“Photosynthetic water splitting (or oxygen evolution) is one of the most important reactions on the planet, since it is the source of nearly all the atmosphere's oxygen […] The mechanism of water oxidation is still not fully elucidated, but we know many details about this process.” Source: Wikipedia
“Photosynthetic organisms capture visible light in their light-harvesting complex and transfer the excitation energy to the reaction center which stores the energy from the photon in chemical bonds. This process occurs with nearly perfect efficiency.” […] “The high efficiency of this process is not yet understood today.” Gain and loss in open quantum systems, Institute for Quantum Science and Engineering, Texas A&M University, and Max Planck Institute for the Physics of Complex Systems, 2017
The efficiency is thought to be very close to 100% (exceptionally high for any complex system) which has led some researchers to suggests that quantum 'superposition' effects and/or may be in operation with respect to the way that photons are somehow channelled towards the most efficient pathways within the light-harvesting complexes. These theories have not yet been generally accepted.