## 4 Applications of Photosynthetic Systems
### Environmental Science Applications
The applications, opportunities and challenges of using microalgae group Chlorophyta for various energy and environmental applications have been reviewed in [167], highlighting some critical aspects such as the applicability of Chlorophyta in industrial and domestic waste water treatment, and removal of contaminating nutrients. Recent research advances in the waste water treatment utilizing the freshwater monocultures of filamentous algae have been reviewed and critically analyzed in [168], mainly focusing on microalgae and polyculture systems. Research gaps in translating this technology to large-scale system design, including species selection criteria, the effect of nutrient type and loading conditions, inorganic carbon supply, algae-bacteria interactions and parameters, viz., pond depth, mixing and harvesting regimes were identified along with providing a future road map for maximizing productivity and waste water treatment efficiency [168]. Furthermore, the green algae (Chlorella Kessleri) is one of the most common light harvesting complexes found in the water which can be used to measure the purity of water utilizing rigorous study of photosynthesis phenomena in them. This can be done by measuring the concentration of oxygen produced by photosynthesis in water containing green algae [169; 170; 171]. The detailed understanding and analysis of quantum coherence and spectral density in the light harvesting complexes would also be helpful in the understanding of processes pertinent to the solar and photovoltaic cells [172; 173; 174; 175; 176; 177; 178]. Recently, different theoretical models used for describing energy absorption and transmission in solar cells and photosynthetic systems, including the FMO complex, have been critically analyzed in [179]. This study highlights that the use of sinks, traps or any artificial relaxation process in the standard theoretical models of solar energy conversion, which is developed for studying the energy transfer to the reaction center in photosynthetic systems may contradict to the second law of thermodynamics. These findings could invalidate several existing models used for studying solar energy conversion and raise significant concerns regarding some of the earlier drawn conclusions. A possible solution to address this issue has been put forward in [179] by providing a thermodynamically-consistent version of the model that explicitly describes parts of the reaction center and employs a Hamiltonian transfer to describe the energy absorption and transmission instead of a decay rate or sink term. Owing to the difficulties to probe molecular self-assembling and packing structures at the atomic level by experimental techniques, theoretical simulations are becoming a useful tool in our better understanding of the structure-property relationship of the electronic processes for organic solar cells [180; 181; 182; 183; 184; 185]. Recent advances in the theoretical simulations for organic solar cells ranging from the molecular dynamics simulated packing structures to the electronic processes computed by quantum-chemical, in combination with kinetic Monte Carlo, simulations have been reviewed in [180]. The future perspective and challenges associated with the prediction of electrical characteristics and photoelectric conversion efficiencies of organic solar cells from molecular structures utilizing theoretical simulations have also been highlighted in [180].