Oxygenic photosynthesis, that takes place in cyanobacteria algae and plants, provides most of the food and fuel on earth. The light stage of this process is driven by two photosystems. Photosystem II (PSII) that oxidizes water to O2 and 4 H+ and photosystem I (PSI) which in the light provides the most negative redox potential in nature that can drive numerous reactions including CO2 assimilation and hydrogen (H2) production. The structure of most of the complexes involved in oxygenic photosynthesis was solved in several laboratories including our own. Utilizing our plant PSI crystals we were able to generate a light dependent electric potential of up to 100 V. We will develop this system for designing biological based photoelectric devices. Recently, we discovered a marine phage that carries an operon encoding all PSI subunits. Generation, in synechocystis, of a phage-like PSI enabled the mutated complex to accept electrons from additional sources like respiratory cytochromes. This way a novel photorespiration, where PSI can substitute for cytochrome oxidase, is created. The wild type and mutant synechocystis PSI were crystallized and solved, confirming the suggested structural consequences. Moreover, several structural alterations in the mesophilic PSI were recorded. We designed a hydrogen producing bioreactor where the novel photorespiration will enable to utilize the organic material of the cell as an electron source for H2 production. We propose that in conjunction of engineering a Cyanobacterium strain with a temperature sensitive PSII, enhancing rates in its respiratory chain an efficient and sustainable hydrogen production can be achieved.