# Photo-Fermentative Bacteria Used for Hydrogen Production
## 3 Discussion  
This review discusses the microbes used in the photo-fermentative hydrogen production and the ideal conditions for increasing the efficiency and rates of hydrogen production are discussed. Initially, we considered the discussion based on exergy calculations. This is a very realistic approach in wastewater treatment plants where operating costs, energy losses, or fees per volume of water treated are critical. However, most of the studies reviewed do not provide essential data for such calculations, e.g., volatile fatty acid formation, amounts of CO\({}_{2}\) produced, heat losses, heating, agitation, or other energy costs. We have, therefore, chosen to report all results in a homogeneous format, expressed as mmol of hydrogen per liter of substrate per hour (mM/h), a parameter that is also of great interest at an industrial level. Table 5 gives a summary of the strains of different bacterial species with their maximum hydrogen production rate, providing a useful guide for comparison purposes. The ranges are shown in Figure 5.  
A comparative analysis of hydrogen-producing bacteria by photo-fermentation showed that _R. palustris_ is the preferred species for hydrogen production under certain environmental conditions due to its hydrogen production rate. A hydrogen production rate of 3.2 mM/h was observed by the CQK01 strain of this bacterium using glucose as a substrate in a serum bottle at pH 7 and a temperature of 30 \({}^{\circ}\)C, followed by its NCIMB 11774 strain using 50 mM glycerol as a substrate in a 1 L fluidized bed PBR, giving a hydrogen production rate of 3.23 mM/h. The results obtained for this, and for all the bacteria analyzed in this review, show that most of these microbes work well around a pH of 7, but the temperature and the type of bioreactor used in the research studies play a major role in the yield of hydrogen production, as can be observed in Table 1 and also in the microbial analysis as well. The same strain CQK01 of _R. palustris_ with 56 mM of glucose as the substrate, at pH 7, and with a tungsten lamp of 31.6 W/m\({}^{2}\) irradiance provided a hydrogen  
Figure 5: Ranges of maximum hydrogen production rates from different bacterial strains. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **Bacteria** & **Strain** & **Substrate (mM)** & **Maximum Hydrogen Production Rate** \\  & & & & **(mM/h)** \\ \hline
[36] & & CQK01 & Glucose (55) & 2.72 \\
[40] & _R. palustris_ & NCIMB 11774 & Glycerol (50) & 3.23 \\
[46] & & CQK01 & Glucose (44.7) & 2.5 \\
[48] & & P4 & Glucose (27.8) & 3.2 \\
[53] & _R. capsulata_ & & Acetate (32) & 0.79 \\  & & & Propionate (5) & 0.71 \\
[59] & & JP91 & Glucose (56) & 1.6–2.3 \\
[60] & _R. capsulatus_ & JP91 & Glucose (35) & 2.6 \\
[74] & _R. sphaeroides_ & DSM 158 & Lactic acid (40) & 8.7 \\
[76] & & HY01 & Acetate (25) + butyrate (34) & 7.0 \\
[21] & _R. rubrum_ & & Acetate & 0.50 \\
[83] & _R. sulfdophilium_ & TH-102 & & 0.42 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of different bacterial strains with their maximum hydrogen production rate. production rate of 1.75 mM/h at 30 \({}^{\circ}\)C in a PBR made of PMMA [33], while a 2.72 mM/h hydrogen production rate between the range 20-40 \({}^{\circ}\)C, when a cylindrical PMMA PBR with chitosan medium-coated optical fiber was used [36], demonstrating the importance of the bioreactor design, its material, and the temperature at which the reactor is operated for hydrogen production. On the other hand, _R. capsulatus_, another bacterium of the same genus already discussed in the review, has mostly been used with substrates, such as acetate, propionate, butyrate, or their mixtures in PBRs or glass vials, but the studies shown in Table 2 indicate that the hydrogen production rates of these microbes are low, as the light conversion is closely linked to their biosynthesis.