 35 \({}^{\circ}\)C, under similar conditions, the rate was even lower at 0.56 mM/h, demonstrating the dependence of hydrogen production on temperature and bioreactor design and material [52]. _R. capsulatus_, a Gram-negative Rhodobacter extensively considered in this review, is very sensitive to environmental conditions, yet these microbes have been used for hydrogen production. _R. capsulatus_ strain JP91 gave a maximum hydrogen production rate under similar environmental conditions as other photo-fermentative bacteria, with pH 7, 30 \({}^{\circ}\)C, tungsten lamps as a light source, and glucose concentration of 56 and 35 mM as the substrates. The rate of hydrogen production as observed in _R. palustris_ is dependent on the temperature, bioreactor design, volume, and material, with hydrogen produced at a rate of 1.66-2.3 mM/h using a 350 mL continuous photobioreactor and 2.6 mM/h hydrogen in a 100 mL serum bottle in a batch process [59; 60]. On the other hand, acetate used as a substrate does not work well with these species, giving a hydrogen production rate as low as 0.15 mM/h, at pH 8 and temperature \(<\) 30 \({}^{\circ}\)C [66], indicating the specificity required for the optimal metabolic activity of this microbe. _R. spharoides_, a bacterium of the same genus, has shown a better potential than _R. capsulatus_ and even _R. palustris_, with strain HY01 having a hydrogen production rate as high as 7 mM/h at a pH range of 7.0-8.2 and 30 \({}^{\circ}\)C temperature in a 30 mL syringe using a mixture of 25 mM acetate and 34 mM butyrate and halogen lamps for light irradiance [76], and strain DSM158 with a hydrogen production rate 8.7 mM/L with 40 mM of lactic acid as the substrate and tungsten lamps as light sources in a single-walled glass vessel at pH 7 and 30 \({}^{\circ}\)C [74]. These results make this microbe the most efficient of all for hydrogen production, but because its metabolic pathway is very diverse, the environmental conditions under which this bacterium can produce hydrogen are very specific. On the other hand, _R. rubrum_ has only been studied for the exergy analysis in hydrogen production. The substrates used by the bacteria are similar to other PNSBs, including acetate, malate, and glucose, using light sources, such as tungsten lamps, which help to determine and compare the renewability of hydrogen production. Finally, _R. sulfidophilum_ was studied and compared with the other PNSBs in the review. These acid-tolerant bacteria work well in acidic environments and at high temperatures. A strain P5 with acetate as a substrate has given a hydrogen production rate of 0.50 mM/h, and a strain TH-102 with glucose has given a hydrogen rate of 0.42 mM/h [83], making it an efficient hydrogen-producing bacterium. Substrate, pH, temperature, bioreactor design and material, and light sources were identified for each bacterium, and it was found that most of them worked well in a pH range of 6.8-8.0, as a more acidic pH significantly reduces the hydrogen yield due to the inhibition of metabolism by the formation of volatile fatty acids. The effect of the temperature varied among bacteria. R. palustris gave a maximum hydrogen yield at 28 \({}^{\circ}\)C with glycerol as a substrate after 192 h and at 30 \({}^{\circ}\)C with glucose after just 20 h. _R. capsulata_ gave a maximum hydrogen yield at 32 \({}^{\circ}\)C with acetate as a substrate, while _R. capsulatus_ reached the optimum with glucose at 30 \({}^{\circ}\)C, pH 6.8; _R. spharoides_' optimum was at 30 \({}^{\circ}\)C using lactic acid, and _R. sulfidophilum_ produced a maximum of 0.42 mM/h at 72 and 168 h, but using a co-culture it went from 168-216 h. In general, the simpler the substrate, the better the yield and the shorter the time required.