## 4 Applications of Photosynthetic Systems
### Environmental Science Applications  
Over the past years, significant research efforts have been devoted for the production of energy from biomass to meet the increasing energy demands worldwide and keeping a check on the environmental issues. Microorganisms such as cyanobacteria and microalgae possess a significant potential in the production of biofuels, chemicals, and bio-based products for meeting the global energy crisis [143; 144; 145; 146; 147; 148; 149; 150]. Such renewable alternatives would not only limit the reliance on fossil fuel resources \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **HOMO (eV)** & **LUMO (eV)** & **Gap** \\ \hline BChl 1 & \(-\)5.286 & \(-\)3.202 & 2.084 \\ BChl 2 & \(-\)4.669 & \(-\)3.1 & 1.569 \\ BChl 3 & \(-\)4.394 & \(-\)2.1 & 2.294 \\ BChl 4 & \(-\)5.142 & \(-\)3.102 & 2.04 \\ BChl 5 & \(-\)5.499 & \(-\)4.361 & 1.138 \\ BChl 6 & \(-\)5.247 & \(-\)3.091 & 2.156 \\ BChl 7 & \(-\)5.331 & \(-\)3.245 & 2.086 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: HOMO and LUMO energies for the different types of BChls taken from the FMO light harvesting complex. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Excitation** & \multicolumn{3}{c}{**BChl 6**} & \multicolumn{3}{c}{**BChl 7**} \\ \hline  & **Energy (eV)** & **Wavelength (nm)** & **f** & **Energy (eV)** & **Wavelength (nm)** & **f** \\ \hline S0-S1 & 1.9035 & 651.35 & 0.3489 & 0.7744 & 1601.08 & 0.0025 \\ S0-S2 & 2.2434 & 552.67 & 0.1275 & 1.0929 & 1134.41 & 0.0078 \\ S0-S3 & 2.9733 & 416.99 & 0.0007 & 1.1909 & 1041.14 & 0.0172 \\ S0-S4 & 3.0891 & 401.36 & 0.000 & 1.7776 & 697.48 & 0.148 \\ S0-S5 & 3.1408 & 394.75 & 0.0026 & 1.9835 & 625.07 & 0.0146 \\ S0-S6 & 3.3296 & 372.37 & 0.0066 & 2.1139 & 586.51 & 0.357 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Calculation of the excitation energy, wavelength and oscillator strength of the first six excited states for two different BChls calculated by using time-dependent density functional theory. Figure 6: (Color online) **(a)** HOMO and **(b)** LUMO representation for one of the BChls using TDDFT. but could also serve as a stepping stone for effective transition from a petroleum-based economy to a bio-based economy [145; 148]. Notably, the microbial fuel cell represents a promising alternative of electricity generation from the catalysis of microorganisms found in lakes, lagoons, ponds, or even waste water reserves [151; 152; 153]. Microalgae is one of the examples of the photosynthetic bioorganisms that is used for the production of green bioelectricity utilizing microbial fuel cells. Furthermore, owing to the natural abundance of such living microorganisms, the bioelectricity produced is a highly cost-effective and sustainable alternative for meeting the energy demands of the growing population. However, the large-scale industrial translation of this technology is currently limited owing to several biotechnological, economic and environmental issues. Some of the associated bottlenecks, mainly related to its low stability and efficiency, can be effectively tackled by the synergetic combination of synthetic biology and nanotechnology. For example, the native electrogenic capacity and the working lifetimes of microbiological cells can be augmented utilizing the tunability of nanomaterials. More recently, an overview of the different nanomaterials used to enhance bioelectricity generation through improved photosynthesis, extracellular electron transfer and anode performance have been reviewed in [153]. More details about the photosynthetic microbial fuel cells and their integration with the conventional technology of the microbial fuel cell can be found in [154; 155; 156; 157; 158; 159]. The photosynthesis can also be used for energy storage, as described in [160; 161; 162; 163; 164; 165]. The effects of different kinds of light sources on the production of biomass and pigments have been studied in the photosynthetic bacteria waste water treatment in [166].